The Chernobyl disaster was a catastrophic
nuclear accident that occurred on 26 April
1986 at the No. 4 nuclear reactor in the Chernobyl
Nuclear Power Plant, near the city of Pripyat
in the north of the Ukrainian SSR.The accident
occurred during a safety test on a common
Soviet reactor type - the RBMK nuclear power
reactor. The test, a simulation of an electrical
power outage, was to aid the development of
a safety procedure for keeping reactor cooling
water circulating until the emergency generators
could provide power. This gap was about one
minute and had been identified as a potential
safety problem which could cause reactor core
overheating. Three such tests had been conducted
since 1982 but had failed to provide a solution.
On the fourth occasion, which became catastrophic,
the test had been delayed by ten hours, and
the reactor operating shift that had specifically
prepared for the test procedure was replaced
by the next shift. The test supervisor then
failed to follow the test procedure, creating
unstable operating conditions which, combined
with inherent RBMK reactor design flaws and
the intentional disabling of several emergency
safety systems, resulted in an uncontrolled
nuclear chain reaction. A huge amount of energy
was suddenly released, which vapourised superheated
cooling water, rupturing the reactor pressure
vessel in a highly destructive steam explosion,
which was instantly followed by an open-air
reactor core fire.This fire produced considerable
updrafts for about nine days
before being finally contained on 4 May 1986.
The lofted plumes of fission products released
into the atmosphere by the fire precipitated
onto parts of the USSR and western Europe.
The estimated radioactive inventory that was
released during this very hot fire phase approximately
equaled in magnitude the airborne fission
products released in the initial destructive
explosion.The total number of casualties remains
disputed. Estimates of reduced life expectancy
as a result of radiation released are highly
uncertain and vary from 4,000 people in a
United Nations study up to 200,000 reported
by a Greenpeace study. During the accident,
steam-blast effects caused two deaths within
the facility: one immediately after the explosion
and the other compounded by a lethal dose
of ionizing radiation. Over the coming days
and weeks, 134 servicemen were hospitalized
with acute radiation syndrome (ARS), of whom
28 firemen and employees died within months.
Additionally, approximately 14 radiation-induced
cancer deaths among this group of 134 hospitalized
survivors were to follow within the next 10
years. Among the wider population, an excess
of 15 childhood thyroid cancer deaths were
documented as of 2011. Additional time and
research is required to definitively determine
the elevated relative risk of cancer among
the surviving employees, those that were initially
hospitalized with ARS, and the population
at large.The Chernobyl accident is considered
the most disastrous nuclear power plant accident
in history, both in terms of cost and casualties.
It is one of only two nuclear energy accidents
classified as a "level 7 major accident",
the maximum classification on the International
Nuclear Event Scale; the other was the 2011
Fukushima disaster in Japan. The struggle
to safeguard against scenarios that were perceived
as having the potential for greater catastrophe,
together with later decontamination efforts
of the surroundings, ultimately involved over
500,000 liquidators and cost an estimated
18 billion rubles (roughly $30 billion USD
in 1986, or $68 billion USD in 2019 adjusted
for inflation).The remains of the No. 4 reactor
building were soon enclosed in the sarcophagus,
a large shelter designed to reduce the spread
of radioactive contamination from the wreckage
and to protect the site from further weathering.
This was rapidly built and was finished by
December 1986, when the reactor was entering
the cold shutdown phase. The shelter also
provided radiological protection for the crews
of the other undamaged reactors at the power
station, with No. 3 continuing to produce
electricity until 2000. Due to the continued
deterioration of the sarcophagus, both it
and the No. 4 reactor were further enclosed
in 2017 by the Chernobyl New Safe Confinement,
a larger, state of the art enclosure, designed
and built by an international team. This structure
has the ability to facilitate the removal
of both the sarcophagus and the reactor debris,
while containing the radioactive contamination.
The accident prompted safety upgrades on all
remaining Soviet-designed RBMK reactors, of
which 10 continue to power electric grids
as of 2019.
== Overview ==
The disaster began during a systems test on
26 April 1986, in reactor No. 4 of the V.
I. Lenin Nuclear Power Plant, near Pripyat
and in proximity to the administrative border
with Belarus and the Dnieper River. There
was a sudden and unexpected power surge. When
operators attempted an emergency shutdown,
a much larger spike in power output occurred.
This second spike led to a reactor vessel
rupture and a series of steam explosions.
These events exposed the graphite moderator
of the reactor to air, causing it to ignite.
For the next week, the resulting fire sent
long plumes of highly radioactive dust into
the atmosphere which caused radioactive fallout
over an extensive geographical area, including
Pripyat. The plumes drifted over large parts
of the western Soviet Union and Europe. According
to official post-Soviet data, about 60% of
the fallout landed in Belarus.
Thirty-six hours after the accident, Soviet
officials established a 10-kilometre exclusion
zone, which resulted in the rapid evacuation
of 49,000 people, primarily from Pripyat,
the nearest large population centre.During
the accident the wind changed direction; the
fact that the different plumes from the reactor
had different ratios of radioisotopes in them
indicates that the relative release rates
of different elements from the accident site
was changing.As plumes and subsequent fallout
continued to be generated, the evacuation
zone was increased from 10 to 30 km about
one week after the accident. A further 68,000
persons were evacuated, including from the
town of Chernobyl itself. The surveying and
detection of isolated fallout hotspots outside
this zone over the following year eventually
resulted in 135,000 long-term evacuees in
total agreeing to be moved. The years between
1986 and 2000 saw the near tripling in the
total number of permanently resettled persons
from the most severely contaminated areas
to approximately 350,000.The accident raised
the already heightened concerns about fission
reactors worldwide, and while most concern
was focused on those of the same unusual design,
hundreds of disparate nuclear reactor proposals,
including those under construction at Chernobyl,
reactors numbers 5 and 6, were eventually
cancelled. With ballooning costs as a result
of new nuclear reactor safety system standards
and the legal and political costs in dealing
with the increasingly hostile/anxious public
opinion, there was a precipitous drop in the
rate of new startups after 1986.The accident
also raised concerns about the cavalier safety
culture in the Soviet nuclear power industry,
slowing industry growth and forcing the Soviet
government to become less secretive about
its procedures. The government coverup of
the Chernobyl disaster was a catalyst for
glasnost, which "paved the way for reforms
leading to the Soviet collapse." Numerous
structural and construction quality issues
as well as deviations from the original design
of the plant were known to KGB at least since
1973 and passed to the Central Committee which
did not take any actions and classified it.
== Accident ==
On 26 April 1986, at 01:23:40 (Moscow Summer
Time; UTC+04:00), reactor no. 4 suffered a
catastrophic power increase, leading to explosions
in its core. As the reactor had not been encased
by any kind of hard containment vessel, this
dispersed large quantities of radioactive
isotopes into the atmosphere and caused an
open-air fire that increased the emission
of radioactive particles carried by the smoke.
The accident occurred during an experiment
scheduled to test the viability of a potential
safety emergency core cooling system, which
required a normal reactor shutdown procedure.
=== Steam turbine tests ===
In steady-state operation, a significant fraction
(over 6%) of the power from a nuclear reactor
is derived not from fission but from the decay
heat of its accumulated fission products.
This heating continues for some time after
the chain reaction has been stopped (e.g.
following an emergency SCRAM) and active cooling
may be required to prevent core meltdown.
RBMK reactors like those at Chernobyl use
water as a coolant. Reactor 4 at Chernobyl
consisted of about 1,600 individual fuel channels,
each of which required coolant flow of 28
metric tons (28,000 litres or 7,400 US gallons)
per hour.Since cooling pumps still require
electricity and must run for some time after
an emergency shutdown or SCRAM, in the event
of a power grid failure, each of Chernobyl's
reactors had three backup diesel generators.
The backup generators could start up in 15
seconds, but took 60–75 seconds to attain
full speed and generate the 5.5‑megawatt
(MW) output required to run one main pump.To
maintain cooling during this one-minute power
gap—considered an unacceptable safety risk—it
had been theorized that rotational energy
from the steam turbine (as it wound down under
residual steam pressure) could be used to
generate the required electrical power. Analysis
indicated that this residual momentum and
steam pressure might be sufficient to run
the coolant pumps for 45 seconds, bridging
the gap between an external power failure
and the full availability of the emergency
generators.This capability still needed to
be confirmed experimentally, and previous
tests had ended unsuccessfully. An initial
test carried out in 1982 indicated that the
excitation voltage of the turbine-generator
was insufficient; it did not maintain the
desired magnetic field after the turbine trip.
The system was modified, and the test was
repeated in 1984 but again proved unsuccessful.
In 1985, a test was conducted a third time
but also yielded negative results. The test
procedure was to be run again in 1986, and
scheduled to take place during a maintenance
shutdown of Reactor 4.The test focused on
the switching sequences of the electrical
supplies for the reactor. The test procedure
was expected to begin with an automatic emergency
shutdown. No detrimental effect on the safety
of the reactor was anticipated, so the test
programme was not formally coordinated with
either the chief designer of the reactor (NIKIET)
or the scientific manager. Instead, it was
approved only by the director of the plant
(and even this approval was not consistent
with established procedures).According to
the test parameters, the thermal output of
the reactor should have been no lower than
700 MW at the start of the experiment. If
test conditions had been maintained as prescribed,
the procedure would almost certainly have
been carried out safely; the eventual disaster
resulted from attempts to boost the reactor
output once the experiment had been started
and an operational misstep had let the output
fall too low, inconsistent with approved procedure.The
Chernobyl power plant had been in operation
for two years without the capability to ride
through the first 60–75 seconds of a total
loss of electric power, and thus lacked an
important safety feature. The station managers
presumably wished to correct this at the first
opportunity, which may explain why they continued
the test even when serious problems arose,
and why the requisite approval for the test
had not been sought from the Soviet nuclear
oversight regulator (even though there was
a representative at the complex of four reactors).The
experimental procedure was intended to run
as follows:
The reactor was to be running at a low power
level, between 700 MW and 800 MW
The steam-turbine generator was to be run
up to full speed
When these conditions were achieved, the steam
supply for the turbine generator was to be
closed off
Turbine generator performance was to be recorded
to determine whether it could provide the
bridging power for coolant pumps until the
emergency diesel generators were sequenced
to start and provide power to the cooling
pumps automatically
After the emergency generators reached normal
operating speed and voltage, the turbine generator
would be allowed to continue to freewheel
down
=== 
Conditions ===
The conditions to run the test were established
before the day shift of 25 April 1986. The
day-shift workers had been instructed in advance
and were familiar with the established procedures.
A special team of electrical engineers was
present to test the new voltage regulating
system. As planned, a gradual reduction in
the output of the power unit began at 01:06
a.m. on 25 April, and the power level had
reached 50% of its nominal 3,200 MW thermal
level by the beginning of the day shift.
At this point, another regional power station
unexpectedly went offline, and the Kiev electrical
grid controller requested that the further
reduction of Chernobyl's output be postponed,
as power was needed to satisfy the peak evening
demand. The Chernobyl plant director agreed,
and postponed the test. Despite this delay,
preparations for the test not affecting the
reactor's power were carried out, including
the disabling of the emergency core cooling
system or ECCS, a passive/active system of
core cooling intended to provide water to
the core in a loss-of-coolant accident. Given
the other events that unfolded, the system
would have been of limited use, but its disabling
as a "routine" step of the test is indicative
of the lack of attention to safety in the
test. In addition, had the reactor been shut
down for the day as planned, it is possible
that more preparation would have been taken
in advance of the test.
At 23:04, the Kiev grid controller allowed
the reactor shutdown to resume. This delay
had some serious consequences: the day shift
had long since departed, the evening shift
was also preparing to leave, and the night
shift would not take over until midnight,
well into the job. According to plan, the
test should have been finished during the
day shift, and the night shift would only
have had to maintain decay heat cooling systems
in an otherwise shut-down plant.The night
shift had very limited time to prepare for
and carry out the experiment. A further rapid
decrease in the power level from 50% was executed
during the shift change-over. Anatoly Dyatlov,
deputy chief-engineer of the entire Chernobyl
Nuclear Power Plant, was present to supervise
and direct the experiment; as he out-ranked
all other supervisory personnel present, his
orders and instructions overrode any objections
of other senior personnel present during the
test and its preparation. (In 1987, Dyatlov
would be found guilty "of criminal mismanagement
of potentially explosive enterprises" and
sentenced to ten years imprisonment—of which
he would serve five—for the role that his
oversight of the experiment played in the
ensuing accident.) Serving under Dyatlov,
Alexander Akimov was chief of the night shift,
and Leonid Toptunov was the operator responsible
for the reactor's operational regimen, including
the movement of the control rods. Toptunov
was a young engineer who had worked independently
as a senior engineer for approximately three
months.The test plan called for a gradual
decrease in power output from reactor 4 to
a thermal level of 700–1000 MW. An output
of 700 MW was reached at 00:05 on 26 April.
Due to the reactor's production of a fission
byproduct, xenon-135, which is a reaction-inhibiting
neutron absorber, core power continued to
decrease in the absence of further operator
action—a process known as reactor poisoning.
In steady-state operation this is avoided
because xenon-135 is "burned off" as quickly
as it is created from decaying iodine-135
by the absorption of neutrons from the ongoing
chain reaction, becoming highly stable xenon-136.
With the reactor power reduced, previously
produced high quantities of iodine-135 were
decaying into the neutron-absorbing xenon-135
faster than the now reduced neutron flux could
burn it off. As the reactor power output dropped
further, to approximately 500 MW, the power
suddenly fell into an unintended near-shutdown
state, with a power output of 30 MW thermal
or less. The exact circumstances that caused
the power fall are unknown because Akimov
died in hospital on 10 May and Toptunov on
14 May; early reports attributed it to Toptunov
mistakenly inserting the control rods too
far into the core, but it has also been suggested
it was due to an equipment failure.The reactor
was now producing 5 percent of the minimum
initial power level prescribed for the test.
Control-room personnel decided to raise power
by disconnecting most of the reactor control
rods from the automatic control rod regulation
system and manually extracting the majority
of rods to their upper limits. Several minutes
elapsed between their extraction and the point
at which the power output began to increase
and subsequently stabilize at 160–200 MW
(thermal), a much lower level than the prescribed
700 MW. The rapid reduction in the power during
the initial shutdown, and the subsequent operation
at a level of less than 200 MW led to increased
poisoning of the reactor core by the accumulation
of xenon-135. This prevented the rise of reactor
power, and made it necessary to extract additional
control rods from the reactor core in order
to counteract the poisoning.
The operation of the reactor at the low power
level (and high poisoning level) was accompanied
by unstable core temperatures and coolant
flow, and possibly by instability of neutron
flux, which triggered alarms. The control
room received repeated emergency signals regarding
the levels in the steam/water separator drums,
and large excursions or variations in the
flow rate of feed water, as well as from relief
valves opened to relieve excess steam into
a turbine condenser, and from the neutron
power controller. Between 00:35 and 00:45,
emergency alarm signals concerning thermal-hydraulic
parameters were ignored, apparently to preserve
the reactor power level.When a power level
of 200 MW was reattained, preparation for
the experiment continued. As part of the test
plan, extra water pumps were activated at
01:05 on 26 April, increasing the water flow.
The increased coolant flow rate through the
reactor produced an increase in the inlet
coolant temperature of the reactor core (the
coolant no longer having sufficient time to
release its heat in the turbine and cooling
towers), which now more closely approached
the nucleate boiling temperature of water,
reducing the safety margin.
The flow exceeded the allowed limit at 01:19,
triggering an alarm of low steam pressure
in the steam separators. At the same time,
the extra water flow lowered the overall core
temperature and reduced the existing steam
voids in the core and the steam separators.
Since water weakly absorbs neutrons (and the
higher density of liquid water makes it a
better absorber than steam), the activation
of the additional pumps decreased the reactor
power. The crew responded by turning off two
of the circulation pumps to reduce feedwater
flow, in an effort to increase steam pressure,
and by removing more manual control rods to
maintain power.The combined effect of these
various actions was an extremely unstable
reactor configuration. Nearly all of the 211
control rods had been extracted manually,
including all but 18 of the "fail-safe" manually
operated rods of the minimum 28 that were
supposed to remain fully inserted to control
the reactor even in the event of a loss of
coolant. While the emergency scram system
that would insert all control rods to shut
down the reactor could still be activated
manually (through the "AZ-5" button), the
automated system that would ordinarily do
the same had been mostly disabled to maintain
the power level, and many other automated
and even passive safety features of the reactor
had been bypassed. The reduction of reactor
coolant pumping left little safety margin;
any power excursion could produce boiling,
thereby reducing neutron absorption by the
water. The reactor configuration was outside
the safe operating envelope prescribed by
the designers. If anything pushed it into
supercriticality, it would be unable to recover
automatically.
=== Experiment and explosion ===
At 1:23:04 a.m., the test began. Four of the
main circulating pumps (MCP) were active (of
the eight total, six are normally active under
regular operation). The steam to the turbines
was shut off, beginning a run-down of the
turbine generator. The diesel generators started
and sequentially picked up loads; the generators
were to have completely picked up the MCPs'
power needs by 01:23:43. In the interim, the
power for the MCPs was to be supplied by the
turbine generator as it coasted down. As the
momentum of the turbine generator decreased,
so did the power it produced for the pumps.
The water flow rate decreased, leading to
increased formation of steam voids (bubbles)
in the core.
Unlike western light-water reactors, the RBMK
had a positive void coefficient of reactivity
at low power levels, meaning that when water
began to boil and produce voids in the coolant,
the nuclear chain reaction increased instead
of decreasing. Given this characteristic,
the No. 4 RBMK reactor operation was now at
risk of spiraling into a positive feedback
loop, in which the formation of steam voids
would reduce the ability of the liquid water
coolant to absorb neutrons, increasing the
reactor's power output, causing yet more water
to flash into steam, and yielding a further
power increase. Throughout most of the experiment
the automatic control system successfully
counteracted this positive feedback, inserting
control rods into the reactor core to limit
the power rise. However, this system had control
of only 12 rods, as nearly all the others
had been manually retracted.
At 1:23:40, as recorded by the SKALA centralized
control system, a SCRAM (emergency shutdown)
of the reactor was initiated. The SCRAM was
started when the EPS-5 button (also known
as the AZ-5 button) of the reactor emergency
protection system was pressed: this engaged
the drive mechanism on all control rods to
fully insert them, including the manual control
rods that had been withdrawn earlier. The
reason why the EPS-5 button was pressed is
not known, whether it was done as an emergency
measure in response to rising temperatures,
or simply as a routine method of shutting
down the reactor upon completion of the experiment.
One view is that the SCRAM may have been ordered
as a response to the unexpected rapid power
increase, although there is no recorded data
showing this. Some have suggested that the
button was not manually pressed, that the
SCRAM signal was automatically produced by
the emergency protection system, but the SKALA
registered a manual SCRAM signal. Despite
this, the question as to when or even whether
the EPS-5 button was pressed has been the
subject of debate. There have been assertions
that the manual SCRAM was initiated due to
the initial rapid power acceleration. Others
have suggested that the button was not pressed
until the reactor began to self-destruct,
while others believe that it happened earlier
and under calm conditions.When the EPS-5 button
was pressed, the insertion of control rods
into the reactor core began. The control rod
insertion mechanism moved the rods at 0.4
metres per second (1.3 ft/s), so that the
rods took 18 to 20 seconds to travel the full
height of the core, about 7 metres (23 ft).
A bigger problem was the design of the RBMK
control rods, each of which had a graphite
neutron moderator section attached to its
end to boost reactor output by displacing
water when the control rod section had been
fully withdrawn from the reactor. That is,
when a control rod was at maximum extraction,
a neutron-moderating graphite extension was
centered in the core with 1.25 metres (4.1
ft) columns of water above and below it. Consequently,
injecting a control rod downward into the
reactor in a SCRAM initially displaced (neutron-absorbing)
water in the lower portion of the reactor
with (neutron-moderating) graphite. Thus,
an emergency SCRAM initially increased the
reaction rate in the lower part of the core
as the graphite extensions of rods moving
down in the reactor displaced water coolant.
This behaviour was discovered when the initial
insertion of control rods in another RBMK
reactor at Ignalina Nuclear Power Plant in
1983 induced a power spike, but as the subsequent
SCRAM of that reactor was successful, the
subsequently disseminated information had
been deemed of little importance.
A few seconds into the SCRAM, a power spike
did occur and the core overheated, causing
some of the fuel rods to fracture, blocking
the control rod columns and jamming the control
rods at one-third insertion, with the graphite
water-displacers still in the lower part of
the core. Within three seconds the reactor
output rose above 530 MW.The subsequent course
of events was not registered by instruments;
it has been reconstructed through mathematical
simulation. Per the simulation, the power
spike would have caused an increase in fuel
temperature and steam buildup, leading to
a rapid increase in steam pressure. This caused
the fuel cladding to fail, releasing the fuel
elements into the coolant, and rupturing the
channels in which these elements were located.Then,
according to some estimations, the reactor
output jumped to around 30,000 MW thermal,
ten times its normal operational output. The
last reading on the control panel was 33,000
MW. It was not possible to reconstruct the
precise sequence of the processes that led
to the destruction of the reactor and the
power unit building, but a steam explosion,
like the explosion of a steam boiler from
excess vapour pressure, appears to have been
the next event. There is a general understanding
that it was explosive steam pressure from
the damaged fuel channels escaping into the
reactor's exterior cooling structure that
caused the explosion that destroyed the reactor
casing, tearing off and blasting the upper
plate, to which the entire reactor assembly
is fastened, through the roof of the reactor
building. This is believed to be the first
explosion that many heard. This explosion
ruptured further fuel channels, as well as
severing most of the coolant lines feeding
the reactor chamber, and as a result the remaining
coolant flashed to steam and escaped the reactor
core. The total water loss in combination
with a high positive void coefficient further
increased the reactor's thermal power.
A second, more powerful explosion occurred
about two or three seconds after the first;
this explosion dispersed the damaged core
and effectively terminated the nuclear chain
reaction. This explosion also compromised
more of the reactor containment vessel and
ejected hot lumps of graphite moderator. The
ejected graphite and the demolished channels
still in the remains of the reactor vessel
caught fire on exposure to air, greatly contributing
to the spread of radioactive fallout and the
contamination of outlying areas.According
to observers outside Unit 4, burning lumps
of material and sparks shot into the air above
the reactor. Some of them fell onto the roof
of the machine hall and started a fire. About
25 percent of the red-hot graphite blocks
and overheated material from the fuel channels
was ejected. Parts of the graphite blocks
and fuel channels were out of the reactor
building. As a result of the damage to the
building an airflow through the core was established
by the high temperature of the core. The air
ignited the hot graphite and started a graphite
fire.After the larger explosion, a number
of employees at the power station went outside
to get a clearer view of the extent of the
damage. One such survivor, Alexander Yuvchenko,
recounts that once he stepped outside and
looked up towards the reactor hall, he saw
a "very beautiful" laser-like beam of light
bluish light caused by the ionization of air
that appeared to "flood up into infinity".
==== Cause of the second explosion ====
There were initially several hypotheses about
the nature of the second explosion. One view
was that the second explosion was caused by
the combustion of hydrogen, which had been
produced either by the overheated steam-zirconium
reaction or by the reaction of red-hot graphite
with steam that produced hydrogen and carbon
monoxide. Another hypothesis, by Checherov,
published in 1998, was that the second explosion
was a thermal explosion of the reactor as
a result of the uncontrollable escape of fast
neutrons caused by the complete water loss
in the reactor core. A third hypothesis was
that the second explosion was another steam
explosion. According to this version, the
first explosion was a more minor steam explosion
in the circulating loop, causing a loss of
coolant flow and pressure that in turn caused
the water still in the core to flash to steam;
this second explosion then caused the majority
of the damage to the reactor and containment
building.
The force of the second explosion and the
ratio of xenon radioisotopes released after
the accident (a vital tool in nuclear forensics)
indicated to Yuri V. Dubasov in a 2009 publication
(suggested before him by Checherov in 1998),
that the second explosion could have been
a nuclear power transient resulting from core
material melting in the absence of its water
coolant and moderator. Dubasov argues that
the reactor did not simply undergo a runaway
delayed-supercritical/exponential increase
in power into the multi-gigawatt power range.
This permitted a dangerous "positive feedback"/runaway
condition, given the lack of inherent safety
stops when power levels began to increase
above the commercial level.Although a positive-feedback
power excursion that increased until the reactor
disassembled itself by means of its internal
energy and external steam explosions is the
more accepted explanation for the cause of
the explosions, Dubasov argues instead that
a runaway prompt supercriticality occurred,
with the internal physics being more similar
to the explosion of a fizzled nuclear weapon,
and that this failed/fizzle event produced
the second explosion.This nuclear fizzle hypothesis,
then mostly defended by Dubasov, was examined
further in 2017 by retired physicist Lars-Erik
De Geer in an analysis that puts the hypothesized
fizzle event as the more probable cause of
the first explosion. The more energetic second
explosion, which produced the majority of
the damage, has been estimated by Dubasov
in 2009 as equivalent to 40 billion joules
of energy, the equivalent of about ten tons
of TNT. Both the 2009 and 2017 analyses argue
that the nuclear fizzle event, whether producing
the second or first explosion, consisted of
a prompt chain reaction (as opposed to the
consensus delayed neutron mediated chain-reaction)
that was limited to a small portion of the
reactor core, since expected self-disassembly
occurs rapidly in fizzle events.
==== Fire ====
Contrary to safety regulations, bitumen, a
combustible material, had been used in the
construction of the roof of the reactor building
and the turbine hall. Ejected material ignited
at least five fires on the roof of the adjacent
reactor 3, which was still operating. It was
imperative to put those fires out and protect
the cooling systems of reactor 3. Inside reactor
3, the chief of the night shift, Yuri Bagdasarov,
wanted to shut down the reactor immediately,
but chief engineer Nikolai Fomin would not
allow this. The operators were given respirators
and potassium iodide tablets and told to continue
working. At 05:00, Bagdasarov made his own
decision to shut down the reactor, leaving
only those operators there who had to work
the emergency cooling systems.
=== Radiation levels ===
The ionizing radiation levels in the worst-hit
areas of the reactor building have been estimated
to be 5.6 roentgens per second (R/s), equivalent
to more than 20,000 roentgens per hour. A
lethal dose is around 500 roentgens (~5 Gray
(Gy) in modern radiation units) over 5 hours,
so in some areas, unprotected workers received
fatal doses in less than a minute. However,
a dosimeter capable of measuring up to 1,000
R/s was buried in the rubble of a collapsed
part of the building, and another one failed
when turned on. All remaining dosimeters had
limits of 0.001 R/s and therefore read "off
scale". Thus, the reactor crew could ascertain
only that the radiation levels were somewhere
above 0.001 R/s (3.6 R/h), while the true
levels were much higher in some areas.Because
of the inaccurate low readings, the reactor
crew chief Alexander Akimov assumed that the
reactor was intact. The evidence of pieces
of graphite and reactor fuel lying around
the building was ignored, and the readings
of another dosimeter brought in by 04:30 were
dismissed under the assumption that the new
dosimeter must have been defective. Akimov
stayed with his crew in the reactor building
until morning, sending members of his crew
to try to pump water into the reactor. None
of them wore any protective gear. Most, including
Akimov, died from radiation exposure within
three weeks.
== Immediate crisis management ==
=== 
Fire containment ===
Shortly after the accident, at 01:45, firefighters
arrived to try to extinguish the fires. First
on the scene was a Chernobyl Power Station
firefighter brigade under the command of Lieutenant
Volodymyr Pravik, who died on 9 May 1986 of
acute radiation sickness. They were not told
how dangerous radioactive the smoke and the
debris were, and may not even have known that
the accident was anything more than a regular
electrical fire: "We didn't know it was the
reactor. No one had told us."Grigorii Khmel,
the driver of one of the fire engines, later
described what happened:
"We arrived there at 10 or 15 minutes to two
in the morning ... We saw graphite scattered
about. Misha asked: "Is that graphite?" I
kicked it away. But one of the fighters on
the other truck picked it up. "It's hot,"
he said. The pieces of graphite were of different
sizes, some big, some small, enough to pick
them up [...] We didn't know much about radiation.
Even those who worked there had no idea. There
was no water left in the trucks. Misha filled
a cistern and we aimed the water at the top.
Then those boys who died went up to the roof—Vashchik,
Kolya and others, and Volodya Pravik ... They
went up the ladder ... and I never saw them
again.”
Anatoli Zakharov, a fireman stationed in Chernobyl
since 1980, offers a different description
in 2008:
"I remember joking to the others, 'There must
be an incredible amount of radiation here.
We'll be lucky if we're all still alive in
the morning.'"
He also stated:
"Of course we knew! If we'd followed regulations,
we would never have gone near the reactor.
But it was a moral obligation—our duty.
We were like kamikaze."
The immediate priority was to extinguish fires
on the roof of the station and the area around
the building containing Reactor No. 4 to protect
No. 3 and keep its core cooling systems intact.
The fires were extinguished by 5:00, but many
firefighters received high doses of radiation.
The fire inside reactor 4 continued to burn
until 10 May 1986; it is possible that well
over half of the graphite burned out.The fire
was extinguished by a combined effort of helicopters
dropping over 5,000 metric tons (4,900 long
tons; 5,500 short tons) of sand, lead, clay,
and neutron-absorbing boron onto the burning
reactor and injection of liquid nitrogen.
It is now known that virtually none of the
neutron absorbers reached the core. Historians
estimate that about 600 Soviet pilots risked
dangerous levels of radiation to fly the thousands
of flights needed to cover reactor No. 4 in
this attempt to seal off radiation.From eyewitness
accounts of the firefighters involved before
they died (as reported on the CBC television
series Witness), one described his experience
of the radiation as "tasting like metal",
and feeling a sensation similar to that of
pins and needles all over his face. (This
is similar to the description given by Louis
Slotin, a Manhattan Project physicist who
died days after a fatal radiation overdose
from a criticality accident.)
The explosion and fire threw hot particles
of the nuclear fuel and also far more dangerous
fission products, radioactive isotopes such
as caesium-137, iodine-131, strontium-90,
and other radionuclides, into the air: the
residents of the surrounding area observed
the radioactive cloud on the night of the
explosion.
Equipment assembled included remote-controlled
bulldozers and robot-carts that could detect
radioactivity and carry hot debris. Valery
Legasov, first deputy director of the Kurchatov
Institute of Atomic Energy in Moscow, said
in 1987:
But we learned that robots are not the great
remedy for everything. Where there was very
high radiation, the robot ceased to be a robot—the
electronics quit working.
=== Evacuation ===
The nearby city of Pripyat was not immediately
evacuated. The townspeople, in the early hours
of the morning, at 01:23 local time, went
about their usual business, completely oblivious
to what had just happened. However, within
a few hours of the explosion, dozens of people
fell ill. Later, they reported severe headaches
and metallic tastes in their mouths, along
with uncontrollable fits of coughing and vomiting.
As the plant was run by authorities in Moscow,
the government of Ukraine did not receive
prompt information on the accident.Valentyna
Shevchenko, then Chairman of the Presidium
of Verkhovna Rada Supreme Soviet of the Ukrainian
SSR, recalls that Ukraine's acting Minister
of Internal Affairs Vasyl Durdynets phoned
her at work at 09:00 to report current affairs;
only at the end of the conversation did he
add that there had been a fire at the Chernobyl
nuclear power plant, but it was extinguished
and everything was fine. When Shevchenko asked
"How are the people?", he replied that there
was nothing to be concerned about: "Some are
celebrating a wedding, others are gardening,
and others are fishing in the Pripyat River".Shevchenko
then spoke over the phone to Volodymyr Shcherbytsky,
Head of the Central Committee of the Communist
Party of Ukraine and de facto head of state,
who said he anticipated a delegation of the
state commission headed by the deputy chairman
of the Council of Ministers of USSR.
A commission was established later in the
day to investigate the accident. It was headed
by Valery Legasov, First Deputy Director of
the Kurchatov Institute of Atomic Energy,
and included leading nuclear specialist Evgeny
Velikhov, hydro-meteorologist Yuri Izrael,
radiologist Leonid Ilyin, and others. They
flew to Boryspil International Airport and
arrived at the power plant in the evening
of 26 April. By that time two people had already
died and 52 were hospitalized. The delegation
soon had ample evidence that the reactor was
destroyed and extremely high levels of radiation
had caused a number of cases of radiation
exposure. In the early daylight hours of 27
April, approximately 36 hours after the initial
blast, they ordered the evacuation of Pripyat.
Initially it was decided to evacuate the population
for three days; later this was made permanent.By
11:00 on 27 April, buses had arrived in Pripyat
to start the evacuation. The evacuation began
at 14:00. A translated excerpt of the evacuation
announcement follows:
For the attention of the residents of Pripyat!
The City Council informs you that due to the
accident at Chernobyl Power Station in the
city of Pripyat the radioactive conditions
in the vicinity are deteriorating. The Communist
Party, its officials and the armed forces
are taking necessary steps to combat this.
Nevertheless, with the view to keep people
as safe and healthy as possible, the children
being top priority, we need to temporarily
evacuate the citizens in the nearest towns
of Kiev region. For these reasons, starting
from 27 April 1986, 14:00 each apartment block
will be able to have a bus at its disposal,
supervised by the police and the city officials.
It is highly advisable to take your documents,
some vital personal belongings and a certain
amount of food, just in case, with you. The
senior executives of public and industrial
facilities of the city has decided on the
list of employees needed to stay in Pripyat
to maintain these facilities in a good working
order. All the houses will be guarded by the
police during the evacuation period. Comrades,
leaving your residences temporarily please
make sure you have turned off the lights,
electrical equipment and water and shut the
windows. Please keep calm and orderly in the
process of this short-term evacuation.
To expedite the evacuation, residents were
told to bring only what was necessary, and
that they would remain evacuated for approximately
three days. As a result, most personal belongings
were left behind, and remain there today.
By 15:00, 53,000 people were evacuated to
various villages of the Kiev region. The next
day, talks began for evacuating people from
the 10-kilometre (6.2 mi) zone. Ten days after
the accident, the evacuation area was expanded
to 30 kilometres (19 mi). This "exclusion
zone" has remained ever since, although its
shape has changed and its size has been expanded.
=== Delayed announcement ===
Evacuation began long before the accident
was publicly acknowledged by the Soviet Union.
In the morning of 28 April, radiation levels
set off alarms at the Forsmark Nuclear Power
Plant in Sweden, over 1,000 kilometres (620
mi) from the Chernobyl Plant. Workers at Forsmark
reported the case to the Swedish Radiation
Safety Authority, which determined that the
radiation had originated elsewhere. That day,
the Swedish government contacted the Soviet
government to inquire about whether there
had been a nuclear accident in the Soviet
Union. The Soviets initially denied it, and
it was only after the Swedish government suggested
they were about to file an official alert
with the IAEA, that the Soviet government
admitted an accident took place at Chernobyl.At
first, the Soviets only conceded that a minor
accident had occurred, but once they began
evacuating over 100,000 people, the full scale
of the situation was realized by the global
community. At 21:02 the evening of 28 April,
a 20-second announcement was read in the TV
news programme Vremya: "There has been an
accident at the Chernobyl Nuclear Power Plant.
One of the nuclear reactors was damaged. The
effects of the accident are being remedied.
Assistance has been provided for any affected
people. An investigative commission has been
set up." This was the entirety of the announcement
of the accident. The Telegraph Agency of the
Soviet Union (TASS) then discussed Three Mile
Island and other American nuclear accidents,
an example of the common Soviet tactic of
emphasizing foreign disasters when one occurred
in the Soviet Union. The mention of a commission,
however, indicated to observers the seriousness
of the incident, and subsequent state radio
broadcasts were replaced with classical music,
which was a common method of preparing the
public for an announcement of a tragedy.Around
the same time, ABC News released its report
about the disaster. Shevchenko was the first
of the Ukrainian state top officials to arrive
at the disaster site early on 28 April. There
she spoke with members of medical staff and
people, who were calm and hopeful that they
could soon return to their homes. Shevchenko
returned home near midnight, stopping at a
radiological checkpoint in Vilcha, one of
the first that were set up soon after the
accident.There was a notification from Moscow
that there was no reason to postpone the 1
May International Workers' Day celebrations
in Kiev (including the annual parade), but
on 30 April a meeting of the Political bureau
of the Central Committee of the CPSU took
place to discuss the plan for the upcoming
celebration. Scientists were reporting that
the radiological background level in Kiev
was normal. At the meeting, which was finished
at 18:00, it was decided to shorten celebrations
from the regular 3.5–4 hours to under 2
hours. Several buildings in Pripyat were officially
kept open after the disaster to be used by
workers still involved with the plant. These
included the Jupiter Factory which closed
in 1996 and the Azure Swimming Pool, used
by the liquidators for recreation during the
clean-up, which closed in 1998.
=== Steam explosion risk ===
Two floors of bubbler pools beneath the reactor
served as a large water reservoir for the
emergency cooling pumps and as a pressure
suppression system capable of condensing steam
in case of a small broken steam pipe; the
third floor above them, below the reactor,
served as a steam tunnel. The steam released
by a broken pipe was supposed to enter the
steam tunnel and be led into the pools to
bubble through a layer of water. After the
disaster, the pools and the basement were
flooded because of ruptured cooling water
pipes and accumulated firefighting water,
and constituted a serious steam explosion
risk.
The smoldering graphite, fuel and other material
above, at more than 1,200 °C (2,190 °F),
started to burn through the reactor floor
and mixed with molten concrete from the reactor
lining, creating corium, a radioactive semi-liquid
material comparable to lava. If this mixture
had melted through the floor into the pool
of water, it was feared it could have created
a serious steam explosion that would have
ejected more radioactive material from the
reactor. It became necessary to drain the
pool.The bubbler pool could be drained by
opening its sluice gates. However, the valves
controlling it were underwater, located in
a flooded corridor in the basement. Volunteers
in wetsuits and respirators (for protection
against radioactive aerosols) and equipped
with dosimeters, entered the knee-deep radioactive
water and managed to open the valves. These
were the engineers Alexei Ananenko and Valeri
Bezpalov (who knew where the valves were),
accompanied by the shift supervisor Boris
Baranov. Upon succeeding, all risk of a further
steam explosion was eliminated. All three
men were awarded the Order For Courage by
Ukrainian President Petro Poroshenko in May
2018.Research by Andrew Leatherbarrow, author
of Chernobyl 01:23:40, determined that the
frequently recounted story which suggests
that all three men died just days after the
incident is false. Alexei Ananenko continues
to work in the nuclear energy industry, and
rebuffs the growth of the Chernobyl media
sensationalism surrounding him.
While Valeri Bezpalov was found to still be
alive by Leatherbarrow, the 65-year-old Baranov
had lived until 2005 and had died of heart
failure.
Once the bubbler pool gates were opened by
the Ananenko team, fire brigade pumps were
then used to drain the basement. The operation
was not completed until 8 May, after 20,000
metric tons (20,000 long tons; 22,000 short
tons) of highly radioactive water were pumped
out.
With the bubbler pool gone, a meltdown was
less likely to produce a powerful steam explosion.
To do so, the molten core would now have to
reach the water table below the reactor. To
reduce the likelihood of this, it was decided
to freeze the earth beneath the reactor, which
would also stabilize the foundations. Using
oil well drilling equipment, the injection
of liquid nitrogen began on 4 May. It was
estimated that 25 metric tons of liquid nitrogen
per day would be required to keep the soil
frozen at −100 °C (−148 °F). This idea
was soon scrapped.
As an alternative, coal miners were deployed
to excavate a tunnel below the reactor. From
there the bottom room where the cooling system
would have been installed, was filled with
concrete to strengthen the foundation below
the reactor.It is likely that intense alpha
radiation hydrolysed the water, generating
a low-pH hydrogen peroxide (H2O2) solution
akin to an oxidizing acid. Conversion of bubbler
pool water to H2O2 is confirmed by the presence
in the Chernobyl lavas of studtite and metastudtite,
the only minerals that contain peroxide.
=== Debris removal ===
In the months after the explosion, attention
turned to removing the radioactive debris
from the roof. The worst of the radioactive
debris was collected inside what was left
of the reactor, however it was estimated that
there was approximately 100 tons of debris
on that roof that resulted from the explosion
and which had to be removed to enable the
safe construction of the 'sarcophagus' – a
concrete structure which would entomb the
reactor and reduce radioactive dust being
released into the atmosphere. The initial
plan was to use robots to clear the debris
off the roof. The Soviets used approximately
60 remote-controlled robots, most of them
built in the Soviet Union although many failed
due to the effect of high levels of radiation
on their electronic controls.Consequently,
the most highly radioactive materials were
shoveled by Chernobyl liquidators from the
military wearing heavy protective gear (dubbed
"bio-robots" by the military); these soldiers
could only spend a maximum of 40-90 seconds
working on the rooftops of the surrounding
buildings because of the extremely high doses
of radiation given off by the blocks of graphite
and other debris. Though the soldiers were
only supposed to perform the role of the "bio-robot"
a maximum of once, some soldiers reported
having done this task five or six times. Only
10% of the debris cleared from the roof was
performed by robots with the other 90% removed
by approximately 5,000 men who absorbed, on
average, an estimated dose of 25 rem (250
mSv) of radiation each.At the time there was
still fear that the reactor could re-enter
a self-sustaining nuclear chain-reaction and
explode again, and a new containment structure
was planned to prevent rain entering and triggering
such an explosion, and to prevent further
release of radioactive material. This was
the largest civil engineering task in history,
involving a quarter of a million construction
workers who all reached their official lifetime
limits of radiation. Ukrainian filmmaker Vladimir
Shevchenko captured film footage of an Mi-8
helicopter as its main rotor collided with
a nearby construction crane cable, causing
the helicopter to fall near the damaged reactor
building and killing its four-man crew on
2 October 1986. By December 1986, a large
concrete sarcophagus had been erected to seal
off the reactor and its contents. A unique
"clean up" medal was given to the workers.
Although many of the radioactive emergency
vehicles were buried in trenches, many of
the vehicles used by the liquidators, including
the helicopters, still remain parked in a
field in the Chernobyl area. Scavengers have
since removed many functioning, but highly
radioactive, parts.Liquidators worked under
deplorable conditions, poorly informed and
with poor protection. Many, if not most of
them, exceeded radiation safety limits. Some
exceeded limits by over 100 times—leading
to rapid death.During the construction of
the sarcophagus, a scientific team re-entered
the reactor as part of an investigation dubbed
"Complex Expedition", to locate and contain
nuclear fuel in a way that could not lead
to another explosion. These scientists manually
collected cold fuel rods, but great heat was
still emanating from the core. Rates of radiation
in different parts of the building were monitored
by drilling holes into the reactor and inserting
long metal detector tubes. The scientists
were exposed to high levels of radiation and
radioactive dust.After six months of investigation,
in December 1986, they discovered with the
help of a remote camera an intensely radioactive
mass in the basement of Unit Four, more than
two metres wide, which they called "the elephant's
foot" for its wrinkled appearance. The mass
was composed of melted sand, concrete and
a large amount of nuclear fuel that had escaped
from the reactor. The concrete beneath the
reactor was steaming hot, and was breached
by now-solidified lava and spectacular unknown
crystalline forms termed chernobylite. It
was concluded that there was no further risk
of explosion.The official contaminated zones
became stage to a massive clean-up effort
lasting seven months. The official reason
for such early (and dangerous) decontamination
efforts, rather than allowing time for natural
decay, was that the land must be re-peopled
and brought back into cultivation. Indeed,
within fifteen months 75% of the land was
under cultivation, even though only a third
of the evacuated villages were resettled.
Defence forces must have done much of the
work. Yet this land was of marginal agricultural
value. According to historian David Marples,
the administration had a psychological purpose
for the clean-up: they wished to forestall
panic regarding nuclear energy, and even to
restart the Chernobyl power station.
== Causes ==
=== INSAG-1 report, 1986 ===
The first official explanation of the accident,
later acknowledged to be erroneous, was published
in August 1986. It effectively placed the
blame on the power plant operators. To investigate
the causes of the accident the IAEA created
a group known as the International Nuclear
Safety Advisory Group (INSAG), which in its
report of 1986, INSAG-1, on the whole also
supported this view, based on the data provided
by the Soviets and the oral statements of
specialists. In this view, the catastrophic
accident was caused by gross violations of
operating rules and regulations. "During preparation
and testing of the turbine generator under
run-down conditions using the auxiliary load,
personnel disconnected a series of technical
protection systems and breached the most important
operational safety provisions for conducting
a technical exercise."The operator error was
probably due to their lack of knowledge of
nuclear reactor physics and engineering, as
well as lack of experience and training. According
to these allegations, at the time of the accident
the reactor was being operated with many key
safety systems turned off, most notably the
Emergency Core Cooling System (ECCS), LAR
(Local Automatic control system), and AZ (emergency
power reduction system). Personnel had an
insufficiently detailed understanding of technical
procedures involved with the nuclear reactor,
and knowingly ignored regulations to speed
test completion.
The developers of the reactor plant considered
this combination of events to be impossible
and therefore did not allow for the creation
of emergency protection systems capable of
preventing the combination of events that
led to the crisis, namely the intentional
disabling of emergency protection equipment
plus the violation of operating procedures.
Thus the primary cause of the accident was
the extremely improbable combination of rule
infringement plus the operational routine
allowed by the power station staff.
In this analysis of the causes of the accident,
deficiencies in the reactor design and in
the operating regulations that made the accident
possible were set aside and mentioned only
casually. Serious critical observations covered
only general questions and did not address
the specific reasons for the accident.
The following general picture arose from these
observations, and several procedural irregularities
also helped to make the accident possible,
one of which was insufficient communication
between the safety officers and the operators
in charge of the experiment being run that
night.
The reactor operators disabled safety systems
down to the generators, which the test was
really about. The main process computer, SKALA,
was running in such a way that the main control
computer could not shut down the reactor or
even reduce power. Normally the computer would
have started to insert all of the control
rods. The computer would have also started
the "Emergency Core Protection System" that
introduces 24 control rods into the active
zone within 2.5 seconds, which is still slow
by 1986 standards. All control was transferred
from the process computer to the human operators.
On the subject of the disconnection of safety
systems, Valery Legasov said, in 1987, "It
was like airplane pilots experimenting with
the engines in flight."This view is reflected
in numerous publications and also artistic
works on the theme of the Chernobyl accident
that appeared immediately after the accident,
and for a long time remained dominant in the
public consciousness and in popular publications.
=== INSAG-7 report, 1992 ===
Ukraine has declassified a number of KGB documents
from the period between 1971 and 1988 related
to the Chernobyl plant, mentioning for example
previous reports of structural damages caused
by negligence during construction of the plant
(such as splitting of concrete layers) that
were never acted upon. They document over
29 emergency situations in the plant during
this period, 8 of which were caused by negligence
or poor competence on the part of personnel.In
1991 a Commission of the USSR State Committee
for the Supervision of Safety in Industry
and Nuclear Power reassessed the causes and
circumstances of the Chernobyl accident and
came to new insights and conclusions. Based
on it, in 1992 the IAEA Nuclear Safety Advisory
Group (INSAG) published an additional report,
INSAG-7, which reviewed "that part of the
INSAG-1 report in which primary attention
is given to the reasons for the accident,"
and was included the USSR State Commission
report as Appendix I.In this INSAG report,
most of the earlier accusations against staff
for breach of regulations were acknowledged
to be either erroneous, based on incorrect
information obtained in August 1986, or less
relevant. This report reflected a different
view of the main reasons for the accident,
presented in Appendix I. According to this
account, the operators' actions in turning
off the Emergency Core Cooling System, interfering
with the settings on the protection equipment,
and blocking the level and pressure in the
separator drum did not contribute to the original
cause of the accident and its magnitude, although
they may have been a breach of regulations.
In fact, turning off the emergency system
designed to prevent the two turbine generators
from stopping was not a violation of regulations.Human
factors, however, contributed to the conditions
that led to the disaster. These included operating
the reactor at a low power level—less than
700 MW—a level documented in the run-down
test programme, and operating with a small
operational reactivity margin (ORM). The 1986
assertions of Soviet experts notwithstanding,
regulations did not prohibit operating the
reactor at this low power level.However, regulations
did forbid operating the reactor with a small
margin of reactivity. Yet "post-accident studies
have shown that the way in which the real
role of the ORM is reflected in the Operating
Procedures and design documentation for the
RBMK-1000 is extremely contradictory", and
furthermore, "ORM was not treated as an operational
safety limit, violation of which could lead
to an accident".According to the INSAG-7 Report,
the chief reasons for the accident lie in
the peculiarities of physics and in the construction
of the reactor. There are two such reasons:
The reactor had a dangerously large positive
void coefficient of reactivity. The void coefficient
is a measurement of how a reactor responds
to increased steam formation in the water
coolant. Most other reactor designs have a
negative coefficient, i.e. the nuclear reaction
rate slows when steam bubbles form in the
coolant, since as the vapour phase in the
reactor increases, fewer neutrons are slowed
down. Faster neutrons are less likely to split
uranium atoms, so the reactor produces less
power (a negative feedback). Chernobyl's RBMK
reactor, however, used solid graphite as a
neutron moderator to slow down the neutrons,
and the water in it, on the contrary, acts
like a harmful neutron absorber. Thus neutrons
are slowed down even if steam bubbles form
in the water. Furthermore, because steam absorbs
neutrons much less readily than water, increasing
the intensity of vapourization means that
more neutrons are able to split uranium atoms,
increasing the reactor's power output. This
makes the RBMK design very unstable at low
power levels, and prone to suddenly increasing
energy production to a dangerous level. This
behaviour is counter-intuitive, and this property
of the reactor was unknown to the crew.
A more significant flaw was in the design
of the control rods that are inserted into
the reactor to slow down the reaction. In
the RBMK reactor design, the lower part of
each control rod was made of graphite and
was 1.3 metres (4.3 ft) shorter than necessary,
and in the space beneath the rods were hollow
channels filled with water. The upper part
of the rod, the truly functional part that
absorbs the neutrons and thereby halts the
reaction, was made of boron carbide. With
this design, when the rods are inserted into
the reactor from the uppermost position, the
graphite parts initially displace some water
(which absorbs neutrons, as mentioned above),
effectively causing fewer neutrons to be absorbed
initially. Thus for the first few seconds
of control rod activation, reactor power output
is increased, rather than reduced as desired.
This behaviour is counter-intuitive and was
not known to the reactor operators.Other deficiencies
besides these were noted in the RBMK-1000
reactor design, as were its non-compliance
with accepted standards and with the requirements
of nuclear reactor safety. While INSAG-1 and
INSAG-7 reports both identified operator error
as an issue of concern, the INSAG-7 identified
that there were numerous other issues that
were contributing factors that led to the
incident. These contributing factors include:
The plant was not designed to safety standards
in effect and incorporated unsafe features
"Inadequate safety analysis" was performed
There was "insufficient attention to independent
safety review"
"Operating procedures not founded satisfactorily
in safety analysis"
Safety information not adequately and effectively
communicated between operators, and between
operators and designers
The operators did not adequately understand
safety aspects of the plant
Operators did not sufficiently respect formal
requirements of operational and test procedures
The regulatory regime was insufficient to
effectively counter pressures for production
There was a "general lack of safety culture
in nuclear matters at the national level as
well as locally"
=== Analysis ===
Both views were heavily lobbied by different
groups, including the reactor's designers,
power plant personnel, and the Soviet and
Ukrainian governments. According to the IAEA's
1986 analysis, the main cause of the accident
was the operators' actions. But according
to the IAEA's 1993 revised analysis the main
cause was the reactor's design. One reason
there were such contradictory viewpoints and
so much debate about the causes of the Chernobyl
accident was that the primary data covering
the disaster, as registered by the instruments
and sensors, were not completely published
in the official sources.
Once again, the human factor had to be considered
as a major element in causing the accident.
INSAG notes that both the operating regulations
and staff handled the disabling of the reactor
protection easily enough: witness the length
of time for which the ECCS was out of service
while the reactor was operated at half power.
INSAG's view is that it was the operating
crew's deviation from the test programme that
was mostly to blame. "Most reprehensibly,
unapproved changes in the test procedure were
deliberately made on the spot, although the
plant was known to be in a very different
condition from that intended for the test."As
in the previously released report INSAG-1,
close attention is paid in report INSAG-7
to the inadequate (at the moment of the accident)
"culture of safety" at all levels. Deficiency
in the safety culture was inherent not only
at the operational stage but also, and to
no lesser extent, during activities at other
stages in the lifetime of nuclear power plants
(including design, engineering, construction,
manufacture, and regulation). The poor quality
of operating procedures and instructions,
and their conflicting character, put a heavy
burden on the operating crew, including the
chief engineer. "The accident can be said
to have flowed from a deficient safety culture,
not only at the Chernobyl plant, but throughout
the Soviet design, operating and regulatory
organizations for nuclear power that existed
at that time."
== Environmental effects ==
=== Spread of radioactive substances ===
Although no informing comparisons can be made
between the accident and a strictly air burst-fuzed
nuclear detonation, it has still been approximated
that about four hundred times more radioactive
material was released from Chernobyl than
by the atomic bombing of Hiroshima and Nagasaki.
By contrast the Chernobyl accident released
about one hundredth to one thousandth of the
total amount of radioactivity released during
the era of nuclear weapons testing at the
height of the Cold War, 1950–1960s, with
the 1/100 to 1/1000 variance due to trying
to make comparisons with different spectrums
of isotopes released. Approximately 100,000
square kilometres (39,000 sq mi) of land was
significantly contaminated with fallout, with
the worst hit regions being in Belarus, Ukraine
and Russia. Slighter levels of contamination
were detected over all of Europe except for
the Iberian Peninsula.The initial evidence
that a major release of radioactive material
was affecting other countries came not from
Soviet sources, but from Sweden. On the morning
of 28 April, workers at the Forsmark Nuclear
Power Plant (approximately 1,100 km (680 mi)
from the Chernobyl site) were found to have
radioactive particles on their clothes.It
was Sweden's search for the source of radioactivity,
after they had determined there was no leak
at the Swedish plant, that at noon on 28 April,
led to the first hint of a serious nuclear
problem in the western Soviet Union. Hence
the evacuation of Pripyat on 27 April, 36
hours after the initial explosions, was silently
completed before the disaster became known
outside the Soviet Union. The rise in radiation
levels had at that time already been measured
in Finland, but a civil service strike delayed
the response and publication.
Contamination from the Chernobyl accident
was scattered irregularly depending on weather
conditions, much of it deposited on mountainous
regions such as the Alps, the Welsh mountains
and the Scottish Highlands, where adiabatic
cooling caused radioactive rainfall. The resulting
patches of contamination were often highly
localized, and water-flows across the ground
contributed further to large variations in
radioactivity over small areas. Sweden and
Norway also received heavy fallout when the
contaminated air collided with a cold front,
bringing rain. There was also groundwater
contamination.Rain was purposely seeded over
10,000 square kilometres (3,900 sq mi) of
the Belorussian SSR by the Soviet air force
to remove radioactive particles from clouds
heading toward highly populated areas. Heavy,
black-coloured rain fell on the city of Gomel.
Reports from Soviet and Western scientists
indicate that Belarus received about 60% of
the contamination that fell on the former
Soviet Union. However, the 2006 TORCH report
stated that half of the volatile particles
had landed outside Ukraine, Belarus, and Russia.
A large area in Russia south of Bryansk was
also contaminated, as were parts of northwestern
Ukraine. Studies in surrounding countries
indicate that over one million people could
have been affected by radiation.Recently published
data from a long-term monitoring program (The
Korma Report II) shows a decrease in internal
radiation exposure of the inhabitants of a
region in Belarus close to Gomel. Resettlement
may even be possible in prohibited areas provided
that people comply with appropriate dietary
rules.
In Western Europe, precautionary measures
taken in response to the radiation included
seemingly arbitrary regulations banning the
importation of certain foods but not others.
In France some officials stated that the Chernobyl
accident had no adverse effects. Official
figures in southern Bavaria in Germany indicated
that some wild plant species contained substantial
levels of caesium, which were believed to
have been passed onto them during their consumption
by wild boars, a significant number of which
already contained radioactive particles above
the allowed level.
=== Radioactive release ===
Like many other releases of radioactivity
into the environment, the Chernobyl release
was controlled by the physical and chemical
properties of the radioactive elements in
the core. Particularly dangerous are the highly
radioactive fission products, those with high
nuclear decay rates that accumulate in the
food chain, such as some of the isotopes of
iodine, caesium and strontium. Iodine-131
and caesium-137 are responsible for most of
the radiation exposure received by the general
population.Detailed reports on the release
of radioisotopes from the site were published
in 1989 and 1995, with the latter report updated
in 2002.
At different times after the accident, different
isotopes were responsible for the majority
of the external dose. The remaining quantity
of any radioisotope, and therefore the activity
of that isotope, after 7 decay half-lives
have passed, is less than 1% of its initial
magnitude, and it continues to reduce beyond
0.78% after 7 half-lives to 0.10% remaining
after 10 half-lives have passed and so on.
(Some radionuclides have decay products that
are likewise radioactive, which is not accounted
for here.) The release of radioisotopes from
the nuclear fuel was largely controlled by
their boiling points, and the majority of
the radioactivity present in the core was
retained in the reactor.
All of the noble gases, including krypton
and xenon, contained within the reactor were
released immediately into the atmosphere by
the first steam explosion. The atmospheric
release of xenon-133, with a half-life of
5 days, is estimated at 5200 PBq.
50 to 60% of all core radioiodine in the reactor,
about 1760 PBq (1760×1015 becquerels), or
about 0.4 kilograms (0.88 lb), was released,
as a mixture of sublimed vapour, solid particles,
and organic iodine compounds. Iodine-131 has
a half-life of 8 days.
20 to 40% of all core caesium-137 was released,
85 PBq in all. Caesium was released in aerosol
form; caesium-137, along with isotopes of
strontium, are the two primary elements preventing
the Chernobyl exclusion zone being re-inhabited.
8.5×1016 Bq equals 24 kilograms of caesium-137.
Cs-137 has a half-life of 30 years.
Tellurium-132, half-life 78 hours, an estimated
1150 PBq was released.
An early estimate for total nuclear fuel material
released to the environment was 3±1.5%; this
was later revised to 3.5±0.5%. This corresponds
to the atmospheric emission of 6 metric tons
(5.9 long tons; 6.6 short tons) of fragmented
fuel.Two sizes of particles were released:
small particles of 0.3 to 1.5 micrometres,
each an individually unrecognizable small
dust or smog sized particulate matter and
larger settling dust sized particles that
therefore were quicker to fall-out of the
air, of 10 micrometres in diameter. These
larger particles contained about 80% to 90%
of the released high boiling point or non-volatile
radioisotopes; zirconium-95, niobium-95, lanthanum-140,
cerium-144 and the transuranic elements, including
neptunium, plutonium and the minor actinides,
embedded in a uranium oxide matrix.
The dose that was calculated is the relative
external gamma dose rate for a person standing
in the open. The exact dose to a person in
the real world who would spend most of their
time sleeping indoors in a shelter and then
venturing out to consume an internal dose
from the inhalation or ingestion of a radioisotope,
requires a personnel specific radiation dose
reconstruction analysis.
=== Environmental residual radioactivity ===
==== Water bodies ====
The Chernobyl nuclear power plant is located
next to the Pripyat River, which feeds into
the Dnieper reservoir system, one of the largest
surface water systems in Europe, which at
the time supplied water to Kiev's 2.4 million
residents, and was still in spring flood when
the accident occurred. The radioactive contamination
of aquatic systems therefore became a major
problem in the immediate aftermath of the
accident.In the most affected areas of Ukraine,
levels of radioactivity (particularly from
radionuclides 131I, 137Cs and 90Sr) in drinking
water caused concern during the weeks and
months after the accident, though officially
it was stated that all contaminants had settled
to the bottom "in an insoluble phase" and
would not dissolve for 800–1000 years. Guidelines
for levels of radioiodine in drinking water
were temporarily raised to 3,700 Bq/L, allowing
most water to be reported as safe, and a year
after the accident it was announced that even
the water of the Chernobyl plant's cooling
pond was within acceptable norms. Despite
this, two months after the disaster the Kiev
water supply was abruptly switched from the
Dnieper to the Desna River. Meanwhile, massive
silt traps were constructed, along with an
enormous 30-metre (98 ft) deep underground
barrier to prevent groundwater from the destroyed
reactor entering the Pripyat River.Bio-accumulation
of radioactivity in fish resulted in concentrations
(both in western Europe and in the former
Soviet Union) that in many cases were significantly
above guideline maximum levels for consumption.
Guideline maximum levels for radiocaesium
in fish vary from country to country but are
approximately 1000 Bq/kg in the European Union.
In the Kiev Reservoir in Ukraine, concentrations
in fish were several thousand Bq/kg during
the years after the accident.
In small "closed" lakes in Belarus and the
Bryansk region of Russia, concentrations in
a number of fish species varied from 100 to
60,000 Bq/kg during the period 1990–92.
The contamination of fish caused short-term
concern in parts of the UK and Germany and
in the long term (years rather than months)
in the affected areas of Ukraine, Belarus,
and Russia as well as in parts of Scandinavia.Groundwater
was not badly affected by the Chernobyl accident
since radionuclides with short half-lives
decayed away long before they could affect
groundwater supplies, and longer-lived radionuclides
such as radiocaesium and radiostrontium were
adsorbed to surface soils before they could
transfer to groundwater. However, significant
transfers of radionuclides to groundwater
have occurred from waste disposal sites in
the 30 km (19 mi) exclusion zone around Chernobyl.
Although there is a potential for transfer
of radionuclides from these disposal sites
off-site (i.e. out of the 30 km (19 mi) exclusion
zone), the IAEA Chernobyl Report argues that
this is not significant in comparison to current
levels of washout of surface-deposited radioactivity.
==== Flora and fauna ====
After the disaster, four square kilometres
(1.5 sq mi) of pine forest directly downwind
of the reactor turned reddish-brown and died,
earning the name of the "Red Forest". Some
animals in the worst-hit areas also died or
stopped reproducing. Most domestic animals
were removed from the exclusion zone, but
horses left on an island in the Pripyat River
6 km (4 mi) from the power plant died when
their thyroid glands were destroyed by radiation
doses of 150–200 Sv. Some cattle on the
same island died and those that survived were
stunted because of thyroid damage. The next
generation appeared to be normal.
A robot sent into the reactor itself has returned
with samples of black, melanin-rich radiotrophic
fungi that are growing on the reactor's walls.Of
the 440,350 wild boar killed in the 2010 hunting
season in Germany, approximately one thousand
were contaminated with levels of radiation
above the permitted limit of 600 becquerels
of Cesium per kg, due to residual radioactivity
from Chernobyl. While all animal meat contains
a natural level of potassium 40 at a similar
level of activity, with both wild and farm
animals in Italy containing "415 ± 56 becquerels
kg−1 dw" of that naturally occurring gamma
emitter. The cesium contamination issue has
historically reached some uniquely isolated
and high levels approaching 20,000 Becquerels
of Cesium per kg in some specific tests, however
as it has not been observed in the wild boar
population of Fukushima after the 2011 accident.
Evidence exists to suggest that the wild German
and Ukrainian boar population are in a unique
location were they have subsisted on a diet
high in plant or fungi sources that biomagnifies
or concentrates radio-cesium, with the most
well known food source the consumption of
the outer shell or wall of the "deer-truffle"/Elaphomyces
which along with magnifying radio-cesium also
magnifies or concentrates natural soil concentrations
of Arsenic.The Norwegian Agricultural Authority
reported that in 2009 a total of 18,000 livestock
in Norway required uncontaminated feed for
a period before slaughter, to ensure that
their meat had an activity below the government
permitted value of cesium per kg deemed suitable
for human consumption. This contamination
was due to residual radioactivity from Chernobyl
in the plants they graze on in the wild during
the summer. 1,914 sheep required uncontaminated
feed for a time before slaughter during 2012,
and these sheep were located in just 18 of
Norway's municipalities, a decrease of 17
from the 35 municipalities affected animals
were located in during 2011 (117 municipalities
were affected during 1986).The after-effects
of Chernobyl were expected to be seen for
a further 100 years, although the severity
of the effects would decline over that period.
Scientists report this is due to radioactive
caesium-137 isotopes being taken up by fungi
such as Cortinarius caperatus which is in
turn eaten by sheep while grazing.The United
Kingdom was forced to restrict the movement
of sheep from upland areas when radioactive
caesium-137 fell across parts of Northern
Ireland, Wales, Scotland and northern England.
In the immediate aftermath of the disaster
in 1986, the movement of a total of 4,225,000
sheep was restricted across a total of 9,700
farms, to prevent contaminated meat entering
the human food chain. The number of sheep
and the number of farms affected has decreased
since 1986, Northern Ireland was released
from all restrictions in 2000 and by 2009
369 farms containing around 190,000 sheep
remained under the restrictions in Wales,
Cumbria and northern Scotland. The restrictions
applying in Scotland were lifted in 2010,
while those applying to Wales and Cumbria
were lifted during 2012, meaning no farms
in the UK remain restricted because of Chernobyl
fallout.The legislation used to control sheep
movement and compensate farmers (farmers were
latterly compensated per animal to cover additional
costs in holding animals prior to radiation
monitoring) was revoked during October and
November 2012 by the relevant authorities
in the UK.
== Human impact ==
In the accident’s aftermath 237 people suffered
from acute radiation sickness, of whom 31
died within the first three months. In 2005,
the Chernobyl Forum, composed of the IAEA,
other UN organizations and the governments
of Belarus, Russia and Ukraine, published
a report on the radiological environmental
and health consequences of the Chernobyl accident.
On the death toll, the report states that
28 emergency workers ("liquidators") died
from acute radiation syndrome, including beta
burns, and 15 patients died from thyroid cancer
in the following years, and it roughly estimated
that cancer deaths caused by Chernobyl may
reach a total of about 4,000 among the 5 million
persons residing in the contaminated areas.
The report projected cancer mortality "increases
of less than one per cent" (~0.3%) on a time
span of 80 years, cautioning that this estimate
was "speculative" since at this time only
a few cancer deaths are linked to the Chernobyl
disaster. The report says it is impossible
to reliably predict the number of fatal cancers
arising from the incident as small differences
in assumptions can result in large differences
in the estimated health costs. The report
says it represents the consensus view of the
eight UN organizations.
Of all 66,000 Belarusian emergency workers,
by the mid-1990s their government reported
that only 150 (roughly 0.2%) died. In contrast,
5,722 casualties were reported among Ukrainian
clean-up workers up to the year 1995, by the
National Committee for Radiation Protection
of the Ukrainian Population.The four most
harmful radionuclides spread from Chernobyl
were iodine-131, caesium-134, caesium-137
and strontium-90, with half-lives of 8.02
days, 2.07 years, 30.2 years and 28.8 years
respectively. The iodine was initially viewed
with less alarm than the other isotopes, because
of its short half-life, but it is highly volatile,
and now appears to have travelled furthest
and caused the most severe health problems
in the short term. Strontium, on the other
hand, is the least volatile of the four, and
of main concern in the areas near Chernobyl
itself. Iodine tends to become concentrated
in thyroid and milk glands, leading, among
other things, to increased incidence of thyroid
cancers. Caesium tends to accumulate in vital
organs such as the heart, while strontium
accumulates in bones, and may thus be a risk
to bone-marrow and lymphocytes. Radiation
is most damaging to cells that are actively
dividing. In adult mammals cell division is
slow, except in hair follicles, skin, bone
marrow and the gastrointestinal tract, which
is why vomiting and hair loss are common symptoms
of acute radiation sickness.
=== Assessment difficulties ===
By the year 2000, the number of Ukrainians
claiming to be radiation 'sufferers' (poterpili)
and receiving state benefits had jumped to
3.5 million, or 5% of the population. Many
of these are populations resettled from contaminated
zones, or former or current Chernobyl plant
workers. According to IAEA-affiliated scientific
bodies, these apparent increases of ill health
result partly from economic strains on these
countries and poor health-care and nutrition;
also, they suggest that increased medical
vigilance following the accident has meant
that many cases that would previously have
gone unnoticed (especially of cancer) are
now being registered.The World Health Organization
states, "children conceived before or after
their father's exposure showed no statistically
significant differences in mutation frequencies".
This statistically insignificant increase
was also seen by independent researchers analyzing
the children of the Chernobyl liquidators.On
farms in Narodychi Raion of Ukraine it is
claimed that in the first four years of the
disaster nearly 350 animals were born with
gross deformities such as missing or extra
limbs, missing eyes, heads or ribs, or deformed
skulls; in comparison, only three abnormal
births had been registered in the five years
prior. The two primary individuals involved
with the attempt to suggest that the mutation
rate among animals was, and continues to be,
higher in the Chernobyl zone, are the Anders
Moller and Timothy Mousseau group. Apart from
continuing to publish experimentally unrepeatable
and discredited papers, Mousseau routinely
gives talks at the Helen Caldicott organized
symposiums for "Physicians for Social Responsibility",
an anti-nuclear advocacy group, devoted to
bring about a "nuclear free planet". Moreover,
in years past Moller was previously caught
and reprimanded for publishing papers that
crossed the scientific "misconduct"/"fraud"
line. The duo have more recently attempted
to publish meta-analyses in which the primary
references they weigh-up, analyze and draw
their conclusions from is their own prior
papers along with the discredited book Chernobyl:
Consequences of the Catastrophe for People
and the Environment.In 1996, geneticist colleagues
Ronald Chesser and Robert Baker published
a paper on the thriving vole population within
the exclusion zone, in which the central conclusion
of their work was essentially that "The mutation
rate in these animals is hundreds and probably
thousands of times greater than normal", this
claim occurred after they had done a comparison
of the mitochondrial DNA of the "Chernobyl
voles" with that of a control group of voles
from outside the region. These alarming conclusions
led the paper to appear on the front cover
of the prestigious journal Nature, however
not long after publication Chesser & Baker
discovered a fundamental error in their research
in which they had incorrectly classified the
species of vole, and therefore were comparing
the genetics of two entirely different vole
species to start with.
==== Abortions ====
Following the accident, journalists mistrusted
many medical professionals (such as the spokesman
from the UK National Radiological Protection
Board), and in turn encouraged the public
to mistrust them. Throughout the European
continent, due to this media-driven framing
of the slight contamination and in nations
where abortion is legal, many requests for
induced abortions, of otherwise normal pregnancies,
were obtained out of fears of radiation from
Chernobyl, including an excess number of abortions
in Denmark in the months following the accident.
In Greece, following the accident many obstetricians
were unable to resist requests from worried
pregnant mothers over fears of radiation.
Although it was determined that the effective
dose to Greeks would not exceed 1 mSv (100
mrem), a dose much lower than that which could
induce embryonic abnormalities or other non-stochastic
effects, there was an observed 2,500 excess
of otherwise wanted pregnancies being terminated,
probably out of fear in the mother of radiation
risk. A slightly above the expected number
of requested induced abortions occurred in
Italy.Worldwide, an estimated excess of about
150,000 elective abortions may have been performed
on otherwise healthy pregnancies out of unfounded
fears of radiation from Chernobyl, according
to Robert Baker and ultimately a 1987 article
published by Linda E. Ketchum in the Journal
of Nuclear Medicine which mentions but does
not reference an IAEA source on the matter.The
available statistical data excludes the Soviet–Ukraine–Belarus
abortion rates, as they are presently unavailable.
From the available data, an increase in the
number of abortions in what were healthy developing
human offspring in Denmark occurred in the
months following the accident, at a rate of
about 400 cases. In Greece, there was an observed
2,500 excess of otherwise wanted pregnancies
being terminated. In Italy, a "slightly" above
the expected number of induced abortions occurred,
approximately 100.
As the increase in radiation in Denmark was
so low...the public debate and anxiety among
the pregnant women and their husbands "caused"
more fetal deaths in Denmark than the accident.
This underlines the importance of public debate,
the role of the mass media and of the way
in which National Health authorities participate
in this debate.
No evidence of changes in the prevalence of
human deformities/birth congenital anomalies
which might be associated with the accident,
are apparent in Belarus or the Ukraine, the
two republics which had the highest exposure
to fallout. In Sweden, and Finland where no
increase in abortion rates occurred, it was
likewise determined that "no association between
the temporal and spatial variations in radioactivity
and variable incidence of congenital malformations
[was found]." A similar null increase in the
abortion rate and a healthy baseline situation
of no increase in birth defects was determined
by assessing the Hungarian Congenital Abnormality
Registry, Findings also mirrored in Austria.
Larger, "mainly western European" data sets
approaching a million births in the EUROCAT
database, divided into "exposed" and control
groups were assessed in 1999. As no Chernobyl
impacts were detected, the researchers conclude
"in retrospect the widespread fear in the
population about the possible effects of exposure
on the unborn fetus was not justified". Despite
studies from Germany and Turkey, the only
robust evidence of negative pregnancy outcomes
that transpired after the accident were these
elective abortion indirect effects, in Greece,
Denmark, Italy etc., due to the anxieties
created.In very high doses, it was known at
the time that radiation can cause a physiological
increase in the rate of pregnancy anomalies,
but unlike the dominant linear-no threshold
model of radiation and cancer rate increases,
it was known, by researchers familiar with
both the prior human exposure data and animal
testing, that the "Malformation of organs
appears to be a deterministic effect with
a threshold dose" below which, no rate increase
is observed. This teratology (birth defects)
issue was discussed by Frank Castronovo of
the Harvard Medical School in 1999, publishing
a detailed review of dose reconstructions
and the available pregnancy data following
the Chernobyl accident, inclusive of data
from Kiev's two largest obstetrics hospitals.
Castronovo concludes that "the lay press with
newspaper reporters playing up anecdotal stories
of children with birth defects" is, together
with dubious studies that show selection bias,
the two primary factors causing the persistent
belief that Chernobyl increased the background
rate of birth defects. When the vast amount
of pregnancy data does not support this perception
as no women took part in the most radioactive
liquidator operations, no in-utero individuals
would have been expected to have received
a threshold dose.In one small behavioral study
in 1998, with low statistical power and limited
Multivariate analysis which akin to the widely
published Hiroshima and Nagasaki studies,
investigated and selected the children; who
were in utero during the rapidly dividing
and therefore radiosensitive phase of neurogenesis(8
to 16 weeks of gestation), and whose mothers
were evacuated from some of the more energetic
hot-spot parts of the Chernobyl exclusion
zone following the accident. From a random
selection of 50 individuals in late-childhood
in 1998, a low quality statistically-significant
increase in the rate of severe IQ reduction
was found, with a threshold of a suggested
~ 0.30 Sv(300 mSv) as a thyroid dose to the
developing human head, for the beginning emergence
of cerebral disorder.The Chernobyl liquidators,
essentially an all-male civil defense emergency
workforce, would go on to father normal children,
without an increase in developmental anomalies
or a statistically significant increase in
the frequencies of germline mutations in their
progeny. This normality is similarly seen
in the children of the survivors of the Goiana
accident.
=== Сancer assessments ===
A 
report by the International Atomic Energy
Agency examines the environmental consequences
of the accident. The United Nations Scientific
Committee on the Effects of Atomic Radiation
has estimated a global collective dose of
radiation exposure from the accident "equivalent
on average to 21 additional days of world
exposure to natural background radiation";
individual doses were far higher than the
global mean among those most exposed, including
530,000 primarily male recovery workers (the
Chernobyl liquidators) who averaged an effective
dose equivalent to an extra 50 years of typical
natural background radiation exposure each.Estimates
of the number of deaths that will eventually
result from the accident vary enormously;
disparities reflect both the lack of solid
scientific data and the different methodologies
used to quantify mortality—whether the discussion
is confined to specific geographical areas
or extends worldwide, and whether the deaths
are immediate, short term, or long term.
In 1994, 31 deaths were directly attributed
to the accident, all among the reactor staff
and emergency workers. As of the 2008 report
by the United Nations Scientific Committee
on the Effects of Atomic Radiation, and the
total number of confirmed deaths from radiation
was 64 and was expected to continue to rise.
The Chernobyl Forum predicts that the eventual
death toll could reach 4,000 among those exposed
to the highest levels of radiation (200,000
emergency workers, 116,000 evacuees and 270,000
residents of the most contaminated areas);
this figure is a total causal death toll prediction,
combining the deaths of approximately 50 emergency
workers who died soon after the accident from
acute radiation syndrome, 15 children who
have died of thyroid cancer and a future predicted
total of 3935 deaths from radiation-induced
cancer and leukaemia.In a peer-reviewed paper
in the International Journal of Cancer in
2006, the authors expanded the discussion
on those exposed to all of Europe (but following
a different conclusion methodology to the
Chernobyl Forum study, which arrived at the
total predicted death toll of 4,000 after
cancer survival rates were factored in) they
stated, without entering into a discussion
on deaths, that in terms of total excess cancers
attributed to the accident:
The risk projections suggest that by now [2006]
Chernobyl may have caused about 1000 cases
of thyroid cancer and 4000 cases of other
cancers in Europe, representing about 0.01%
of all incident cancers since the accident.
Models predict that by 2065 about 16,000 cases
of thyroid cancer and 25,000 cases of other
cancers may be expected due to radiation from
the accident, whereas several hundred million
cancer cases are expected from other causes.
Two anti-nuclear advocacy groups have publicized
non-peer-reviewed estimates that include mortality
estimates for those who were exposed to even
smaller amounts of radiation. The Union of
Concerned Scientists (UCS) calculated that,
among the hundreds of millions of people exposed
worldwide, there will be an eventual 50,000
excess cancer cases, resulting in 25,000 excess
cancer deaths, excluding thyroid cancer. However,
these calculations are based on a simple linear
no-threshold model multiplication and the
misapplication of the collective dose, which
the International Commission on Radiological
Protection (ICRP) states "should not be done"
as using the collective dose is "inappropriate
to use in risk projections".Along similar
lines to the UCS approach, the 2006 TORCH
report, commissioned by the European Greens
political party, likewise simplistically calculates
an eventual 30,000 to 60,000 excess cancer
deaths in total, around the globe.
In 2004, the UN collaborative, Chernobyl Forum,
revealed thyroid cancer among children to
be one of the main health impacts from the
Chernobyl accident. This is due to the ingestion
of contaminated dairy products, along with
the inhalation of the short-lived, highly
radioactive isotope, Iodine-131. In that publication,
more than 4,000 cases of childhood thyroid
cancer were reported. It is important to note
that there was no evidence of an increase
in solid cancers or leukemia. It said that
there was an increase in psychological problems
among the affected population. The WHO's Radiation
Program reported that the 4,000 cases of thyroid
cancer resulted in nine deaths.According to
the United Nations Scientific Committee on
the Effects of Atomic Radiation, up to the
year 2005, an excess of over 6,000 cases of
thyroid cancer had been reported. That is,
over the estimated pre-accident baseline thyroid
cancer rate, more than 6,000 casual cases
of thyroid cancer have been reported in children
and adolescents exposed at the time of the
accident, a number that is expected to increase.
They concluded that there is no other evidence
of major health impacts from the radiation
exposure.Well-differentiated thyroid cancers
are generally treatable, and when treated
the five-year survival rate of thyroid cancer
is 96%, and 92% after 30 years. the United
Nations Scientific Committee on the Effects
of Atomic Radiation had reported 15 deaths
from thyroid cancer in 2011. The International
Atomic Energy Agency (IAEA) also states that
there has been no increase in the rate of
birth defects or abnormalities, or solid cancers—such
as lung cancer—corroborating the assessments
by the UN committee. UNSCEAR raised the possibility
of long term genetic defects, pointing to
a doubling of radiation-induced minisatellite
mutations among children born in 1994. However,
the risk of thyroid cancer associated with
the Chernobyl accident is still high according
to published studies.The German affiliate
of the ultra-anti-nuclear energy organization,
the International Physicians for the Prevention
of Nuclear War suggest that 10,000 people
are affected by thyroid cancer as of 2006
and that 50,000 cases are expected in the
future.
=== Other health disorders ===
Fred Mettler, a radiation expert at the University
of New Mexico, puts the number of worldwide
cancer deaths outside the highly contaminated
zone at perhaps 5,000, for a total of 9,000
Chernobyl-associated fatal cancers, saying
"the number is small (representing a few percent)
relative to the normal spontaneous risk of
cancer, but the numbers are large in absolute
terms". The same report outlined studies based
on data found in the Russian Registry from
1991 to 1998 that suggested that "of 61,000
Russian workers exposed to an average dose
of 107 mSv about 5% of all fatalities that
occurred may have been due to radiation exposure."The
report went into depth about the risks to
mental health of exaggerated fears about the
effects of radiation. According to the IAEA
the "designation of the affected population
as "victims" rather than "survivors" has led
them to perceive themselves as helpless, weak
and lacking control over their future". The
IAEA says that this may have led to behaviour
that has caused further health effects.Fred
Mettler commented that 20 years later: "The
population remains largely unsure of what
the effects of radiation actually are and
retain a sense of foreboding. A number of
adolescents and young adults who have been
exposed to modest or small amounts of radiation
feel that they are somehow fatally flawed
and there is no downside to using illicit
drugs or having unprotected sex. To reverse
such attitudes and behaviours will likely
take years, although some youth groups have
begun programs that have promise." In addition,
disadvantaged children around Chernobyl suffer
from health problems that are attributable
not only to the Chernobyl accident, but also
to the poor state of post-Soviet health systems.The
United Nations Scientific Committee on the
Effects of Atomic Radiation (UNSCEAR), part
of the Chernobyl Forum, have produced their
own assessments of the radiation effects.
UNSCEAR was set up as a collaboration between
various United Nation bodies, including the
World Health Organization, after the atomic
bomb attacks on Hiroshima and Nagasaki, to
assess the long-term effects of radiation
on human health.
=== Radiation exposure deaths ===
The number of potential deaths arising from
the Chernobyl disaster is heavily debated.
The WHO's prediction of 4000 future cancer
deaths in surrounding countries is based on
the Linear no-threshold model (LNT), which
assumes that the damage inflicted by radiation
at low doses is directly proportional to the
dose. Radiation epidemiologist Roy Shore contends
that estimating health effects in a population
from the LNT model "is not wise because of
the uncertainties".According to the Union
of Concerned Scientists the number of excess
cancer deaths worldwide (including all contaminated
areas) is approximately 27,000 based on the
same LNT.Another study critical of the Chernobyl
Forum report was commissioned by Greenpeace,
which asserted that the most recently published
figures indicate that in Belarus, Russia and
Ukraine the accident could have resulted in
10,000–200,000 additional deaths in the
period between 1990 and 2004. The Scientific
Secretary of the Chernobyl Forum criticized
the report's reliance on non-peer-reviewed
locally produced studies. Although most of
the study's sources were from peer-reviewed
journals, including many Western medical journals,
the higher mortality estimates were from non-peer-reviewed
sources, while Gregory Härtl (spokesman for
the WHO) suggested that the conclusions were
motivated by ideology.Chernobyl: Consequences
of the Catastrophe for People and the Environment
is a 2007 Russian publication that concludes
that there were 985,000 premature deaths as
a consequence of the radioactivity released.
The results were criticized by M. I. Balonov
from the Institute of Radiation Hygiene in
St. Petersburg, who described them as biased,
drawing from sources which were difficult
to independently verify and lacking a proper
scientific base. Balanov expressed his opinion
that "the authors unfortunately did not appropriately
analyze the content of the Russian-language
publications, for example, to separate them
into those that contain scientific evidence
and those based on hasty impressions and ignorant
conclusions".According to U.S. Nuclear Regulatory
Commission member and Professor of Health
Physics Kenneth Mossman, the "LNT philosophy
is overly conservative, and low-level radiation
may be less dangerous than commonly believed."
Yoshihisa Matsumoto, a radiation biologist
at the Tokyo Institute of Technology, cites
laboratory experiments on animals to suggest
there must be a threshold dose below which
DNA repair mechanisms can completely repair
any radiation damage. Mossman suggests that
the proponents of the current model believe
that being conservative is justified due to
the uncertainties surrounding low level doses
and it is better to have a "prudent public
health policy".Another significant issue is
establishing consistent data on which to base
the analysis of the impact of the Chernobyl
accident. Since 1991 large social and political
changes have occurred within the affected
regions and these changes have had significant
impact on the administration of health care,
on socio-economic stability, and the manner
in which statistical data is collected. Ronald
Chesser, a radiation biologist at Texas Tech
University, says that "the subsequent Soviet
collapse, scarce funding, imprecise dosimetry,
and difficulties tracking people over the
years have limited the number of studies and
their reliability".
== Social economic effect ==
It is difficult to establish the total economic
cost of the disaster. According to Mikhail
Gorbachev, the Soviet Union spent 18 billion
rubles (the equivalent of US$18 billion at
that time, or $35.7 billion in today's dollars)
on containment and decontamination, virtually
bankrupting itself. In 2005, the total cost
over 30 years for Belarus alone was estimated
at US$235 billion; about $297 billion in today's
dollars given inflation rates.Ongoing costs
are well known; in their 2003–2005 report,
The Chernobyl Forum stated that between 5%
and 7% of government spending in Ukraine is
still related to Chernobyl, while in Belarus
over $13 billion is thought to have been spent
between 1991 and 2003, with 22% of national
budget having been Chernobyl-related in 1991,
falling to 6% by 2002. In 2018, Ukraine spent
5–7% of its national budget on recovery
activities related to the Chernobyl disaster.
Overall economic loss is estimated at $235
billion in Belarus. Much of the current cost
relates to the payment of Chernobyl-related
social benefits to some 7 million people across
the three countries.A significant economic
impact at the time was the removal of 784,320
ha (1,938,100 acres) of agricultural land
and 694,200 ha (1,715,000 acres) of forest
from production. While much of this has been
returned to use, agricultural production costs
have risen due to the need for special cultivation
techniques, fertilizers and additives.Politically,
the accident gave great significance to the
new Soviet policy of glasnost, and helped
forge closer Soviet–US relations at the
end of the Cold War, through bioscientific
cooperation. The disaster also became a key
factor in the Union's eventual 1991 dissolution,
and a major influence in shaping the new Eastern
Europe.Both Ukraine and Belarus, in their
first months of independence, lowered legal
radiation thresholds from the Soviet Union's
previous, elevated thresholds (from 35 rems
per lifetime under the USSR to 7 rems per
lifetime in Ukraine and 0.1 rems per year
in Belarus).
== Aftermath ==
Following the accident, questions arose about
the future of the plant and its eventual fate.
All work on the unfinished reactors 5 and
6 was halted three years later. However, the
trouble at the Chernobyl plant did not end
with the disaster in reactor 4. The damaged
reactor was sealed off and 200 cubic meters
(260 cu yd) of concrete was placed between
the disaster site and the operational buildings.
The work was managed by Grigoriy Mihaylovich
Naginskiy, the deputy chief engineer of Installation
and Construction Directorate – 90. The Ukrainian
government continued to let the three remaining
reactors operate because of an energy shortage
in the country.
=== Decommissioning ===
In October 1991, a fire broke out in the turbine
building of reactor 2; the authorities subsequently
declared the reactor damaged beyond repair,
and it was taken offline. Reactor 1 was decommissioned
in November 1996 as part of a deal between
the Ukrainian government and international
organizations such as the IAEA to end operations
at the plant. On 15 December 2000, then-President
Leonid Kuchma personally turned off Reactor
3 in an official ceremony, shutting down the
entire site.
=== Confinement ===
Soon after the accident, the reactor building
was quickly encased by a mammoth concrete
sarcophagus in a notable feat of construction
under severe conditions. Crane operators worked
blindly from inside lead-lined cabins taking
instructions from distant radio observers,
while gargantuan-sized pieces of concrete
were moved to the site on custom-made vehicles.
The purpose of the sarcophagus was to stop
any further release of radioactive particles
into the atmosphere, mitigate damage should
the core go critical and explode, and provide
safety for the continued operations of adjacent
reactors 1, 2 and 3.The concrete sarcophagus
was never intended to last very long, with
a lifespan of only 30 years. On 12 February
2013, a 600 m2 (6,500 sq ft) section of the
roof of the turbine-building collapsed, adjacent
to the sarcophagus, causing a new release
of radioactivity and temporary evacuation
of the area. At first it was assumed that
the roof collapsed because of the weight of
snow, however the amount of snow was not exceptional,
and the report of a Ukrainian fact-finding
panel concluded that the collapse was the
result of sloppy repair work and aging of
the structure. Experts warned the sarcophagus
itself was on the verge of collapse.In 1997,
the international Chernobyl Shelter Fund was
founded to design and build a more permanent
cover for the unstable and short-lived sarcophagus.
It received over €810 million and was managed
by the European Bank for Reconstruction and
Development (EBRD). The new shelter was named
the New Safe Confinement and construction
began in 2010. It is a metal arch 105 metres
(344 ft) high and spanning 257 metres (843
ft) built on rails adjacent to the reactor
4 building so that it could be slid over top
the existing sarcophagus. The New Safe Confinement
was completed in 2016 and slid into place
over top the sarcophagus on November 29. The
huge steel arch was moved into place over
several weeks. Unlike the original sarcophagus,
the New Safe Confinement is designed to allow
the reactor to be safely dismantled using
remotely operated equipment.
=== Waste management ===
As of 2006, some fuel remained in the reactors
at units 1 through 3, most of it in each unit's
spent fuel pool, as well as some material
in a small spent fuel interim storage facility
pond (ISF-1). In 1999 a contract was signed
for construction of a radioactive waste management
facility to store 25,000 used fuel assemblies
from units 1–3 and other operational wastes,
as well as material from decommissioning units
1–3 (which will be the first RBMK units
decommissioned anywhere). The contract included
a processing facility able to cut the RBMK
fuel assemblies and to put the material in
canisters, which were to be filled with inert
gas and welded shut.
The canisters were to be transported to dry
storage vaults, where the fuel containers
would be enclosed for up to 100 years. This
facility, treating 2,500 fuel assemblies per
year, would be the first of its kind for RBMK
fuel. However, after a significant part of
the storage structures had been built, technical
deficiencies in the concept emerged, and the
contract was terminated in 2007. The interim
spent fuel storage facility (ISF-2) will now
be completed by others by mid-2013.Another
contract has been let for a liquid radioactive
waste treatment plant, to handle some 35,000
cubic metres (1,200,000 cu ft) of low- and
intermediate-level liquid wastes at the site.
This will need to be solidified and eventually
buried along with solid wastes on site. In
January 2008, the Ukrainian government announced
a 4-stage decommissioning plan that incorporates
the above waste activities and progresses
towards a cleared site.
==== Fuel-containing materials ====
According to official estimates, about 95%
of the fuel in Reactor 4 at the time of the
accident (about 180 metric tons (180 long
tons; 200 short tons)) remains inside the
shelter, with a total radioactivity of nearly
18 million curies (670 PBq). The radioactive
material consists of core fragments, dust,
and lava-like "fuel containing materials"
(FCM)—also called "corium"—that flowed
through the wrecked reactor building before
hardening into a ceramic form.
Three different lavas are present in the basement
of the reactor building: black, brown, and
a porous ceramic. The lava materials are silicate
glasses with inclusions of other materials
within them. The porous lava is brown lava
that dropped into water and thus cooled rapidly.
It is unclear how long the ceramic form will
retard the release of radioactivity. From
1997 to 2002 a series of published papers
suggested that the self-irradiation of the
lava would convert all 1,200 metric tons (1,200
long tons; 1,300 short tons) into a submicrometre
and mobile powder within a few weeks.It has
been reported that the degradation of the
lava is likely to be a slow and gradual process
rather than sudden and rapid. The same paper
states that the loss of uranium from the wrecked
reactor is only 10 kg (22 lb) per year; this
low rate of uranium leaching suggests that
the lava is resisting its environment. The
paper also states that when the shelter is
improved, the leaching rate of the lava will
decrease.
=== Exclusion zone ===
An area originally extending 30 kilometres
(19 mi) in all directions from the plant is
officially called the "zone of alienation".
It is largely uninhabited, except for about
300 residents who have refused to leave. The
area has largely reverted to forest, and has
been overrun by wildlife because of a lack
of competition with humans for space and resources.
Even today, radiation levels are so high that
the workers responsible for rebuilding the
sarcophagus are only allowed to work five
hours a day for one month before taking 15
days of rest. As of 2016, 187 locals had returned
and were living permanently in the zone.In
2011 Ukraine opened up the sealed zone around
the Chernobyl reactor to tourists who wish
to learn more about the tragedy that occurred
in 1986. Sergii Mirnyi, a radiation reconnaissance
officer at the time of the accident, and now
an academic at National University of Kyiv-Mohyla
Academy in Kiev, Ukraine, has written about
the psychological and physical effects on
survivors and visitors, and worked as an advisor
to Chernobyl tourism groups.
=== Forest fire concerns ===
During the dry seasons, a perennial concern
is forests that have been contaminated by
radioactive material catching on fire. The
dry conditions and build-up of debris make
the forests a ripe breeding ground for wildfires.
Depending on the prevailing atmospheric conditions,
the fires could potentially spread the radioactive
material further outwards from the exclusion
zone in the smoke. In Belarus, the Bellesrad
organization is tasked with overseeing the
food cultivation and forestry management in
the area.
== Recovery projects ==
The Chernobyl Trust Fund was created in 1991
by the United Nations to help victims of the
Chernobyl accident. It is administered by
the United Nations Office for the Coordination
of Humanitarian Affairs, which also manages
strategy formulation, resources mobilization,
and advocacy efforts. Beginning 2002, under
the United Nations Development Programme,
the fund shifted its focus from emergency
assistance to long-term development.The Chernobyl
Shelter Fund was established in 1997 at the
Denver 23rd G8 summit to finance the Shelter
Implementation Plan (SIP). The plan calls
for transforming the site into an ecologically
safe condition by means of stabilization of
the sarcophagus followed by construction of
a New Safe Confinement (NSC). While the original
cost estimate for the SIP was US$768 million,
the 2006 estimate was $1.2 billion. The SIP
is being managed by a consortium of Bechtel,
Battelle, and Électricité de France, and
conceptual design for the NSC consists of
a movable arch, constructed away from the
shelter to avoid high radiation, to be slid
over the sarcophagus. The NSC was moved into
position in November 2016 and is expected
to be completed in late-2017.In 2003, the
United Nations Development Programme launched
the Chernobyl Recovery and Development Programme
(CRDP) for the recovery of the affected areas.
The programme was initiated in February 2002
based on the recommendations in the report
on Human Consequences of the Chernobyl Nuclear
Accident. The main goal of the CRDP's activities
is supporting the Government of Ukraine in
mitigating long-term social, economic, and
ecological consequences of the Chernobyl catastrophe.
CRDP works in the four most Chernobyl-affected
areas in Ukraine: Kyivska, Zhytomyrska, Chernihivska
and Rivnenska.
The International Project on the Health Effects
of the Chernobyl Accident was created and
received US$20 million, mainly from Japan,
in hopes of discovering the main cause of
health problems due to 131I radiation. These
funds were divided among Ukraine, Belarus,
and Russia, the three main affected countries,
for further investigation of health effects.
As there was significant corruption in former
Soviet countries, most of the foreign aid
was given to Russia, and no positive outcome
from this money has been demonstrated.Chernobyl
Children International (CCI) is a UN-accredited,
non-profit, international development, medical,
and humanitarian organization that works with
children, families and communities that continue
to be affected by the economic outcome of
the Chernobyl accident. The organization's
founder and chief executive is Adi Roche.
The CCI was founded in 1991 in response to
an appeal from Ukrainian and Belarusian doctors
for aid. Roche then began organizing 'rest
and recuperation' holidays for a few Chernobyl
children. It works closely with the Belarusian
government, the United Nations, and many thousands
of volunteers worldwide to deliver a broad
range of economic supports to the children
and the wider community. It also acts as an
advocate for the rights of those affected
by the Chernobyl explosion, and engages in
research and outreach activities to encourage
the rest of the world to remember the victims
and understand the long-term impact on their
lives. More than 18,000 Ukrainian children
affected by the disaster have been treated
at Cuba's Tarará resort town since 1990.
== Commemoration ==
The Front Veranda (1986), a lithograph by
Susan Dorothea White in the National Gallery
of Australia, exemplifies worldwide awareness
of the event. Heavy Water: A Film for Chernobyl
was released by Seventh Art in 2006 to commemorate
the disaster through poetry and first-hand
accounts. The film secured the Best Short
Documentary at Cinequest Film Festival as
well as the Rhode Island "best score" award
along with a screening at Tate Modern.Chernobyl
Way is an annual rally run on 26 April by
the opposition in Belarus as a remembrance
of the Chernobyl disaster.
== Nuclear debate ==
The Chernobyl accident attracted a great deal
of interest. Because of the distrust that
many people had in the Soviet authorities,
a great deal of debate about the situation
at the site occurred in the First World during
the early days of the event. Because of defective
intelligence based on satellite imagery, it
was thought that unit number three had also
suffered a dire accident. Journalists mistrusted
many professionals, and they in turn encouraged
the public to mistrust them.In Italy, the
Chernobyl accident was reflected in the outcome
of the 1987 referendum. As a result of that
referendum, Italy began phasing out its nuclear
power plants in 1988, a decision that was
effectively reversed in 2008. A 2011 referendum
reiterated Italians' strong objections to
nuclear power, thus abrogating the government's
decision of 2008.
In Germany, the Chernobyl accident led to
the creation of a federal environment ministry,
after several states had already created such
a post. The minister was given the authority
over reactor safety as well, which the current
minister still holds as of 2015. The events
are also credited with strengthening the anti-nuclear
movement in Germany, which culminated in the
decision to end the use of nuclear power that
was made by the 1998–2005 Schröder government.In
direct response to the Chernobyl disaster,
a conference to create a Convention on Early
Notification of a Nuclear Accident was called
in 1986 by the International Atomic Energy
Agency. The resulting treaty has bound signatory
member states to provide notification of any
nuclear and radiation accidents that occur
within its jurisdiction that could affect
other states, along with the Convention on
Assistance in the Case of a Nuclear Accident
or Radiological Emergency.
In the United States people became motivated
by the Chernobyl disaster to investigate and
draw parallels to their own government's nuclear
problems. The Hanford Site has become known
as "America's Chernobyl" and many parallels
have been drawn between the causes of the
Chernobyl accident and the causes of the catastrophic
and ongoing situation at Hanford. Additionally,
Chernobyl, along with the space shuttle Challenger
disaster, the Three Mile Island accident,
and the Bhopal disaster have been used together
as case studies, both by the US government
and by third parties, in research concerning
the root causes of such disasters, such as
sleep deprivation and mismanagement.
== See also ==
List of Chernobyl-related articles
List of industrial disasters
Lists of nuclear disasters and radioactive
incidents
Nuclear and radiation accidents and incidents
