Good morning, everybody.
In the last two lectures, I gave you some
ideas about the safety principles which are
followed in the design of all nuclear reactors
and nuclear establishments.
I also talked to you about the approaches
why how the safety is really gone into a very
large depth, what is meant of the defense
in depth approach wherein we find out what
are all the events which can happen, how it
can happen for a particular design, what sort
of events can happen and then we see that
the effect of those events are minimal.
But now how do you know what events can happen.
One is from the design, we can try to postulate
failures, but besides this, there is a wealth
of data which is available based on the operating
experience of different nuclear power plants
and radiation facilities.
Here it is very important that we take duo
note of all such events which have happened
in the other plants and then see that such
an event cannot happen.
Okay, should such an event happen, can I take
some approaches in the design by which the
consequences could be mitigated, all such
thoughts need to be given to your design.
Now, let us look at what sort of events have
been there.
As I mentioned in the lecture on safety principles
that we have to have a technical safety objective
that we take measures to prevent accidents
and in the case accidents happen, we should
be able to mitigate their consequences to
a very large extent, such that any radiation
release is of a very low probability and that
in any radioactive release is within the prescribed
limits.
So essentially we are looking at any event
should have minimal radiological consequence.
So let us look, one is as I said the operating
experience on similar installations need to
be considered.
So we have to ask ourselves a question, can
it happen in my design.
If yes, then how to prevent or mitigate.
So this approach needs to be that.
Now, thanks to the safety approaches, we have
safety conventions under the auspices of the
IAEA and there is a method of reporting any
deviation from the normal.
So any unusual occurrences happening in the
plant, they may not have had any consequences,
but any occurrence which is not in the design
is called as an unusual occurrence.
As I mentioned, we have data which are reported
from different countries and it is a practice
in all the nuclear establishments to report
any unusual occurrence, there is any deviation
from the normal to the IAEA and so you have
the data bank, you have the data bank of the
World Association of Nuclear Operators, you
have the Failure Data Base of the Japanese
Atomic Energy Agency and USNRC.
In India we have the Atomic Energy Regulatory
Board which monitors all such unusual occurrences
in all the radiation establishments.
This lecture finally would look into different
events which have happened, but not all the
events, some events.
Whenever we look at an event, the severity
of the event is important and public must
get an idea what is the level of that event.
For example, you take an earthquake, whenever
there is an earthquake, it is said it is 6.2
on the Richter scale.
When you talk about temperature, you say it
is 34 degree centigrade that is on the Celsius
scale.
If it is Fahrenheit, it is a Fahrenheit scale.
So there is a scale; here we have the International
Nuclear Event Scale which explains the significance
of the event.
It could be from different activities, but
the significance is known by the level and
how the levels are classified, we have seven
levels.
Levels 1, 2, 3 are called as incidents and
4 to 7 are accidents, and which is below level
1 that is 0, they are just deviations, minor
deviations.
From this, you can easily conclude that level
7 would be the maximum accident, you are right.
Let us just see what is level 7.
It is a major accident with large external
radioactivity release of the order of thousands
of terabecquerels and a good amount of damage
to the plant.
Level 6, again a serious accident but the
radioactivity release is less, something like
thousands to tens of becquerels.
Coming to level 5, the accident has good amount
of offsite consequences.
Again, it is related to the radioactivity
release of the order of hundreds to thousands
of becquerels and severe damage.
Then level 4 talks to you about events wherein
the site, the plant site has the problems
issues, that is activities only restricted
to the plant site.
So there is no offsite risk.
Of course, installation has damaged.
Level 3 would be a serious accident but not
with release of radioactivity.
Level 2 would be an incident and Level 1,
an anomaly.
Now, as I mentioned to you, the different
incidents cover not only the nuclear power
plants; they also cover the fuel cycle facilities
also, all the fuel cycle facilities where
radiation is involved, like reprocessing,
fuel fabrication, and all such areas.
Just to give you an idea of different events
which have happened in different scales, topmost
you see the Fukushima reactor accident.
It is IAEA scale 5.
It happened in Japan after the emergency core
cooling failed after a big earthquake and
tsunami.
Then near about the same place called Onagawa
in Japan nothing happened; there was a fire
only after tsunami.
Then in an irradiation facility in Belgium
in 2006, worker has got a high dose of radiation.
If you then move down in the Hungary in the
Paks nuclear power station, spent fuel rod
ruptured and fuel pellets split, bringing
out the radioactivity.
Then we have the famous Tokaimura event in
Japan in 1999; it is IAEA 4 where there is
a fatal overexposure that means overexposure
of so much after a criticality accident in
a reprocessing facility and the worker died,
two or three workers died.
I will give you the details later.
Before this, of course, you had the Chernobyl
accident which was placed at level 7.
We will look into all these accidents one
by one.
Oh, we already have; Chernobyl which really
had an impact on the environment.
Then level 6, we have an event from a reprocessing
facility where an explosion occurred and radioactivity
split out.
Then the Windscale event, I will describe
to you, Tokaimura, then radiological barriers.
The next set of events like Three Mile Island,
the environment was not affected, but the
radiological barrier, the final barrier was
intact.
Then we had the other events like you know
Saint Laurent, France where one fueled channel
melted, but there was no activity release.
There have been some other activities, actions
or events which had taken place but they really
they have not affected the environment because
the defense in depth approach had been followed.
Of course, to the common man, common public,
TMI, Chernobyl, and Fukushima are the most
important events which will be remembered
for a long time and there have been some events
like a near-miss event wherein it could have
become a larger event had it not been carefully
watched.
There is one event in the David Besse nuclear
plant in USA and also we had a fire in the
Narora Atomic Power station in India.
So from all these things, we have learnt a
lot of lessons which we have already implemented
into our power plant designs.
TMI, Three Mile Island is a pressurized water
reactor.
So you have the core here, the steam comes
out, goes like this.
It is not steam, I am sorry, it is pressurized
water.
It goes like this, exchanges heat to light
water in another steam generator, and comes
back and pumped back.
Now there is a pressurizer which maintains
the pressure of this system.
We want to have the higher pressure so that
boiling is avoided in the reactor core that
is uniqueness of the pressurized water reactor.
So what happened?
This accident was a quite serious accident
in the US commercial history.
So lot of changes were subsequently brought
in the training of operators, in the response
planning.
Really it was, it happened in 1979 and really
opened the eyes of many of the designers and
many of the operators to improve, not that
things were bad, but how to improve and you
know, always there is chance or there is a
scope for improvement at every stage and that
is what.
Not that there was something very badly done,
everything was okay, some thoughts into some
type of events which could have caused were
not you know effectively put it.
What happened?
The accident happened in March 28, 1979, and
there was a failure in the non-nuclear section
in the steam water system of the plant.
The main feed water pumps stopped due to some
fault.
Because of that the flow to the steam generator
was not there.
So the turbine tripped automatically, the
reactor tripped.
Of course, for the reactor to trip the pressure
in the primary system increased and the pressure
in the primary system increased beyond a certain
level, the reactor tripped.
After some time the pressure started relieving
through a pressure relief valve in the pressurizer.
After some time when the pressure had fallen
down, normally the relief valve should have
set back into the position.
However, even though the operator felt that
the valve would have closed, it was still
open and no signals were available to the
operator to really confirm that the valve
has closed.
So what happened, water steam was going out
through the pressure relief valve outside
not to the core and this caused overheating
of the core.
Now, to have an idea of the level of water
in the core, there is no instrument which
shows the level of water in the core, but
the level in the pressurizer was seen by the
operator.
Now, let us look back what would have happened.
See here because of the loss of cooling the
temperature increased and here the temperature
increase finally resulted in steam production
and this steam production lifted this mass
of water, whereas it was getting released
continuously.
So looking at the pressurizer level rising,
the operator thought, oh, it is full of water.
The core is full of water.
So what he did?
The emergency core cooling pumps, he really
tripped the pump so finally there was cooling
still absent to the core.
The fuel pins ruptured, some of the fuel periods
belted.
In fact, the hydrogen generated due to the
reaction of the zirconium clad and the water
came out, but luckily for us nothing happened.
It didn't reach explosion levels.
So effectively, you can look at there have
been design deficiencies, no proper signals
were available instrumentation signals are
available to the operator to assess the state
of the plant.
Then personal error in the sense that the
person, the operating personal could have
looked at some other aspects really before
turning off the cooling to the core, because
removing the cooling to the core is a very,
very important step before which they should
have done.
So what was the effect?
Upgrading was done, the plant design was strengthened,
and operator training was improved by a very
good amount so then identification of more
problems and sharing of the information more
and more.
Apparently some similar event that happened
in another plant quite some time back, but
unfortunately that information was not shared.
So maybe if it had been shared during the
training period apparently the operators may
have been better, put to save the situation.
Next, so we have seen the American contribution,
now let us go to the Russian reactor, Chernobyl.
This Chernobyl is a boiling water reactor,
but it is a pressure tube boiling water reactor.
So there is a tube, pressure tube, it is not
a pressure vessel.
So pressure tube in which you have the fuel,
you have the fuel assembly and steam is produced
at the outlet the steam water mixture goes
here, get separated, steam goes to the turbine,
and then runs, and then this is the circulating
pump.
This is a boiling water reactor.
Now what happened?
There is a feature in all the Russian plants
that should there be a power failure and the
turbine would trip but the turbine would be
coasting down, turbine has got a large mass.
So it has got inertia so it will coast down.
So their idea is to generate power from that
mechanical energy, variable frequency, variable
voltage power and see whether you could run
the main circulating pumps or let us say auxiliary
feed pumps to cool the core.
In fact, they say that this can cool core
for nearly few minutes.
This feature is present in most of their plants.
But here, in this Chernobyl reactor, they
just wanted to perform a test to see whether
this feature is existing and up to how much
time it can take in this particular plant.
Idea is good, but unfortunately a series of
mistakes happened which really resulted in
a big accident.
So at the end of the accident, there was a
big explosion, 31 people died and the 31 people
who died, most of them were firefighters and
people around 30 kilometers were evacuated
and significant amount of radioactivity was
released to the environment.
So as I mentioned, the aim of the test was
ability of the reactor’s turbine generator
to generate electrical power to power the
emergency core cooling system in the case
of loss of external electric power.
So what happened?
The main pumps everything was started, but
the emergency core cooling system which normally
would have come up by itself was deactivated,
so that when the reactor is at a very low
power at that time it should not come.
But when they were reducing the power to do
the test, unfortunately, they pushed the rod
too much so the power became very small.
Now when the power was very small, it is not
good because this type of design below about
7-10% of power level, it has got a positive
coefficient, positive reactivity coefficient
in that means that in case the temperature
increases, there will be a positive reactivity.
So normally this reactor is not supposed to
be operated at that power.
So the operators try to take out the control
rods but in spite of their control rods taking
everything, they couldn't really come back
to a higher power.
But of course, the operators were not aware
that this positive coefficient exists and
we must not do tests at that power.
Now the operators what do you called closed
the valve to the turbine and the turbine started
coasting down.
The recirculation pumps got the power from
the generator, but they are also slowing down,
the cooling water flow stopped, and as I mentioned,
this was a region in which any rise in temperature
would cause a positive reactivity, means it
would go to increase the neutron chain reactions,
and the power started to rise.
At this stage operators tried to put in the
control rods but the speed of the control
rods was not very good; they were not able
to suppress the reactivity rise and there
was a burst, fuel temperature’s increased,
fuel ruptured and within 40 to 44 seconds
after the experiment started, we had the explosion
of the reactor building and lot of radioactivity
plume coming out.
So everything was over in less than a minute.
So the fuel failed due to increased -- pressure
tubes failed, water steam came out, and high
temperature steam coming in contact with a
graphite, the graphite also caught fire, the
fire again increased the explosive force,
upper part of the reactor was taken off, and
the reactor building was really not a containment
building.
That is another reason, that was really not
a good containment building; it was just a
reactor building that also exploded and brought
everything all the radioactive materials to
the public.
So one of the learned thing was this is a
reactor without self-control ability.
So in fact, no other country has this design.
That is one thing; it’s a good thing for
us.
These were basically plutonium producing reactors
for the Russians, however, this reactor had
the maximum availability in the Russian nuclear
power plants.
So the thickness of the pipes or the pressure
tubes was also less; they were not strong.
So the containment was also not pressure proof.
S o this is basically a large thing in the
design.
Then of course, other mistakes that cutting
off the emergency core cooling system was
also another mistake by the operator, all
compounded, but basic design had the flaws.
Now, let us go to the Fukushima reactor.
Fukushima reactor is a boiling water reactor.
So it is a pressure vessel boiling water reactor
unlike the Chernobyl reactor, here all the
core elements are there, steam is produced,
so then that is goes to the turbine, and here
at the top, they have got the spent fuel pool
that is the fuel which has seen enough burn
up in the core is kept in this tank which
is full of continuous cooling water so that
its temperature is kept low.
Of course, we have the different containments;
the reactor pressure vessel, you have the
dry well, the wet well and of course, the
concrete building, etcetera.
Here let us see what happened.
In 2011, March 11th, there was an earthquake,
had a magnitude of nearly 9 on the Richter
scale and apparently the epicenter was in
the depth in the ocean, somewhere 25 kilometers
depth, near east of a place called Sendai
and 372 kilometers of Tokyo.
So this resulted, the movement by earthquake
was there, the seismological signals, vibrations
were this thing, all 11 nuclear plants at
four sites, they shut down, all, everything
went off well.
But then this earthquake resulted in a large
tsunami because it was in the sea, it pushed
up the water and it is estimated that it should
have gone to something like 14 to 15 meters
height.
So it flooded the place, lot of people were
dead, in fact, because of the tsunami and
lot of infrastructure was damaged.
Okay, so what happened to the plant?
The offsite power was lost because of the
snapping of the towers, so there was no offsite
power.
So immediately the emergency diesel generators
started.
So they provided the power supplied to all
the important systems of the plant, including
the cooling of the core and the spent fuel
storage.
Then 40 minutes after everything had practically
settled down, at that time, there was another
tsunami, large tsunami, it inundated the whole
place and the design of the place of the whole
establishment was for a tsunami of eight meters
height based on the data available to them,
based on the previous tsunamis.
But this tsunami was of a higher height.
So finally what happened?
It resulted in the loss of AC electrical power
to all the units because the diesel generators,
everything goes underwater.
So we have a situation called no offsite power
and no onsite power so that is called as station
blackout.
In the nuclear reactor terminology, we call
it as SBO, station blackout.
So cooling was lost in unit 1; unit 2 again
also it happened after 71 hours and unit 3
after 36 hours.
So without the AC pumps, the plant was relying
only on the battery.
There were some diesel driven pumps but they
also got submerged and finally the cooling
was lost to the plant.
Then what happened, the zirconium water reaction
resulted in hydrogen built-up, the built-up
of hydrogen was so much that it damaged the
first and second level of containments and
then it exploded.
Unit 1, 2, unit 1, of course, the expression
was there.
Not only that, even in the spent fuel bay,
lot of fuel pins had failed because of lack
of cooling and hydrogen generated.
So one point is very clear in the plant design
everything was okay but for the fact that
the diesel generators should have been kept
at a bigger height.
In fact, when this accident happened normally
we have a review of all our reactors whether
such things can happen and we just looked
at our own facilities and we found and basically
those facilities which are around near the
sea because this happened due to a tsunami
earthquake in the sea, so we looked at the
Kudankulam power plant.
We had already put the diesel at about 15
to 20 meters height from the sea level.
So apparently we had based on our margins
built in, it was there so we concluded that
such an accident cannot happen.
But we did have a tsunami in 2004 during which
there was a flooding of the Kalpakkam plant
which we shall see, of course, nothing happened,
which we shall see later.
Then there was a Windscale accident in UK.
See the Windscale Pile or you can call as
what do you call, any reactor you called as
a pile.
Graphite was being used as a moderator.
Then this graphite has a property of storing
energy when temperatures are low.
At room temperature, it stores some energy
and then beyond a certain temperature, maybe
500 or 1000 it would release that energy and
that energy will come with a burst.
This actually is called as Wigner energy.
Now this Wigner energy was there and in this
case, when the operator was raising the power,
the temperature was not sensed properly because
it was not located in the correct place.
So apparently, the operator overshot the temperatures
and the Wigner energy got released and it
burnt the graphite, the fuel failed, and it
burned for several days and damaging the portions
of the core.
Of course, operator was very, very thoughtful,
immediately he put in water and quenched the
fire.
However, radioactive gases basically Iodine-131
had got released and the they took care that
milk which was distributed within about a
200 square mile area they didn't use it, but
otherwise nothing specific.
But there is a story, they said no explosion
happened, reactor fire was extinguished but
the engineer’s hair turned white in the
control room.
Then we had an accident which is again related
to release of radioactivity in Siberia in
the Tomk-7.
Some reprocessing experiments were going on
and there was an explosion, so whatever was
the fissile material, everything came out
that contained plutonium and this release
was approximately six gigabecquerel of Pu-239
and 30 terabecquerels of other radionuclides
and nearly 160 onsite workers were actually
exposed to this radiation.
However, the total dose was 50 millisieverts
only, we have a limit for 100 millisieverts
in five years.
So that way it was not much but it did create
a radioactivity release.
Then the Hungary Paks plant which I mentioned.
While the fuel rods were undergoing cleaning,
it fell down and the fuel pellets came out
and the thoughtful operator added boric acid
so that they don’t become critical because
if we put boric acid it will absorb the neutrons
and there will be no chance of any situation
becoming critical.
Then the reprocessing plant at Sellafield,
here see the incident 20 tons of uranium and
160 kg of plutonium dissolved in a large amount
of nitric acid, but there is a pipe which
was leaking for several months, it was within
a stainless steel sump, and that there was
getting into another some outside the plant.
So when the reactivity increased people realized
that something has happened inside the plant,
but nothing much to worry about.
Now, let us see the accidents which will happen
with a lesser degree that we will take care
in the next lecture.
Thank you.
