The Japan Nuclear Incident: An Overview
Part I
As you know, in March of 2011, there was a
serious nuclear emergency at the Fukushima
Daiichi nuclear power plant in Japan. And
you probably also know that the U.S. Nuclear
Regulatory Commission geared up its emergency
response organization to provide support to
that country and to help this country understand
what was happening overseas.
Hello. I’m Rick Hasselberg with the NRC’s
Office of Nuclear Security and Incident Response. 
It’s my job to train and prepare the NRC’s
Reactor Safety Team to respond to emergencies
that might occur here in the United States.
During the Fukushima event, I was part of
the NRC team that responded to the events
in Japan.
In this first video, I’m going to explain
what happened at the Fukushima site. In the
second video, I’m going to explain how the
NRC responded to the incident.
But first let me offer some general background
about nuclear power plants.
Nuclear power plants produce electricity.
They do this through the nuclear process of
uranium fission. That process produces very
hot steam that rotates an electrical generator.
The fission process also creates radioactive
materials. Nuclear power plants are designed
to contain almost all of the radioactive material
they generate. That material is sealed up
inside of the metal fuel rods that make up
the nuclear reactor’s core.
But radioactive materials produce heat as
they decay. That heat, if not removed – even
if the plant is already shutdown – can destroy
the reactor core and release the radioactive
materials to the environment.
Overheating reactor fuel can also generate
flammable hydrogen gas. The greater the amount
of fuel damage, the greater the amount of
hydrogen that is generated. Hydrogen, when
mixed with air at the wrong concentrations,
can burn. At even higher concentrations, hydrogen
can explode. This reality will become important
when I discuss what happened at Fukushima.
If the reactor core is damaged, the building
that contains the reactor then becomes essential
in limiting the radiological impact of the
event. The containment for plants like Fukushima
consists of a large steel structure that surrounds
the reactor vessel, and a steel, donut-shaped
structure located below the reactor in the
shape of a donut. That lower structure is
partially filled with reactor cooling water,
and is called the wetwell. The upper structure
is dry and is called the drywell. The drywell
and wetwell are connected by a series of pipes.
Together, these structures form what is known
as the reactor’s primary containment. Surrounding
and supporting the primary containment is
a secondary containment, also called the reactor
building.
The Fukushima Daiichi site consists of six
Boiling Water Reactors. On March 11, 2011,
only three of the site’s six units were
operating. The other three units were shutdown
for different reasons – and had different
amounts of fuel in their reactor vessels.
At the moment of tsunami impact, the three
and four-story tall structures facing the
sea and the turbine buildings at the site
were slammed by waves 47 feet above sea level
-- much higher than the 19-foot tsunami the
site was designed to withstand. 
The waves destroyed AC power generating and
distribution equipment, and flooded several
battery rooms. Many structures facing the
sea were completely destroyed
The combination of the 9.0 earthquake followed
by the huge tsunami knocked out nearly everything.
Back-up power systems worked for a while,
but the tsunami took out almost all vital
AC power generating and distribution equipment. 
Without power, nearly all the redundant and
diverse cooling systems and containment support
systems were lost. Any emergency plans or
procedures that relied on electric power were
useless. Any expectations that relief personnel
could provide the type of assistance that
was needed were also in vain.
The failure of core cooling systems resulted
in core heat-up. Decay heat from the reactor
core increased coolant temperature. The water
boiled faster. More boiling led to higher
pressure and eventually, safety valves opened
to relieve excess pressure and prevent catastrophic
failure of piping and components. When those
valves opened, they reduced the temperature
and pressure in the reactor vessel but they
increased the temperature and pressure in
the containment wet wells. With no way to
pump cooling water into the reactors, there
was no way to make up for the water that was
now exiting the reactor as high pressure steam.
With no source of cooling water, reactor vessel
water level decreased as steam continued to
be discharged into the containment wet wells.
At different times, all three operating units
reached the point at which the top of the
reactor fuel rods was no longer covered with
water. At this point, the incident was well
into what we call a severe accident sequence.
Sometime after uncovering of the top of the
fuel, portions of the fuel rods reached a
temperature of 2200 degrees. The fuel rods
are made of a zirconium alloy called zircalloy.
They contain the reactor’s uranium fuel
pellets and prevent the release of almost
all of the radioactive fission products. At
temperatures above 2200 degrees, the zirconium
in the fuel rods reacts violently with steam
to produce four bad things:
Rapid fuel rod degradation results in the
inability to contain all of the fission products.
This results in the rapid release of some
of the fission products, especially some fission
product gases. As if fuel rod temperatures
weren't high enough, the zirconium-water reaction
actually accelerates itself as it rapidly
increases fuel rod temperatures in the course
of the chemical reaction. And one more thing
– the zirconium-water reaction creates hydrogen
gas.
Hydrogen, when mixed with air in the right
concentrations, can be flammable or even explosive.
At Units 1, 2 and 3, a huge amount of hydrogen
was generated as the fuel rods were violently
consumed by the self-sustaining zirconium-water
reaction. 
Core temperatures continued to rise. You could
hardly call them fuel rods anymore, but some
of the materials that used to be inside the
fuel rods were reaching 3000, 4000, 5000 degrees.
At those temperatures, five more bad things
happened:
Localized melting of the fuel – the hottest
locations in the core start to melt. Relocation
of the fuel materials – What started out
as melting at a few locations became a sizzling
mass of molten material. As that occurs, many
of the fission products that had been trapped
inside the uranium fuel pellets were now released.
The radiological impact of the core melt is
huge. Materials that had been in solid form,
sealed inside of fuel rods, were now in either
liquid or gaseous form, are they were on
the move. As the temperature went up, more
and more radioactive material became available
for release. The temperature of that sizzling
mass of highly radioactive core material was
higher than the melting point of the steel
reactor vessel. So, if they came in contact,
the steel reactor vessel would likely fail.
This is the classic core melt scenario.
Moving away from the core for a moment, containment
temperature and pressure had been rising for
some time. If some of that energy was not
soon released to the environment, then the
containment might rupture – something you
want to avoid at all costs. Also in the containment,
hydrogen gas has been building up. Recall
that hydrogen is created as the fuel rods
react with steam. The operators needed to
vent containment as quickly as possible. 
For use only under severe accident conditions
like these, a special, non-filtered ventilation
pathway had been installed in these containments.
If this “hardened” vent pathway could
be used, containment’s pressure and temperature
could be lowered, excess hydrogen gas could
be safely released, and the containment could
then be resealed to hold in the worst of the
fission products.
But the hardened vents were not successfully
operated in time to prevent extremely high
pressures from failing the containments in
all three units. With core melt down and containment
failure, Fukushima Units 1, 2 and 3 were unable
to prevent the uncontrolled release of radioactive
material and hydrogen gas from the primary
containments to the secondary containments.
In the secondary containments, hydrogen concentrations
reached flammable and then (in the cases of
Units 1 and 3) explosive concentrations. In
Unit 2, hydrogen ignited before it could reach
an explosive mixture but it burned continuously
for several days.
The events at Fukushima Daiichi were tragic.
On the International Nuclear Event Scale,
the Fukushima accident rates as a seven – a
major accident – based on the amount of
radioactive material released to the environment.
Only one other nuclear event has been rated
at this level – the Chernobyl accident in
1986. Prior to Chernobyl, the Three Mile Island
accident, in 1979, was the most significant
commercial nuclear power plant accident. We
learned a great deal from the accidents at
Three Mile Island and Chernobyl – and we
are learning from Fukushima. By understanding
what went wrong, we will be better able to
identify and implement ways to make nuclear
power plants safer.
