Dr Carle Gibbons
Nuclear Engineer
Sequence of Events
On Friday, 11 March 2011, Japan’s biggest earthquake
ever hit. Three reactors, Units 1, 2 and 3, were in operation at TEPCO’s
Fukushima Daiichi nuclear power plant with Unit 4’s fuel removed to a
fuel storage pool next to the reactor, of type shown in Figure 1.
All operating Units went to automatic shutdown by
lowering boron control rods. These have the ability to stop fission
taking place by absorbing neutrons. Neutrons escape from fissile
material (Isotopes of Uranium U235 and in the case of Unit 3 Plutonium
Pu239) and are slowed down by the reactor water coolant to enable them
to be captured by the surrounding fissile atoms (U235 or Pu239). On
capturing a neutron the fissile atom becomes unstable and separates, as
shown in Figure 2, changing to lighter radioactive elements, each having
differing chemical and radiological behavior.
Figure 1 - Section Through a Light - Water Reactor
Automatic shutdown is part of the reactors
‘fail-safe’ measures and does not need electrical power or human
intervention. The plant was hit by a 7 metre high tsunami which consumed
essential power/control equipment and the backup generators. All pumps
then stopped and cooling systems were not able to remove the heat still
being produced by the fuel in the reactors and spent fuel cooling pools.
With no cooling, temperatures and pressure in the reactors rose and
water levels dropped as the coolant evaporated.
Nuclear fuel that has been used in a reactor will
continue to generate heat for a considerable time (possibly for greater
than a year), even when removed from a reactor. Some fission will be
continuing and considerable heat will be generated as the newly formed
fission products go through a process of radioactive decay, Figure 3.
The length of time taken for decay to take place
varies considerably, from milliseconds to billions of years. However
most of the heat is generated from those having a short life with the
heat output reducing from 6.5% to 0.3% of the heat generated at load, in
10 days, as shown in Figure 4.
The two models are displayed in the graph, one being
Retran, which does not relate to any operating history. The Todreas
model assumes 2 years prior operation.
Figure 2 - Uranium Fission Reaction
The reaction by TEPCO after the accident was to
install mobile generators to produce power to stabilise conditions at
Unit 2 and Unit 3. However, there was insufficient power to provide
adequate coolant to Unit 1 and this generated considerable quantities of
hydrogen after the steam reacted with zirconium cladding, which contains
the fuel.
The zirconium removes oxygen from the steam. It is
probable that the aftershock that followed allowed hydrogen to escape
into the reactor hall and exploded in the loft, destroying the fuelling
crane and the upper walls and roof. When the pressure in Unit 2 reached
700kPa the integrity of the containment vessel was threatened and it
became necessary to vent steam. The steam was vented into the
suppression system, which resulted in an explosion and damage to the
suppression system vessel. Radiation levels at the site rose as a
consequence of these initial events with possible releases of
radioactive water (tritium-H3 oxide), Cesium137 and Iodine131.
The Japanese government then declared that, due to
the release of radioactive material to the environment, the accident was
rated as Level 4 on the International Nuclear Events Scale (see Figure
5). This was raised to a Level 5 incident on 18 March 2011 due to a
perceived wider threat. Events are classified at seven levels: Levels 1
- 3 are “incidents” and Levels 4 - 7 “accidents”. Previously the
accident at Windscale (UK) was Level 5, as was 3 Mile Island (USA);
Level 5 and Level 7 at Chernobyl (Ukraine).
Figure 3 - Plutonium Glowing through Radioactive Decay Heat
A situation was also developing where fuel stored in
the fuel cooling pools were becoming exposed due to decaying fuel
heating and boiling off the water. It became clear that fuel inside the
reactors and cooling pools need not be supplied with water as there was
sufficient heat to melt the fuel.
Also steam was reacting with the zirconium cladding
and producing hydrogen, with a risk of further explosions and fires.
Zirconium also reacts with air at high temperature and can combust.
There also became a likelihood that as the fuel melts, contaminated
materials are released from the pools to the atmosphere, delivering
radioactive iodine, cesium and potassium to wide areas in the proximity
or carried further by the wind.
High levels of radiation have been recorded on the
site so far, reporting levels of 4,000 microsieverts/hour. International
Atomic Energy Regulations for Radiation workers limits radiation to
workers in the nuclear industry to 100,000 microsieverts, averaged over
a 5 year period, and the general public as 1000 microsieverts per year.
The effect of dose is illustrated in Table 1.
What’s Next
The situation will remain fluid for some time and
events are likely to worsen, particularly if the fuel cannot receive
sufficient cooling water. This will result in some meltdown of fuel and
escape of harmful radionuclides. Iodine has a radioactive half-life of 8
days and will not remain a problem for long. However, Uranium (U238) and
Plutonium (Pu239) have a half-lives of over a billion years where as
Cesium (Ce137) has a half-life of 30 years and a biological half-life of
70 days (this is the time half the quantity remains in the body).
Figure 4 - Heat Generated by Radiation Decay over Time
Accidental ingestion of caesium-137 can be treated
with the chemical ‘Prussian Blue’ (Iron Ferrocyanide Fe7(CN)18), which
binds to it chemically and then speeds its expulsion from the body.
There will also be significant amounts of Strontium (St90) which has a
half-life of 28.8 years. This is difficult to remove from the body and
is a bone seeker and primary cause of leukemia.
Cesium acts in the same way as potassium and will
contaminate all food (livestock and vegetables) rendering land
contaminated with cesium unfit for any food production. The greatest
radioactive threat to the population as the radiation spreads is one of
contamination. This is why the Japanese government has stressed the
removal of contamination from the skin and clothing.
Precautions, such as taking Potassium Iodine tablets
is important if you are near the Fukushima Daiichi nuclear power plant.
It, however, becomes less important the further one is from the plant. I
do not consider that this America or Thailand, or the rest of the world,
should be worried about significant radiation reaching their shores.
Figure 5 - Illustration of International Nuclear Events Scale (INES)
Watching the clouds of steam rising from Units 3 and
4 plant, as fire units and helicopters attempted to cool the fuel,
indicated that contamination could spread with rising steam. It is also
likely that this steam will produce hydrogen as it comes in contact with
the hot zirconium cladding. At the moment, fuel in cooling ponds also
concern as they are open to the environment. It should be assumed by now
that they are severely damaged with no mechanism to remove them as fuel
handling equipment is destroyed.
At this stage TEPCO should be considering
encapsulating them in sand containing boron and concrete. In using water
to cool fuel, TEPCO should carefully assess the potential of introducing
hazards and the likelihood of an explosion inside the reactor vessels,
exposing the reactor core. Also pumping water into the fuel pond could
result in initially mobilising plutonium and fission products into the
atmosphere.
The greatest impact this accident has to the rest of
the world is on the continuing nuclear debate. The world in reviewing
nuclear should consider that nuclear technology has advanced since
Japan’s oldest nuclear power plant, Fukushima Daiichi, came on line in
1970. Nuclear is only one alternative to be considered, alongside
others, in developing sustainable power sources that offer advancement
and security.
