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).
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.
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 3 - Plutonium Glowing through Radioactive Decay Heat
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 4 - Heat Generated by Radiation Decay over Time
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.
Figure 5 - Illustration of International Nuclear Events Scale (INES)
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.
Effect of Radiation Dose in Milisieverts mSv (1000µSv)
|Exposure||Health Effect||Time to Onset|
|0.3||30days-normal background rad’n|
|500||changes in blood chemistry|
|1, 000||nausea in some people||hours|
|5, 500||Nausea and fatigue common||hours|
|8, 000||hair loss||2-3 weeks|
|40, 000||possible death||within 2 months|
|100, 000||destruction of intestinal lining|
|and death||1-2 weeks|
|300, 000||damage to central nervous system||minutes|
|loss of consciousness and death||hours to days|