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This post was put together with the information available as of March 17 01:00 JST.

 

Professor Yoshiaki Oka (Joint Department of Nuclear Energy, Waseda University)

Former American Nuclear Society board director, former Atomic Energy Society of Japan President, Tokyo University Emeritus Professor

Research field: Nuclear reactor design, reactor physics.

 
A summary of what has happened since March 11:
(the explanations in brackets were added by SMC for clarification)
 
1. Due to the earthquake, the nuclear reactor automatically stopped operating. Electrical transmission from sources outside the power plant stopped (loss of external power supply) and the emergency Diesel Generator (DG) and emergency nuclear reactor cooling system were switched on.“The shape of any plume of material obviously depends on how high it goes – related to temperature of the damaged fuel – and weather.  After Chernobyl, more volatile elements such as radiocaesium and radioiodine spread across Europe.  Less volatile elements such as strontium-90 and plutonium were discharged as microscopic hot particles – tiny bits of reactor material – and were mainly deposited within 10 km of the plant.
 
2. Due to the tsunami that occurred about an hour after the earthquake, the system that drew in seawater for cooling (e.g. pumps) was damaged, and so heat release method was lost (i.e. heat sink loss). This has become an accident that goes beyond what it was designed to withstand and needs to be classified as severe. The DG stopped because the heat being generated could not be removed. The emergency cooling system could not operate for a long time, since there was nowhere for the heat to go. Since there was nowhere for the heat coming out of the radioactive fuel (decay heat), the water temperature and pressure inside the nuclear reactor increased and the water level inside the reactor vessel started falling.
 
3. Severe accident procedures, prepared in the 1990s, were initiated.
 
4. Steam was discharged from the nuclear reactor into the containment vessel, and the gas inside was released into the nuclear reactor building in order to depressurize its inside (i.e. keep the containment functioning). 
(For details about a reactor's structure etc, refer to websites such as The Federation of Electric Power Companies of Japan.)
 
5. In order to stop the water level from dropping, and to remove the heat, the reactor vessel was depressurized and seawater was injected into the pressure vessel using a movable power supply and pump. 
 
6. But because the nuclear reactor's pressure was higher than pressure the pump could handle, water could not be injected properly and temporarily exposed the fuel rods in the nuclear reactor as the heat increased. As a result, reduction of water from zirconium oxidation (an exothermic reaction) in the fuel cladding tube created hydrogen. This hydrogen traveled through the containment vessel and accumulated in the upper part of the nuclear reactor building as the reactor and containment vessel were depressurized.
  On March 12 and 14, explosions occurred at nuclear reactor Unit 1 and 3, and the upper parts of each unit’s building were destroyed. The reactor vessel and its containment vessel are covered by a 2cm thick concrete wall so they were not damaged. (Note that it was not the nuclear reactor that exploded. The nuclear reactor had been stopped.) In regards to Unit 2, it is possible the containment vessel’s containment mechanism was damaged in the hydrogen explosion on March 15.
 
7. Hydrogen can also be made by water radiation decomposition. If the hydrogen recombinator is not working, hydrogen accumulates in the upper part of the nuclear reactor building. It is possible this was the case happening in Unit 4’s spent fuel pool, which had already been inactive when the earthquake hit. Even if a hydrogen explosion occurs in the upper part of the nuclear reactor building, the spent fuel at the bottom of the pool would not be damaged because the pool is deep (7m in depth). When the water level drops, it weakens the radiation shielding function so more water needs to be injected. For example, it is possible to inject water using fire trucks.
 
8. Rising levels of radiation have been observed outside the nuclear plant site, but these values are far lower than the level needed to be considered as a radiation hazard. The majority of radioactive materials (radioactivity) observed off the site are likely to be noble gases. Noble gases have no chemical reactivity, and their density decreases as they spread across the atmosphere. It is not a problem as far as radiation exposure is concerned. It is possible some volatile radioactive material may have attached onto clothes. Volatile radioactive materials dissolve into water as long as the nuclear reactor fuel is covered with water, and are hardly found in the environment. I think that most of those that did get released into the environment were able to do so when the fuel rods were temporarily uncovered from water, since high water temperatures raise the possibility of a release.  
  In the future, depending on the amount of radioactive iodine, it might be necessary to limit intake of vegetables, fruits and milk produced near the site. Iodine tends to accumulate in the thyroid gland of children, and polluted milk needs to be disposed of. I doubt we will reach this level.
  The 1986 nuclear reactor accident in the former Soviet Union involved a reactor core explosion and a massive graphite fire (graphite was a component of the reactor core). This lead to a massive amount of reactor core materials, including iodine, to be released into the environment. (TEPCO's nuclear reactor is a different type and does not use graphite.) Cows grazed on contaminated grass, but because there were no milk restrictions, children who drank that milk accumulated iodine in their thyroid glands and they had to undertake thyroid gland operations.
  The hydrogen explosions to date have not sent large parts of the core flying out, so a huge amount of iodine has not been released into the environment. Since iodine dissolves into water, it is important to cover the nuclear reactor fuel with water.
 
9. Even a small amount of radiation as well as radioactivity can be measured. It is often reported about when numbers are tens of hundreds of times larger than normal, but the numbers that have been reported currently are far lower than the level that causes harm to human health. Tolerence levels are also 50 times lower than numbers needed to start causing health concerns. However, note that the acceptable value for workers at the power plant are different from the public. The workers (operators) work under an agreement regarding risk, and the acceptable value for them is higher than that for the public. Radiation damage is categorized as an occupational accident. Of course, it would be better to try and prevent such damage from happening. Their risk can be reduced if they wear an iron vest/apron and breathing masks while they are working.
 
10. The three explosions observed at Unit 1, Unit 3 and Unit 2 in that order can be thought of as hydrogen burning explosively. With the first two, smoke was pushed out horizontally. With the third explosion involving Unit 2’s containment vessel, the smoke went upwards. The concrete surrounding the containment vessel may have caused the explosive material to be released upwards. If the third explosion was a hyrdovolcanic explosion due to a reaction between the molten core and water, then a larger amount of radioactive material would have been emitted and the radiation levels would have gone up much more. The hydrovolcanic explosion’s plume went up only 200-300m into the air, which is not very high, so I do not think much radioactive material made it to the stratosphere.
 
11. There are program models to help us use wind direction and strength data to calculate how radioactive materials could be diffused into the air, and these should be able to give accurate predictions.
 
12. Rain brings radioactive materials in the atmosphere down to the ground so try not to come in direct contact with rain. It would be best to wash (edible) things that are wet (from the rain) before eating them. There are a number observation points measuring radiation levels so we should be able to predict rain effects easily. 
 
 
A Wrap-Up and what’s going to happen now
 
The safety plan is basically: stop, cool down, and trap. Out of the three, the reactor’s ‘stop’ has thought to have been the moment it had shutdown automatically following the earthquake. ‘Cool down’ is under progress right now as seawater is being poured into the reactor. It is important to keep covering the nuclear reactor fuels with water, while at the same time start removing all of the heat. It is not clear whether power from external sources have been recovered, but if they have, it should be able to stabilize and re-start equipment operations. The final ‘trap’ phase will be achieved once the reactor’s fuel rods are completely immersed in water.
 
There is a need to stop a massive fire from happening in order to prevent radioactive materials from flying and spreading, but there is hardly anything flammable in the nuclear reactor building so the chances of a fire are low. What follows after the prevention of a fire is the prevention of an explosion from hydrogen generated.
 
Evacuation plans had already been drawn out beforehand as a measure for ensuring safety in case of a severe accident. The area around the site carries out an emergency drill once a year.
 
There is no need to evacuate from areas outside the 30km safety zone where people inside are being told to stay indoors or move away.
 
The national and local governments are carrying out the right procedures, and it would be safer to follow their instructions.
 

For more information, I recommend websites such as NHK.

 

 

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