I am hearing reports that people in Hong Kong and that part of the world are asking the question of “is Fukushima going to become a nuclear accident with the same effects as Chernobyl?”
My own view is to say no.
At Fukushima as a result of the earthquake and the tidal wave the electricity supply to the cooling system failed. The power reactors on the site were automatically shut down when the earth quake was detected. As soon as the reactors were shut down the nuclear fission reaction stops in a blink of an eye.
The longer the time between the moment when a power reactor is shut down and when radioactivity leaks out the better, the thing about radioactivity is unlike most nasty things it has a finite lifetime. The longer that radioactivity can be isolated from “man and his environment” the better.
As time goes by the amount of radioactivity becomes smaller, also the amount of heat emitted by the fuel becomes smaller as time goes by.
The fact that the Japanese plant has been shut down under safe conditions for about ten hours before losing all power is a good thing which will mitigate the effects of the accident.
Sadly the standby power systems failed as a result of the tidal wave and then after some hours of running on battery power the emergency systems stopped working for a time.
The supply of water to the cores of the reactors and the ponds where the used fuel is stored was stopped for some time. During this time the fuel in some of the reactors overheated.
The zirconium alloy tubes which held the fuel were damaged; this is one possible source of the hydrogen which has been responsible for a series of explosions. It is important to note that these explosions are not nuclear explosions and they occurred outside the part of the building which is designed to prevent the escape of radioactivity if it escapes from the big metal tank inside which the reactor is.
The worst thing which can happen to the reactors would be for them to continue to heat up, this process would require hours and days to evolve into the worst possible accident which can happen. If no attempts were made to cool the fuel in the reactor then the following events would occur.
1. All the water boils away; this would then expose the fuel to steam or the air.
2. The fuel would then start to heat up; this could provoke reactions between the zirconium fuel cladding and steam which would make more hydrogen. This stage of the accident ends when the fuel has become so hot that it melts. During this stage the more volatile radioisotopes would be able to leave the fuel. These would include isotopes of krypton, xenon, iodine and caesium. The less volatile radioisotopes such as strontium-89, strontium-90 zirconium-95 and the plutoniums will remain in the molten fuel.
3. Once the fuel is molten it would then start to melt a hole in the bottom of the reactor’s pressure vessel. Once it has made a hole then the molten fuel would be able to drop into the lower part of the reactor’s containment. Once the fuel gets into the lower part of the containment it might cause a violent production of steam if it lands in a pool of water. This is the so called china syndrome.
While these events sound perfectly horrible, they take time. They give the men and women who are dealing with the accident time to take actions. By cooling the reactor it will be possible to reverse these stages of the melting.
From what I know about the reactor accident I know that the top of the fuel did dry out and become damaged, right now I do not have the information needed to be able to tell you how much of the fuel has been damaged. So for the top part of the fuel the accident did progress through stage one and part way through stage two.
The easy way to imagine this accident is it is the nuclear version of leaving your pan of soup on the cooker and allowing it to boil dry. It is an accident which progresses slowly.
Chernobyl was a very different accident.
At Chernobyl as a result of a ill-advised experiment in a reactor which had some serious design flaws (which do not exist in BWRs, PWRs or British gas cooled reactors) a vast surge of power occurred. This was due to the rate of the nuclear reaction suddenly skyrocketing. This almighty power surge caused a steam explosion (a BLEVE). The BLEVE then smashed the fuel into small fragments which were thrown out of the reactor.
The big lumps landed close to the reactor and made it more dangerous and difficult to work near the reactor. The small particles were flung high into the sky. This release of smashed fuel resulted in a sudden and great release of radioactivity. I hold the view that it was impossible for any human to stop the initial escape of radioactivity from Chernobyl.
The steam explosion at Chernobyl was followed by a series of fires, these fires included a graphite fire which kept the damaged fuel hot. The boiling water reactor in Japan does not contain graphite so it is impossible for a graphite fire to occur in the reactors. The graphite fire at Chernobyl increased the release of radioactivity from the damaged core.
The graphite fire caused the core to carry on releasing the more volatile radioisotopes for a very long time.
In addition the fact that the core at Chernobyl contained graphite made it more dangerous to use water to put out the fire and cool the wreckage. By adding water to the mixture of graphite and nuclear fuel at Chernobyl a new nuclear reaction could have been provoked.
In the Japanese reactor as no graphite is present this danger of a new nuclear reaction is far lower, as precautionary measure boron has been added to the water which is being used to cool the Japanese reactors. Boron in the form of boric acid is a cheap, chemically safe substance which is well known to be able to absorb neutrons thus inhibiting (poisoning) a nuclear fission reaction.
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