Japan: earthquake, tsunami..and meltdown?

Submitted by Matthew on 23 March, 2011 - 12:21

Get nuclear power’s risks in perspective

The terrible events recently in Japan have resulted in at least 15,000 deaths, of which those attributable to the overheating cores and hydrogen explosions at the Fukushima Daiichi nuclear power plant amount to… zero.

However, the situation at the power plant is potentially more serious if it is not controlled. What has been happening?

Some time ago, the Tokyo Electric Power Company (TEPCO) decided to build nuclear power plants in an earthquake zone. They judged that their design was robust enough to withstand a powerful earthquake. They judged that safety measures were adequate in the case of interruption of the electricity supply to the coolant pumps. They hadn’t considered the possibility of a large tsunami.

The plants are Boiling Water Reactors (BWRs) — sort of giant nuclear kettles. The core contains fuel rods of uranium-235 (235U) and plutonium-239 (239Pu) which undergo fission (atom-splitting) reactions, releasing neutrons, radiation, heat and fission products. The neutrons are fed back into the fuel rods in carefully controlled amounts to sustain a chain reaction, releasing heat which is continuously removed by superheated water under 70 times atmospheric pressure. This is allowed to boil, high pressure steam being used to drive electricity generators.

The radiation is absorbed by the core and cannot escape. It eventually contributes to the heat of the core.

The fission products are smaller atoms, usually radioactive. Most dangerous are caesium-137 (137Cs) and iodine-131 (131I). They are contained within the fuel rods, paradoxically making these more radioactive for a while than the original U or Pu.

So what are the safety features of the Japanese BWRs? If the electricity to the pumps cuts out, the chain reaction must be stopped to prevent the release of more heat. This is done by inserting boron control rods into the core. These absorb neutrons so that new fissions cannot occur. Then residual heat must be removed from the rods. The fact that the coolant water is at about 300 ºC shows that the core heat is considerable. If current is cut to the electric pumps, back-up diesel pumps come into operation. If these fail, batteries operate the pumps electrically. Before these run out, TEPCO assumes the main or diesel pumps will be working again.

What actually happened on 11 March and after was as follows. The buildings withstood one of the most powerful earthquakes in recorded history and the control rods were automatically inserted into the core. However, the electrically powered pumps were disabled when the earthquake felled power lines. Diesel pumps kicked in but were then swamped by an unexpectedly large tsunami. Then the shed-load of batteries took over for a few hours but, when they ran down, neither had the electricity had been restored nor the diesel pumps restarted. The core started to overheat.

This risked damage to the fuel rods, resulting in emission of caesium-137 and iodine-131. The risk of damage was increased as the heat of the core made it difficult to cool it with the seawater that the plant workers and emergency services were trying to dump on the reactors. The water was instantly boiling and being driven off as steam. The danger of the fuel rods melting and emitting even more radioactive substances was growing. It is not clear that this would lead to a more catastrophic breach of the steel containment: this would require temperatures exceeding 1500 ºC. But it would increase the danger to the workers of excessive radiation, and risk spreading radioactive caesium and iodine in the surroundings.

The problem of these substances is two-fold. Caesium compounds are very soluble and chemically similar to compounds of sodium and potassium. Caesium rapidly spreads through the environment and is absorbed by plants and animals which may be part of the human diet. Its half-life is about 30 years, meaning that it takes about 100 years to decay to 10% of its original level. However, except locally, it is unlikely to be particularly hazardous. Iodine is more problematic. It is absorbed easily and passed on to humans in food. The body then concentrates it in the thyroid gland, converting a low general dose of radiation to a much higher specific dose to one tissue. It has a half-life of eight days, making it more radioactive atom for atom than caesium-137 but dropping to less than 1% in two months. Preventative measures can easily be taken, minimising the risks.

It is not clear whether the reactors will be brought under control without substantial emission of radiation. It is clear that TEPCO should have sited the back-up pumps higher to avoid inundation by tsunamis. It is less clear but arguable that an earthquake zone was not a wise choice.

Nevertheless, the minimal injuries and absence of deaths compared with the effect of the earthquake and tsunami should help to put nuclear power’s risks in perspective. And we’re not talking about another Chernobyl.

Update on Chernobyl

According to the UNSCEAR report 20 years after the Chernobyl accident, 134 people got acute radiation syndrome. Of these, 28 died soon after the accident, and 19 subsequently, mostly from illnesses that are unconnected to their exposure.

More than 6,000 cases of thyroid cancer have occurred among people, predominantly children, exposed to radioactive iodine (131I). Not all but the vast majority of these are thought due to this exposure. This resulted from contamination of milk but was not an inevitable result of the Chernobyl accident. As the UNSCEAR report notes drily, “prompt countermeasures were lacking [which] resulted in large doses to the thyroids of members of the general public”.

Iodine is needed to synthesise the hormone thyroxine, which controls metabolism in adults and, crucially, growth in children. It is efficiently extracted from food and concentrated in the thyroid gland. Grazing cows would have eaten grass on which radioactive iodine had fallen and incorporated it into their milk which, of course, would have been drunk fresh largely by… children.

The countermeasures are simple: flood the system with ordinary iodine (127I, since you ask) by giving people tablets containing iodine salts. This was not done by the incompetent bureaucrats of the former Soviet Union and the result was that low whole body doses of 131I were converted into high doses in the thyroid.

The good (or, rather, less bad) news is that thyroid cancer responds well to treatment and only 15 of the 6000+ cases have died. There is also little evidence of more than a slight increase in other cancers. Thus the total of deaths proven to be caused by the worst accident in the history of nuclear power is not many more than 43.

* United Nations Scientific Committee on the Effects of Atomic Radiation, Vol II Annex D Health Effects due to radiation from the Chernobyl accident, 2008 (downloaded from the IAEA website)

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