Japan's worst nuclear accident took place in a plant that produces fuel for the nuclear power industry. These plants take uranium ore and use a complex chemical process that changes the ore into fuel.
Uranium in itself is not particularly radioactive, otherwise it would be too dangerous to mine from the ground. The ore can be handled for short periods without gloves or other protection, but this ore would be of no use in a nuclear reactor because it isn't radioactive enough.
The fuel-processing companies build up the radioactivity by "enriching" the fuel, in effect concentrating the amount of a particular form of uranium - U235 - in the fuel. These reactors work by fission, a reaction in which the U235 is broken apart, in the process releasing energy. One U235 atom on its own wouldn't be noticeable, but if large amounts of U235 are put together, millions of atoms split, in turn causing millions more to split in a chain reaction.
An atom bomb is uncontrolled fission, when the chain reaction occurs instantaneously and all the energy is released in a single burst. Controlled fission occurs in a nuclear power plant, when the chain reaction is "moderated" so that it can proceed at a slow but steady rate.
The Chernobyl accident occurred in a nuclear reactor which contained tonnes of uranium fuel. Mishandling of the reactor and its moderation systems controlling the chain reaction caused a massive explosion, which resulted in a devastating release of highly radioactive materials from deep in the rector core. The explosion was followed by a huge fire which caused tonnes of radioactive material to be spewed into the sky to be carried away by the wind.
Something similar has happened at the plant in Tokaimura, but so far on a much smaller yet none the less appalling scale. Part of the fuel process involves dissolving uranium in nitric acid, but workers in the plant are understood to have put 16kg of uranium into a tank meant to handle only 2.4kg.
This set the scene for, in effect, a mini-Chernobyl. A large amount of uranium was contained in a small space and the acid is thought to have acted as a moderator to help the chain reaction to begin spontaneously. This in turn led to an explosion that blew a hole in the roof and allowed radioactive gases and dust to escape into the air over the plant.
The explosion was a nuclear reaction and would have released a tremendous amount of radioactivity, which in turn has caused at least 14 workers to be taken to hospital.
This explosion should have brought an end to the reaction, but this may not be the case, given the spectacular levels of radiation, up to 15,000 times normal, now being detected up to 2km from the plant. The plant's operators cannot re-enter the building for fear of fresh explosions.
Most of these so-called critical mass incidents blow themselves out, with an explosion and burst of radiation but no subsequent release of radioactive particles into the atmosphere. The Tokaimura explosion was strong enough to breach the roof.
Radioactivity is apparently continuing to escape into the environment, which suggests that some level of uranium fission is continuing inside the plant. The only other way for radioactivity to escape is via a fire - as happened at Chernobyl.
Ireland is not at risk from the Japanese accident, the worst so far experienced by that country's nuclear industry. It is too far away and the release monitored so far is nothing even remotely like the scale of release from Chernobyl.
email: dahlst@irish-times.ie
Further details on the nuclear crisis in Japan will be posted on The Irish Times website as they become available.
Weblinks: www.iaea.or.at (International Atomic Energy Agency); www.greenpeace.org/cnuk.html (directory of campaigns against toxic waste); www.uic.com.au (uranium information centre); and www.pu-investigation.org (Paris-based plutonium investigation site).