Japan's nuclear crisis: the causes and the risks

A British nuclear expert has told The First Post that the explosion at the Fukushima I nuclear plant looks likely to be a "significant nuclear event" with a bigger impact on public health than the 1979 meltdown at Three Mile Island.

John Large, who has been a nuclear analyst since the mid-1960s and is a Fellow of the Royal Society, says the Japanese government may be significantly downplaying the seriousness of the accident – and he believes it could have far-reaching consequences for the nuclear industry worldwide.

What damage was caused?

The blast tore the roof off the building and damaged surrounding walls. Four workers were injured. Japanese authorities told the International Atomic Energy Authority that the explosion happened outside the reactor's primary containment vessel, which appears to be intact.

What is the radiation risk?

Japan's Nuclear and Industrial Safety Agency (Nisa) reported higher levels of radiation around the power station over the weekend and the presence of caesium-137 and iodine-131 in the air. These are radioactive isotopes produced in fission reactions. The isotopes were released when steam was vented from the reactor.

Radiation monitors around the site recorded a radiation level of 500 microSieverts per hour on Saturday afternoon, a quarter of the annual dose the general population is exposed to due to natural background radiation. The level of radiation at the power station's main gate fell on Sunday to a very low level of 3.2 microSieverts.

Fukushima I is just 40 miles from the epicentre of the closest of 248 separate seismic shocks which have taken place in the past week off the coast of Japan. A 'boiling water' plant, it has six reactors.

Dramatic footage of an explosion at the plant's No 1 reactor on Saturday has commanded global attention. Now the Japanese government says it is fighting to prevent a second meltdown at No 3 reactor.

Meanwhile, sister plant Fukushima II, which is just 11km further away from the earthquake epicentre, is also in trouble. Cooling systems have broken at three of its four reactors – and at least one reactor has failed.

Efforts are underway to evacuate 210,000 people living within 20km of Fukushima I. Earlier, on Friday, 2,800 people were taken from their homes by coach.

When will the power station be safe?

The strategy of pumping sea water into nuclear reactors is untested. It could take several days to bring the temperature and pressure of the reactor cores down to within safe limits.

If the cooling fails, the reactors could overheat and cause a total meltdown of the radioactive fuel rods in the core. This would only lead to a major release of radiation if the reactor's containment vessel was breached.

The Japanese authorities have classified the sitiuation as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale (INES). The scale runs from zero for a deviation in normal operations to seven for a major accident. The Three Mile Island incident in 1979 was a five and Chernobyl in 1986 was a seven on the scale.