Japan's nuclear crisis: the causes and the risks

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How did the explosion at the Fukushima No 1 power station in Japan happen? And what are the consequences?
Fukushima nuclear power plant
The aftermath of the explosion in the building housing reactor 1 at the Fukushima No 1 nuclear power plant in Okuma, Japan. Photograph: Digitalglobe/Getty Images

What caused the nuclear crisis?


Why did the building explode?

The makeshift attempt to cool reactor 1 at the power station failed. Heat from the fuel rods in the reactor core led to a build-up of superheated water inside.

On Saturday, engineers released water vapour – which contained radioactive caesium and iodine – from the pressure vessel as an emergency measure. Superheated water can split into hydrogen and oxygen, and it appears that hydrogen escaped during the venting procedure and exploded.

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.

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.

Have people been exposed to radiation?

At least nine people have tested positive for radiation exposure near Fukushima, but a Nisa official said that number could rise to between 70 and 160. Radioactivity can cause a variety of health problems, from a reddening of the skin and increased cancer risk to fatal radiation sickness. Health officials distributed potassium iodide pills, which protect against thyroid cancers, to residents near the power station. Those unable to leave were advised to limit their exposure by staying indoors and switching off air conditioning or wearing a protective mask if outside.

Are the other reactors safe?

On Sunday, engineers vented steam from reactor 3 and began pumping in sea water after its cooling system failed.Authorities said there was a risk of an explosion similar to that in reactor 1. Sea water was being readied to pump into reactor 2.

A separate state of emergency was announced at the nearby Onagawa nuclear power station amid increased levels of radiation, but Japanese officials said this had been carried on the wind from Fukushima.

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 situation as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale.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.