Japanese nuclear situation101

Short and sharp

From The Guardian:
Japan’s nuclear crisis: the causes and the risks
Ian Sample, science correspondent
How did the explosion at the Fukushima No 1 power station in Japan happen? And what are the consequences?

What caused the nuclear crisis?

Problems began when Friday’s massive earthquake knocked out electricity at the Fukushima No 1 power station. Back-up generators kicked in to pump coolant around the reactor cores to prevent the fuel rods from overheating. The generators worked for a short time, but were damaged by the ensuing tsunami, forcing a scramble by engineers to fit mobile battery power units. These were insufficient to cool all of the reactors properly.

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.

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.

Are the other reactors safe?

On Sunday, engineers vented water vapour from reactor 3 and began pumping in sea water after its cooling system failed. Japanese authorities said there was a risk of an explosion similar to that in reactor 1.

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.