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.
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 (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.