The four central reactors at the Fukushima Daiichi power station were the most badly damaged by the tsunami.

Although the reactors all shut down automatically after the quake, problems soon emerged with the cooling systems that keep the fuel rods cool.

A loss of power prevented nuclear workers from pumping water through the cooling system. The fuel rods overheated, causing a series of major problems.

Radiation has leaked from the plant, though it is not clear from exactly where. Reactors 2 and 3 are thought to be the most likely candidates however.

Efforts are continuing to cool the reactors to reduce the risk of any further leaks.

This close up shows the extent of the damage

Reactor 1: Explosion on 12 March, ripped off roof.

Reactor 2: Explosion on 15 March, some internal damage, most likely source of radiation spike that day.

Reactor 3: Explosion on 14 March, roof damaged. Fears damage may have led to radiation spike on 16 March - although later reports said it was unclear what caused the leap in levels.

Reactor 4: Under maintenance at time of quake. At least two fires reported since.

Reactors 5 and 6: Under maintenance at time of quake. Fears overheating may lead to explosions.