Last month the plant's owner, Tepco, finally admitted what many had suspected – that the plant was leaking. Now Japan's Nuclear Regulatory Authority is calling the situation an emergency, and says Tepco's plans to stop the leak are unlikely to work.

The problem is that groundwater is entering the damaged reactor buildings, picking up radioactive elements like caesium and seeping out to sea. Tepco has spent months pumping the water to the surface and storing it in tanks, and sinking wells to lower the water table.

They also have a plan to purify the water, but this is only a partial solution as some radioactive substances, like tritium, cannot be removed.