This prompted Japan's Nuclear Regulation Authority to upgrade the situation from 1 to 3 – a "serious accident" on the 8-point International Nuclear Event Scale.

The crisis reopened questions about how to deal with the flood of radioactive water accumulating at Fukushima. There is a radical option: to filter out as much radioactive material as possible, dilute what's left, and dump it in the Pacific.

This idea was put forward in April by a delegation from the International Atomic Energy Agency (IAEA) after a visit to the site. But the Japanese government and local fishermen are opposed, fearing the waste would destroy their fisheries. A look at the numbers suggests that such fears lack firm justification