That had allowed highly toxic water to trickle away into the ground. Latest reports from the plant suggest some of it may already have reached the nearby Pacific Ocean.

A puddle of the contaminated water was emitting 100 millisieverts an hour of radiation, Kyodo news agency said earlier this week.

Teams of workers at the plant have surrounded the leaking tank with sandbags and have been attempting to suck up large puddles of radioactive water.

But it is a difficult and dangerous job. The water is so radioactive that teams must be constantly rotated and it is clear that most of the toxic water has already disappeared into the ground.

Under the Ines, events have seven categories starting with Level 0 ("without safety significance") and Levels 1-3 denoting "incidents" while Levels 4-7 denote "accidents".

The triple meltdown at Fukushima two years ago was classed as a level 7 incident, one of only two nuclear events ever rated that highly - along with the 1986 Chernobyl disaster in the former Soviet Union.

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