The deepening crisis at the Fukushima plant will be upgraded from a level 1 "anomaly" to a level three "serious incident" on an international scale for radiological releases, a spokesman for Japan's Nuclear Regulation Authority (NRA) said.
That will mark the first time Japan has issued a warning on the International Nuclear Event Scale (INES) since three reactor meltdowns after the massive quake in March 2011.
Water still leaking from the plant is so contaminated that a person standing close to it for an hour would receive five times the annual recommended limit for nuclear workers in a year.
A maximum level 7 was declared at the battered plant after explosions led to a loss of power and cooling two years ago, confirming Fukushima as the worst nuclear accident since Chernobyl a quarter of a century earlier.
Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Fukushima, the plant's operator, Tokyo Electric Power Co said on Tuesday. The leak was classified as an "anomaly" earlier this week.
The NRA's impending assessment upgrade came in a document posted on the agency's website on Wednesday, with formal adoption to follow a meeting that is being held by the authority's commissioners, the NRA spokesman said by telephone.
"Judging from the amount and the density of the radiation in the contaminated water that leaked ... a level 3 assessment is appropriate," the document said.
The leak, which has not been plugged, is so contaminated that a person standing 50 cm (1.6 feet) away would, within an hour, receive a radiation dose five times the average annual global limit for nuclear workers.
After 10 hours, a worker in that proximity to the leak would develop radiation sickness with symptoms including nausea and a drop in white blood cells.
Each one-step INES increase represents a tenfold increase in severity, according to a factsheet on the website of the International Atomic Energy Agency.
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