On Friday, workers dumped more sandbags of zeolite, a radiation absorbing material, into the sea by Unit 3.

FORBES: Conditions May Be Deteriorating at Japanese Nuclear Power Plant

"It is likely that the level of radiation increased sharply due to a fire at Unit 4, " Edano said.

WSJ: Japan's Nuclear Crisis Escalates

The future of the heavily damaged Unit 3 reactor, where the radiation leak occurred after a tube break last year, is not clear.

NPR: San Onofre Submits Draft Linked To Restart Bid

Two of the three workers inside the plant received more than the lethal dose of seven sieverts (a sievert is the unit that measures the intensity of radiation's impact on the body).

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By contrast, sieverts - another unit commonly used during the last month's reporting - measure the likely medical impact of the radiation to which an individual is exposed.

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