Residents line up to buy necessities at a supermarket in downtown Sendai, northeastern Japan, March 16, 2011. [Photo/Agencies]

SENDAI, Japan – The radioactivity in water in one unit of a hobbled nuclear power plant in northeastern Japan has tested 10 million times higher than normal, the plant's operator said Sunday.

Leaked water in Unit 2 of the Fukushima Daiichi plant measured 10 million times higher than usual radioactivity levels when the reactor is operating normally, said Tokyo Electric Power Co spokesman Takashi Kurita.

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Radioactivity in the air in Unit 2 measured at 1,000 millisieverts per hour - four times higher than the occupational limit of 250 millisieverts set by the government, he said.

The readings came as workers grappled with how to remove and store the highly radioactive water pooling in four troubled units at the plant.

The discovery of puddles with radiation levels 10,000 times the norm sparked a temporary evacuation of the plant on Thursday. Two workers who stepped into the water were hospitalized with possible burns.

The development set back feverish efforts to start up a crucial cooling system knocked out in a massive March 11 earthquake and tsunami, but has helped experts get closer to determining the source of the dangerous leak.

Chief Cabinet Secretary Yukio Edano, speaking Sunday on TV talk shows, said the radioactive water is "almost certainly" seeping from a reactor core.