Tokyo Electric Power Co. (TEPCO), owner and operator of a stricken nuclear facility in northeast Japan said Thursday that 520 tons of radioactive water are 20,000 times above the annual permissible level for the plant leaked into the Pacific Ocean in early April.

TEPCO revealed for the first time Thursday that between April 1 and April 6 highly contaminated water containing around 5,000 terabecquerels of radioactive substances leaked from its No. 1 nuclear power plant in Fukushima prefecture into the sea.

But while the level of radiation is far lower than the 370,000 to 630,000 terabecquerels of radioactive substances estimated to have been released into the atmosphere from the plant in the days following the March 11 quake and tsunami, the pace and volume of the contaminated flow from the plant to the sea was considerable.

During the six-day leak, some 520 tons of highly radioactive water is believed to have flowed into the Pacific at a rate of about 4.3 tons per hour, but being that from late March radioactivity in the sea was still evident, the toxic spill could have been far larger, experts said.

TEPCO embarked Tuesday on a one-month effort to shift even more radioactive water from the No. 2 reactor turbine building to a toxic waste disposal unit at the site, with the move likely to lessen the risk of more contaminated water leaking into the ocean.

Hidehiko Nishiyama, spokesman of the government's Nuclear and Industrial Safety Agency said Thursday that the work is going ahead according to plan.

As the nuclear crisis rumbles on, Chief Cabinet Secretary Yukio Edano ordered the health ministry to check women's breast milk for traces of radioactive substances, following a citizen's group saying that four women living near Tokyo had produced breast milk containing small amounts of radioactive iodine-131.

Edano played down the issue saying that he gave the order to the health ministry to appease apprehensive mothers.

On a separate note the top government spokesperson said Thursday that TEPCO will bear the primary burden of paying compensation to those affected by the nuclear crisis and the utility will likely slash annual salaries of its employees, including executives, in a bid to ensure the payments can be made.