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Groundwater full of radiation from reactors that melted down at the Fukushima Dai-ichi nuclear plant has been leaking into the Pacific Ocean, raising concern the toxic water has been flowing into the sea since the disaster at the facility more than two years ago.

Backtracking on previous comments, Tokyo Electric Power Co. (Tepco) confirmed the groundwater leaks Monday night, earning a rebuke from the government the following day to stop the leaks that Chief Cabinet Secretary Yoshihide Suga said were “serious.” Three of the nuclear facility’s six nuclear reactors melted down following an earthquake measuring 9.0 and an ensuing tsunami swamped the facility shutting down its cooling system March 11, 2011.

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Tepco suspected the breach after finding water levels in monitoring wells moving in sync with tidal flows, said spokeswoman Kaoru Suzuki. The operator doesn’t know when the leaks started or how much radiated water drained into the ocean, she said. Water samples suggest contamination remains contained in the port area near the Fukushima plant, Suzuki said.

The finding comes in the same month Japan’s Nuclear Regulation Authority told Tepco to speed up completion of a seawall to block contaminated water that it suspected was leaking into the ocean. While reporting elevated levels of cesium 137 and 134 in the plant’s groundwater, the utility had maintained that there had been no apparent effect on the adjacent seawater.

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The pace of decline in radiation levels slowed in the waters beside the Fukushima plant after June 2011, suggesting there was a leak, said marine science professor Kanda, whose findings are under review for publication in the journal Biogeosciences.

Tepco needs to pinpoint the source of the leak before it can come up with credible measures to stop it, he said.

Reports differ on how much radiation escaped into the atmosphere and the sea after the Fukushima disaster.

The Atmospheric Chemistry and Physics journal in October 2011 estimated the radiation released was about 42 percent of the 1986 nuclear accident at Chernobyl and that most of it fell into the North Pacific Ocean.

In the same month, the Institute for Radiological Protection and Nuclear Safety, funded by the French government, said the Fukushima plant was responsible for the biggest discharge of radioactive material into the ocean in history. Tepco in May last year estimated the total radiation release was about 17 percent of Chernobyl.

Human exposure to radiation at moderate to high levels can lead to cancers, such as leukemia, according to the United Nations Scientific Committee on the Effects of Atomic Radiation. The body, known as UNSCEAR, is in charge of the most comprehensive study of the Fukushima disaster and will deliver its report to the UN in October this year.

Tepco also said in an emailed statement Monday steam from an unknown source first spotted near the fifth floor of the Fukushima plant’s No. 3 reactor building on July 18 was visible again around 9 a.m. By 1:30 p.m., it was no longer visible, the company said.

The steam was probably the result of atmospheric moisture evaporating against the outer wall of the containment vessel, which is warmer than air temperatures, spokesman Yusuke Kunikage.

The utility found no significant changes to the unit’s containment vessel temperature and other readings, it said in the statement.

The handling of highly radioactive water is an issue plaguing Tepco as the utility oversees the plant’s cleanup. Leaks in April raised the prospect the utility might end up to dumping radioactive water in the Pacific.

Last month, Tepco said it had found unsafe levels of radioactivity in groundwater at the Fukushima station. The contaminants were in a monitoring well in a turbine complex at the Dai-ichi plant.

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