TOKYO — The Japanese government Friday published a report on the discharge of more than 10,000 metric tons of low-level radioactive water from the quake-hit Fukushima Daiichi nuclear complex, in a bid to allay concerns among neighboring countries that it was spreading contamination into the ocean.
Plant operator Tokyo Electric Power Co. released a total of 10,393 tons of radioactive water April 4 to 10, according to the report published Friday evening local time by Japan’s Nuclear and Industrial Safety Agency, made up of 1,323 tons of groundwater and 9,070 tons of seawater.
The agency said the analysis showed that the water was only lightly contaminated. Most of the radiation was believed to have come from a series of hydrogen explosions that hit the plant in the first week, which was then brought down in subsequent rainfall, it said.
The government estimated the total amount of radiation contained in the released water at 150 billion becquerels — exceeding the legal limits by about 100 times — depending on the sample taken, according to Hiroki Ishigaki, an official with the agency. The data showed that iodine-131, which has a short half-life of eight days, was the most commonly found isotope, with the longer-lived cesium-134 and cesium-137 at lower but still elevated levels.
The report follows criticism, especially from South Korea and China, that Japan hadn’t provided advance warning of its plans and that the release could pose environmental threats.
“We will make sure that detailed explanations will be provided to neighboring countries that have shown interest in the operation,” said Hidehiko Nishiyama, director-general and spokesman of the NISA.
The report was also distributed to the three closest prefectures to the plant as well as to local fishing cooperatives, which had been sharply critical of the short notice given before the discharge began on April 4.
The decision to release the lightly irradiated water was taken in order to create room for the storage of highly toxic water that resulted from a spraying operation to cool down overheated reactors.
The amount of water used to cool down the reactor totaled 27,000 tons, most of which either evaporated into the atmosphere or leaked through damaged parts of reactors and flooded the nuclear complex, making it all but impossible for workers to do repairs to damaged reactor facilities.
Some of the water also proved highly toxic, sometimes 100 million times more radioactive than the allowable limit. Preventing the leakage of the most toxic water became a top priority for Tepco and the government.
While China and South Korea expressed strong concerns about the operation, Russia has taken a more reserved view.
“We understand that Japan did not have any other choice but to release the radioactive water,” said Vladimir Uiba, head of Russia’s Federal Medical-Biological Agency, at a press conference in the Russian Embassy in Tokyo Friday, before Japan’s nuclear agency released its report. “We would have done the same had we been in the same situation.”
He compared the contamination of seawater by the Fukushima complex with an oil spill in the Gulf of Mexico by BP PLC last year, and said, “The BP oil spill has caused far more serious impact on the environment than the Fukushima accident,” he said.
Meanwhile, the Atomic Energy Society of Japan released a report on its assessment on the Fukushima Daiichi reactors, saying that the nuclear fuel has partially melted and that the molten part settled in granular form at the bottom of the pressure vessel, posing less of a threat that a renewed nuclear reaction could again begin. A reactor’s pressure vessel contains its fuel and is one of the first lines of defense against radioactive release.
Mr. Nishiyama, of Japan’s nuclear agency, neither supported nor rejected the analysis, saying “Many different views have been put forward on the conditions of the reactors, but we consider them as speculation, as nobody has actually looked inside.”