Although higher levels of radiation have been detected in the ocean waters near the plant, the breach discovered Saturday is the first identified direct leak of such high levels of radiation into the sea.
The leak, found at a maintenance pit near the plant's No. 2 reactor, is a fresh reminder of the dangerous consequences of the strategy to cool the reactors and spent fuel storage pools by pumping hundreds of tons of water a day into them. While much of that water has evaporated, a significant portion has also turned into runoff.
Three workers at the plant, operated by Tokyo Electric Power Company, have been injured by stepping into pools of contaminated water inside one reactor complex, while above-normal levels of radiation have been detected in seawater near the plant.
Workers are racing to drain the pools, though they have struggled to figure out how to store the irradiated water. On Saturday, contaminated water was transferred into a barge to free up space in other tanks on land. A second barge also arrived.
Some experts in the nuclear industry are now starting to question the so-called "feed-and-bleed" strategy of pumping the reactors with water, because so much contaminated water is injuring workers and escaping into the ocean.
While the company said it has not identified the original source of contaminated water, experts say it could be from excess runoff from the spent fuel pools or a broken pipe or valve connected to the reactor.
The leaks could also be evidence that the reactor pressure vessel, which holds the nuclear fuel rods, is unable to hold all of the water being poured into it, said Satoshi Sato, a consultant to the nuclear energy industry and a former engineer with General Electric.
"The more water they add, they more problems they are generating," Mr. Sato said. "It's just a matter of time before the leaks into the ocean grow."
Tetsuo Iguchi, a professor in the department of quantum engineering at Nagoya University, said that the leak discovered Saturday raised fears that the contaminated water may be seeping out through many more undiscovered sources. He said unless workers could quickly stop the leaking, Tokyo Electric could be forced to re-evaluate the feed-and-bleed strategy.
"It is crucial to keep cooling the fuel rods, but on the other hand, these leaks are dangerous," Mr. Iguchi said.
"They can't let the plant keep leaking high amounts of radiation for much longer," he said.
Plant workers discovered a crack about eight inches wide in the maintenance pit, which lies between the No. 2 Reactor and the sea and holds cables used to power seawater pumps, Japan's nuclear regulator said.
The air directly above the water leaking into the sea had a radiation reading of more than 1,000 millisieverts an hour, said Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency. Earlier Saturday, Mr. Nishiyama had said that above-normal levels of radioactive materials were detected about 25 miles south of the Fukushima plant, much further than had previously been reported.
The pit was filled with four to eight inches of contaminated water, said the operator of the plant, Tokyo Electric. Highly radioactive water has also been discovered in the reactor's turbine building in the past week.
Workers will try to patch up the crack with concrete, the company said.
Saturday's announcement of a leak came a day after the U.S. Energy Secretary Steven Chu said Reactor No. 2 at the Fukushima plant had suffered a 33 percent meltdown. He cautioned that the figures were "more of a calculation." Mr. Chu also said that roughly 70 percent of the core of Reactor No. 1 had suffered severe damage.
The crisis at the nuclear plant has overshadowed the recovery effort under way in Japan since the 9.0 magnitude quake and tsunami hit the northeastern coast on March 11. The country's National Police Agency said the official death toll from the disaster had surpassed 11,800, while more than 15,500 were listed as missing.
Earlier Saturday, Prime Minister Naoto Kan made his first visit to the region since last month's disaster, where he promised to do everything possible to help. His tour came a day after asking Japan to start focusing on the long hard task of rebuilding the tsunami-shattered prefectures.
"We'll be together with you to the very end," Mr. Kan said during a stop in Rikuzentakata, a town of about 20,000 people that was destroyed on March 11. "Everybody, try your best."
Dressed in a blue work jacket, Kan also visited with refugees stranded in an elementary school and then visited a sports complex about 20 miles south of the disabled nuclear plant. The training facility has been turned into a staging area for firefighters, Self-Defense Forces and workers from Tokyo Electric.
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