The contaminated water will quickly dissipate into the sea and is not expected to cause any health hazard. Nevertheless, the disturbing discovery points at the unexpected problems that can crop up and continue to hamper technicians trying to control the crisis.
Word of the leak came as Prime Minister Naoto Kan toured the town of Rikuzentakata, his first trip to survey damage in one of the dozens of villages, towns and cities slammed by the March 11 tsunami that followed a magnitude 9.0 earthquake.
"The government has been too focused on the Fukushima power plant rather than the tsunami victims. Both deserve attention," said 35-year-old Megumi Shimanuki, who was visiting her family at a community center converted into a shelter in hard-hit Natori, about 100 miles (160 kilometers) from Rikuzentakata.
The double disaster is believed to have left nearly 25,000 dead - 11,800 confirmed. More than 165,000 are still living in shelters, and tens of thousands more still do not have electricity or running water.
Although the government had rushed to provide relief, its attention has been divided by the efforts to stabilize the Fukushima Dai-ichi nuclear plant, which suffered heavy damage and has dragged the country to its worst nuclear crisis since the atomic bombings of Hiroshima and Nagasaki during World War II.
The plant's reactors overheated to dangerous levels after electrical pumps - deprived of electricity - failed to circulate water to keep the reactors cool. A series of almost daily problems have led to substantial amount of radiation leaking in the atmosphere, ground and sea.
On Saturday, workers discovered an 8-inch (20-centimeter) long crack in a maintenance pit that was leaking highly radioactive water into the Pacific Ocean, said Japan Nuclear and Industrial Safety Agency spokesman Hidehiko Nishiyama.
He said the water contaminated with levels of radioactive iodine far above the legal limit found inside the pit could be one of the sources of recent spikes in radioactivity in sea water.
"There could be other similar cracks in the area, and we must find them as quickly as possible," he told reporters.
Soon after the discovery, the plant's operator, Tokyo Electric Power Co., started filling the pit with cement to seal the crack and prevent more contaminated water from seeping into the ocean.
Nuclear safety officials said the crack was likely caused by the quake and may be the source of radioactive iodine that started showing up in the ocean more than a week ago.
People living within 12 miles (20 kilometers) of the plant have been evacuated and the radioactive water will quickly dissipate in the sea, but it was unclear if the leak posed any new danger to workers. People have been uneasy about seafood from the area despite official reassurances that the risk of contamination is low.
The cracked pit houses cables for one of the six nuclear reactors, and the concentration of radioactive iodine was the same as in a puddle of contaminated water found outside the reactor earlier in the week. Because of that, officials believe the contaminated water is coming from the same place, though they are not sure where.
A nuclear plant worker who fell into the ocean Friday while trying to board a barge carrying water to help cool the plant did not show any immediate signs of being exposed to unsafe levels of radiation, nuclear safety officials said Saturday, but they were waiting for test results to be sure.
Radiation worries have compounded the misery for people trying to recover from the tsunami. Kan's visit Saturday to Rikuzentakata did little to alleviate their worries.
"The government fully supports you until the end," Kan told 250 people at an elementary school serving as an evacuation center. He earlier met with the mayor, whose 38-year-old wife was swept away.
He bowed his head for a moment of silence in front of the town hall, one of the few buildings still standing, though its windows are blown out and metal and debris sit tangled out front.
Kan also stopped at the sports complex being used as a base camp for nuclear plant workers, who have been hailed as heroes for laboring in dangerous conditions. He had visited the nuclear crisis zone once before, soon after the quake.
Workers have been reluctant to talk to the media about what they are experiencing, but one who spent several days at the plant described difficult conditions in an anonymous interview published Saturday in the national Mainichi newspaper.
When he was called in mid-March to help restore power at the plant, he said he did not tell his family because he did not want them to worry. But he did tell a friend to notify his parents if he did not return in two weeks.
"I feel very strongly that there is nobody but us to do this job, and we cannot go home until we finish the work," he said.
Early on, the company ran out of full radiation suits, forcing workers to create improvised versions of items such as nylon booties they were supposed to pull over their shoes.
"But we only put something like plastic garbage bags you can buy at a convenience store and sealed them with masking tape," he said.
He said the tsunami littered the area around the plant with dead fish and sharks, and the quake opened holes in the ground that tripped up some workers who could not see through large gas masks. They had to yell at one another to be heard through the masks.
"It's hard to move while wearing a gas mask," he said. "While working, the gas mask came off several times. Maybe I must have inhaled much radiation."
Radiation is also a concern for people living around the plant. In the city of Koriyama, Tadashi and Ritsuko Yanai and their 1-month-old baby have spent the past three weeks in a sports arena converted into a shelter. Baby Kaon, born a week before the quake, has grown accustomed to life there, including frequent radiation screenings, but his parents have not. Their home is fine, but they had to leave because it is six miles (10 kilometers) from the nuclear plant.
Asked if he had anything he would like to say to the prime minister, Tadashi, a 32-year-old father, paused to think and then replied: "We want to go home. That's all, we just want to go home."
In Natori, where about 1,700 people are living in shelters, others had stronger words for Kan. Toru Sato, 57, lost both his wife and his house in the tsunami and said he was bothered that Kan's visit to the quake zone was so brief - about a half day.
"He's just showing up for an appearance," Sato said. "He should spend time to talk to various people, and listen to what they need."
