Radioactive water may have reached the ocean from a water purification unit at Japan's crippled Fukushima No. 1 nuclear facility, the plant's operator said.
A radiology professor said there appeared to be little danger because of the leak.
Tokyo Electric Power Co. officials said an employee discovered the leak Sunday of more than 1,589 cubic feet of water, with some flowing into a drainage ditch leading to the Pacific Ocean, the Mainichi Daily News reported.
Tepco said it was investigating the cause of the leak.
The Fukushima power plant had core meltdowns in some reactors after a March 11 earthquake and tsunami damaged its cooling systems. The reactors since have released massive amounts of radioactive matter into the air, land and sea.
The leaked water has radioactive substances, such as strontium, at between 100,000 and 1 million becquerels per cubic centimeter, emitting surface radiation of about 110 millisieverts hourly, company officials said. The water's strontium content is much higher than allowable for discharge into the ocean.
Strontium can cause bone cancer if injested, The Japan Times reported, but Genichiro Wakabayashi, a radiology professor at Kinki University, downplayed the potential danger of the leak.
He said the amount of radiation that had escaped is "tiny compared to what has escaped" previously in the Fukushima crisis.
"We currently believe the leak will have no effect on the water circulation system or our judgment on whether we've achieved a cold shutdown of the reactors," Tepco official Junichi Matsumoto said.
In a related matter, a government cleaning project to decontaminate radioactive areas around the Fukushima plant was demonstrated to media outlets, Kyodo said.
Crews led by contractor Obayashi Corp. have been working for a week to clean up the area based on a map developed by the Japan Atomic Energy Agency.
Also Sunday, The Miyagi prefectural government began thyroid testing on 83 pre-school and elementary school-age children in Marumori, a town bordering the Fukushima prefecture, to examine any health impact from the nuclear crisis.