TOKYO -- Japan's nuclear watchdog on Wednesday said it is taking the leakage of highly radioactive water at the crippled Fukushima nuclear power plant seriously, and proposed raising the rating to describe it from "an anomaly" to a "serious incident."
The operator of the plant said about 300,000 litres of contaminated water has leaked from one of hundreds of steel tanks around the wrecked Fukushima Dai-ichi plant. Tokyo Electric Power Co. hasn't figured out how or where the water leaked, but suspects it did so through a seam on the tank.
The leak, the fifth since last year involving tanks of the same design, also raised concerns that this could be the beginning a new disaster -- contaminated water leaking from storage tanks one after another.
"That's what we fear the most. We must remain alert. We should assume that what has happened once could happen again, and prepare for more," watchdog chairman Shunichi Tanaka told a news conference. "We are in a situation where there is no time to waste."
The watchdog proposed at a weekly meeting Wednesday to raise the rating of the leak to level 3 from an earlier level 1 on an International Nuclear and Radiological event scale of eight. The watchdog, however, plans to consult with the U.N. nuclear regulatory agency over whether it is appropriate to use the evaluation scale on the already wrecked Fukushima plant.
The watchdog urged TEPCO to step up monitoring for leaks and take precautionary measures.
During the meeting, officials also revealed that plant workers apparently have overlooked several signs of leaks, suggesting that their twice-daily patrols were largely just a walk. They have not monitored water levels inside tanks, obviously missed a puddle forming at the bottom of the tank earlier, and kept open a valve on an anti-leakage barrier around the tanks.
TEPCO said the leaked water is believed to have mostly seeped into the ground after escaping from the barrier around the tank. It initially said the leak did not pose an immediate threat to the sea because of its distance -- about 500 metres (1,650 feet) -- from the coastline.
But TEPCO reversed that view late Wednesday and acknowledged a possible leak to the sea after detecting high radioactivity inside a gutter extending to the ocean.
The company also said the tank may have been leaking slowly for weeks through a possible flaw on its bottom. That could create extensive soil contamination and a blow to plans to release untainted underground water into the sea as part of efforts to reduce the amount of radioactive water.
Four other tanks of the same design have had similar leaks since last year. The incidents have shaken confidence in the reliability of hundreds of tanks that are crucial for storing water that has been funneled into the broken reactors to keep melted radioactive fuel cool.
The plant suffered multiple meltdowns following a massive earthquake and tsunami in March 2011 -- a level 7 "major accident," the highest on the INES rating and the worst since Chornobyl in 1986.
About 350 of some 1,000 steel tanks built across the plant complex containing nearly 300 million litres of partially treated contaminated water are less-durable ones with rubber seams.
TEPCO says the tanks that have leaked use rubber seams that were intended to last about five years. Ono said TEPCO plans to build additional tanks with welded seams that are more watertight, but will still have to rely on ones with rubber seams.
Figuring out what to do with the radioactive water is among the most pressing issues affecting the cleanup process, which is expected to take decades.
"The growing contaminated water has been one of our biggest concerns since the March 11 accident," said Zengo Aizawa, TEPCO's executive vice-president. "The contaminated water remains a problem that could lead to a crisis."
The leaked water's radiation level, measured about 50 centimetres above the puddle, was about 100 millisieverts per hour -- the maximum cumulative exposure allowed for plant workers over five years, TEPCO spokesman Masayuki Ono said.
Contaminated water that TEPCO has been unable to contain continues to enter the Pacific Ocean at a rate of hundreds of tons per day. Much of that is ground water that has mixed with untreated radioactive water at the plant.
Workers were pumping out the puddle and remaining water in the tank to transfer it to other containers. By Tuesday afternoon they had captured only about 4,000 litres, Ono said.
