TOKYO — Japan’s nuclear watchdog on Wednesday said a leakage of highly radioactive water at the crippled Fukushima nuclear power plant could be the beginning of a new disaster — a series of leaks of contaminated water from storage tanks.
The plant operator has built hundreds of steel tanks to store massive amounts of radioactive water coming from three melted reactors, as well as underground water running into reactor and turbine basements.
Tokyo Electric Power Co. said on Tuesday that about 80,000 gallons of contaminated water leaked from one of the tanks. It said it has not determined how or where the water leaked, but suspects it did so through a seam.
The leak is the fifth, and the worst, since last year involving tanks of the same design at the wrecked Fukushima Daiichi plant, raising concerns that contaminated water could begin leaking from storage tanks one after another.
‘‘We should assume that what has happened once could happen again, and prepare for more,’’ Nuclear Regulation Authority chairman Shunichi Tanaka told a news conference. ‘‘We are in a situation where there is no time to waste.’’
The watchdog also proposed at a weekly meeting Wednesday to raise the rating of the seriousness of the leak to level 3, a ‘‘serious incident,’’ from level 1, ‘‘an anomaly,’’ on an International Nuclear and Radiological event scale of eight.
The watchdog urged Tokyo Electric to step up monitoring for leaks and take precautions.
At 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 antileakage barrier around the tanks.
Tokyo Electric said the leaked water is thought 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 1,650 feet — from the coastline.
But Tokyo Electric 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 in 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.
The leaks have shaken confidence in the reliability of hundreds of tanks that are crucial for storing water that has been pumped into the broken reactors to keep melted radioactive fuel cool.
The plant had multiple meltdowns after a quake and tsunami in March 2011 — a level 7 ‘‘major accident’’ and the worst since Chernobyl in 1986.