The operator of the wrecked Fukushima nuclear plant said it had found another leak of contaminated water on Wednesday, piling pressure on the utility to curb the problem as it seeks permission to release water to the sea.
The Fukushima Daiichi Nuclear Power plant was hit by a 9.0 magnitude earthquake and tsunami more than two years ago, triggering nuclear reactor meltdowns and explosions.
Tokyo Electric Power, or Tepco, said a worker patrolling the area spotted the leak just after noon, with droplets of contaminated water leaking out between the tank's circular steel structure.
Shunichi Tanaka, chief of Japan's new nuclear regulator, set up after its predecessor was discredited in the 2011 disaster, told a news conference that Tepco should deal with the problem immediately. But he said the regulator did not regard the matter as serious.
The latest leak was acknowledged after Tepco said earlier this week it had detected radioactive cesium in groundwater flowing into the plant -- overturning an early finding that contamination was negligible.
A spate of similar incidents and power outages plagued the facility in March and April. The incidents represent another setback for the company as it tries to reassure the public and the government that it can manage the problem of tainted water.
The plant's still precarious state also serves as a reminder of the poor management of the worst nuclear accident since Chernobyl in 1986 as Prime Minister Shinzo Abe's government presses ahead with a drive to export Japan's nuclear technology.
The discovery that groundwater is being contaminated before it enters the damaged reactor buildings complicates efforts to persuade local authorities and fishermen that the groundwater is safe enough to be dumped into the ocean.
About 400 metric tons (441 tons) of groundwater flow daily into the reactor buildings only to be mixed with highly contaminated water that comes from cooling the melted fuel.
The government ordered Tepco last week to increase storage capacity of water tanks and construct a wall of frozen earth around the four reactor buildings to stem the flow of groundwater seeping into the plant.
Leaks were found in underground storage pits in April, prompting Tepco to speed up the construction of sturdier above-ground tanks. The leak discovered on Wednesday was from one of the sturdier tanks.
Tepco is also trying to construct a bypass that would route the water away from the plant and into the ocean. That plan requires the agreement of fishermen who oppose plans to dump 100 metric tons of groundwater a day from the plant into the sea.
(Reporting by Mari Saito, Risa Maeda and Kenichiro Hamada; Editing by Ron Popeski)