TOKYO (Reuters) - Japanese officials are readying a new approach to cooling reactors at the Fukushima nuclear plant after discovering an Olympic swimming pool-sized pond of radioactive water in the basement of a unit crippled by the March earthquake and tsunami.
The discovery has forced officials to abandon their original plan to bring under control the No. 1 reactor at the Fukushima Daiichi plant as they focus on how to deal with the rising pool that some experts see as a threat to groundwater and the Pacific coast.
Despite the setback, Japanese nuclear safety officials and the plant's operator, Tokyo Electric Power (TEPCO), plan to stick to a target of stabilising the plant and bringing its reactors to a state of "cold shutdown" by January.
At that point, the fuel at the core of the reactors would have dropped in temperature and no longer be capable of boiling the surrounding water.
"We want to preserve the timetable, but at the same time we're going to have to change our approach," Goshi Hosono, an adviser to Prime Minister Naoto Kan, told a television talk show on Sunday.
Some outside experts have questioned whether the initial timetable for Fukushima was too optimistic. The 9.0 magnitude earthquake and the tsunami that followed unleashed the worst nuclear accident since Chernobyl in Ukraine in 1986.
"We would be cautious about saying the danger is over until the decontamination and cleanup of the site are well under way with no more leakage," Serge Gas, a spokesman for the Paris-based Nuclear Energy Agency, said in an email.
Tepco is scheduled to provide an update on progress on Tuesday. In a pair of television appearances on Sunday, Hosono said the government would announce its own timetable then as well.
Tepco is preparing to pay compensation to thousands of residents, farmers, fisherman and businesses for the disaster under a plan directed and partly funded by the government.
The earthquake and ensuing 15-metre (46.5-foot) tsunami devastated Japan's northeastern coast, killing more than 15,000 people. A further 9,500 are still missing.
Details emerged about the state of the No. 1 reactor in the past week. Progress to bring the unit under control has been seen as a test case for how quickly work on three other damaged reactors can proceed.
Among the revelations: the fuel in the reactor melted down after the earthquake and dropped to the bottom of the pressure vessel at the reactor's core about 16 hours after the quake struck.
A robot on the first floor of the reactor building on Friday recorded radiation of 2,000 millisieverts per hour. At that level workers could stay in the vicinity for no longer than eight minutes before exceeding exposure limits.
In addition, the reactor's containment vessel has leaked large amounts of radioactive water into the reactor building.
On Saturday, a Tepco worker was able to peer into the basement of the No.1 reactor and saw it had filled to almost half its 11-metre (35-foot) height -- an estimated 3,000 tonnes, larger than the volume of an Olympic swimming pool.
Critics have said that pumping in large amounts of water -- more than 10,000 tonnes in No. 1 reactor alone - could pose grave environmental risks.
Hidehiko Nishiyama, a spokesman for the Nuclear and Industrial Safety Agency, said new steps were being readied to treat and store radioactive water at Fukushima.
As a first step, a massive barge set out for Fukushima on Sunday. The 136-metre long "Mega-Float" had been used as an artificial island for fishing in Shizuoka, south of Tokyo. It will be turned into a floating storage site for water with low levels of radioactivity starting in June.
Among the major risks ahead, experts say, is the prospect of another hydrogen explosion like those believed to have destroyed parts of the buildings housing reactors No. 3 and No. 4.
Officials also remain worried about structural damage to the No. 4 reactor and whether its storage pool for spent fuel rods has sufficient support. A strong aftershock could topple the structure and spill and scatter radioactive fuel on the ground, compounding the crisis, experts have said.
Nishiyama said that Japanese officials "don't believe it is in danger of immediate collapse," but want to shore up the No. 4 reactor with new steel girders and cement.
Additional reporting by Scott DiSavino in New York; Editing by Daniel Magnowski