By Shinichi Saoshiro and Taiga Uranaka
TOKYO (Reuters) - Workers were withdrawn from a reactor building at Japan's earthquake-wrecked nuclear plant on Sunday after potentially lethal levels of radiation were detected in water there, a major setback for the effort to avert a catastrophic meltdown.
The operator of the facility said radiation in the water of the No. 2 reactor was measured at more than 1,000 millisieverts an hour, the highest reading so far in a crisis triggered by a massive earthquake and tsunami on March 11.
That compares with a national safety standard of 250 millisieverts over a year. The U.S. Environmental Protection Agency says a single dose of 1,000 millisieverts is enough to cause hemorrhaging.
"The situation is serious. They have to pump away this water on the floor, get rid of it to lower the radiation. And it's virtually impossible to work, you can only be there for a few minutes," said Robert Finck, radiation protection specialist at the Swedish Radiation Safety Authority.
"It's impossible to say how long it will take before they can gradually take control."
The Japanese government said the overall situation was unchanged at the plant, 240 km (150 miles) north of Tokyo.
"We did expect to run into unforeseen difficulties, and this accumulation of high radioactivity water is one such example," Chief Cabinet Secretary Yukio Edano told a news briefing.
Yukiya Amano, director-general of the International Atomic Energy Agency (IAEA), said the nuclear emergency could go on for weeks, if not months. "This is a very serious accident by all standards," he told the New York Times. "And it is not yet over."[nN2679678]
Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.
At Chernobyl in Ukraine a quarter of a century ago, the worst nuclear accident in the world, it took weeks to stabilize what remained of the reactor that exploded and months to clean up radioactive materials and cover the site with a concrete and steel sarcophagus.
Tokyo Electric Power Co engineers have been working around the clock to stabilize the Fukushima Daiichi plant since the earthquake and tsunami knocked out the back-up power system needed to cool the reactors.
The operation has been suspended several times due to explosions and spiking radiation levels inside the reactors.
Last Thursday, three workers were taken to hospital from reactor No. 3 after stepping in water with radiation levels 10,000 times higher than usually found in a reactor.
LEVELS 10 MILLION TIMES ABOVE NORMAL
The latest scare came as engineers were trying to pump radioactive water out of a turbine unit after it was found in buildings housing three of the reactors.
Officials said the water in No. 2 was found to contain 10 million times the amount of radioactive iodine than is normal in the reactor, but noted the substance had a half-life of less than an hour, meaning it would disappear within a day.
"It's a dramatic headline for sure, but if it is iodine, after three weeks it will be down to about one-tenth (of the radioactivity) and it will be dispersed in the seawater," said Paddy Regan, a nuclear physicist at the University of Surrey in England.
"Ten million times is a massive number but if you were right up close to the fuel rods it would be ten million times normal because normal is (almost) zero."
Radiation levels in the sea off the plant rose on Sunday to 1,850 times normal, from 1,250 on Saturday, Japan's Nuclear and Industrial Safety Agency said.
"Ocean currents will disperse radiation particles and so it will be very diluted by the time it gets consumed by fish and seaweed," said Hidehiko Nishiyama, a senior agency official.
TOKYO RADIATION LEVELS NORMAL
The elevated radiation detected on Sunday was confined to the reactor, and radioactivity in the air beyond the evacuation zone around the plant remained in normal ranges.
In downtown Tokyo, a Reuters reading on Sunday afternoon showed ambient radiation of 0.16 microsieverts per hour, below the global average of naturally occurring background radiation of 0.17-0.39 microsieverts per hour.
Several countries have banned produce and milk from Japan's nuclear crisis zone and are monitoring Japanese seafood over fears of radioactive contamination.
The accident has also triggered concern around the globe about the safety of nuclear power generation. U.N. Secretary-General Ban Ki-moon said it was time to reassess the international atomic safety regime.
The crisis also looks set to claim its first, and unlikely, political casualty. In far away Germany, Chancellor Angela Merkel's party faces a defeat in a key state on Sunday, largely because of her policy U-turns on nuclear power.
OVERSHADOWING RELIEF EFFORT
The drama at the plant has overshadowed a relief and recovery effort from the magnitude 9.0 quake and the huge tsunami it triggered that left more than 27,100 people dead or missing in northeast Japan.
In Otsu, 70 km (42 miles) south of the stricken nuclear facility, the townsfolk are faced with livelihoods derailed by the natural disaster and now the fear of radiation in the air.
Ninety-three-year-old Kou Murata sat cross-legged on the floor of a school classroom, her home for the past fortnight. Surrounded by piles of quilts and blankets, she fretted over what was to become of her in the twilight of her life.
"I am afraid because people are leaving, and we are alone," she said, looking small and frail in a jacket decorated with snowmen.
Murata's daughter, Hisae, said the government had not helped them.
"I want to go back home, but the situation is impossible," she said. "I applied to the government to get a temporary house, but we need a certificate to say the house was destroyed. Now all the temporary houses have been taken. We thought the government would come to us, but we need to go to them."
The first opinion poll to be taken since the disaster showed the approval rating for Prime Minister Naoto Kan had edged higher, to 28.3 percent, but more than half disapproved of how the nuclear crisis had been handled.
Prior to the earthquake, Kan's approval rating had sunk to around 20 percent, opposition parties were blocking budget bills to force a snap election that his party was at risk of losing, and critics inside his own camp were pressing him to quit.
The government estimated last week the material damage from the catastrophe could top $300 billion, making it the world's costliest natural disaster.
(Additional reporting by Kiyoshi Takenaka, Chizu Nomiyama, Elaine Lies and Shinichi Saoshiro in Tokyo, Phil Smith in Otsu, Jon Herskovitz in Kamaishi, Gerard Wynn in London and Alister Doyle in Oslo; Writing by John Chalmers; Editing by Dean Yates)