Potentially lethal levels of radiation were detected in water in a reactor building at Japan's crippled Fukushima nuclear reactor, hampering efforts to head off a potentially catastrophic meltdown.

Plant operator Tokyo Electric Power (TEPCO) said the water radiation level in puddles thought to have leaked from the number two reactor was 1,000 millisieverts per hour, making it necessary to evacuate workers from the reactor turbine building.

TEPCO did retract on Sunday a previous announcement that the radiation in the water was 10 million times higher than normal, saying the mistake was due to confusion between readings of iodine and cobalt in the water. The operator did not retract the 1,000 millisieverts figure.

Olivier Isnard, an expert with the French Institute for Radiological Protection and Nuclear Safety, said the reading was "proof that the reactor core partially melted."

A single dose of 1,000 millisieverts can cause temporary radiation sickness, with nausea and vomiting.

But Japanese nuclear regulators said the radioactive iodine found in the water would disappear within a day, as the substance had a half-life of less than an hour.

Leak yet to be found

Workers are trying to pinpoint the precise source of the leak amid concerns that fuel rod vessels or their valves and pipes may be damaged

Last week several workers suffered burns after being exposed to radioactive water while trying to install cables and get reactor cooling systems up and running again.

TEPCO came under more criticism when the three men were hospitalized after stepping in the water with inadequate protective clothing. The company is also being accused of providing erratic and opaque information.

Long-running emergency

The director general of the International Atomic Energy Agency, Yukija Amano, has told the New York Times that the emergency could go on for weeks, if not months.

Around 500 workers at the plant are struggling to prevent a catastrophic meltdown due to the failure of the backup power system needed to cool the reactors.

The plant was damaged in the huge earthquake and tsunami of March 11, which killed at least 10,668 people in Japan and left 16,574 missing, according to police statistics. The Japanese government has estimated the material damage from the catastrophe at over $300 billion (213.2 billion euros), which would make it the world's most expensive natural disaster.

Author: Timothy Jones (dpa, AFP)
Editor: Martin Kuebler