Tokyo Correspondent Juliet Hindell: Most of the spilt liquid has now been removed

The Japan Atomic Power Company says an 8cm-long crack discovered in a pipe at the Tsuruga nuclear power plant caused radioactive water to flood the facility.

The reactor itself was shut down as soon as the danger was realised but officials at the giant plant, on the Sea of Japan coast in Fukui prefecture, said the coolant water continued to leak out.

Workers had to wait for 14 hours before the room was cool enough for them to be able to enter and investigate and stop the leak.

The pipe was in a device used to remove impurities from the primary cooling water and adjust its temperature.

Cooling water is radioactive because it has direct contact with the nuclear reactor.

Plant officials, joined by several government inspectors, used sound waves on the pipe to determine the cause of the crack.

Japanese Trade and Industry Minister Kaoru Yosano said they would get to the bottom of it.

"Whether this accident occurred because of an unexpected fault or because of the layout of the plant, we need to conduct a thorough investigation to find out the cause," he said.

Not isolated incident

The reactor involved in Monday's breakdown started operating in 1987.

It had leaked water once before in December 1996. No radioactive leak was reported in that accident either.

There have been a number of previous similar accidents at Japanese nuclear power stations.

Japan, a nation poor in natural resources, relies on nuclear power for about one-third of its electricity.

Correspondents say public trust in Japan's nuclear industry has fallen in recent years after a number of accidents and cover-ups by plant operators.

The worst incident was a fire at Tokaimura power plant in 1997, which exposed 37 workers to radiation.