A leak Monday forced Japan's TEPCO to halt the pumping in of decontaminated runoff water being used to cool reactors at its crippled Fukushima Daiichi nuclear plant, a company spokesman said.

The effort to decontaminate and recycle water back into reactors at the plant had already been delayed due to problems with the system used to cleanse liquid that has built up in the three-month effort to contain the crisis.

The system, which uses French and US technology, decontaminates the water, which is then pumped into reactor cores to prevent dangerous rises in temperature.

The water was pumped into reactors 1, 2 and 3 on Monday but the operation that TEPCO calls "circulating injection cooling" was halted after only 90 minutes, a spokesman said.

"We had to stop the operation," a TEPCO spokesman said. "We apologise."

Engineers will check the system, which connects reactors with water purification facilities, on Tuesday in the hope of restarting it as soon as possible, he said.

The success of the system is seen as a key step in helping the company meet its goal of bringing the reactors to safe shutdowns by January 2012 at the latest.

Up until Monday, water to cool reactors had to be brought in from outside the plant. The water is irradiated as soon as it is injected into the damaged reactors as it comes in contact with melted reactor cores, which are emitting high levels of radiation.

TEPCO needs to decontaminate more than 100,000 tonnes of highly radioactive water that has built up during reactor cooling operations and prevented workers from accessing areas of the plant to make repairs.

Highly radioactive water has spilled into the ocean since the plant was crippled in the March 11 disaster, stoking outrage within the local fishing industry as well as neighbouring countries including China and South Korea.

The 9.0 magnitude earthquake triggered a tsunami that smashed into the Fukushima plant and knocked out reactor cooling systems, triggering meltdowns, explosions and radiation leaks in the world's worst nuclear accident since Chernobyl in 1986.

Earlier Prime Minister Naoto Kan's new minister in charge of the Fukushima nuclear disaster Goshi Hosono said it had "taken some time" to stabilise the processing of waste water.

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