TOKYO (AFP) – Japan's TEPCO said it started on Monday pumping in decontaminated runoff water to cool reactors at its crippled Fukushima Daiichi plant, a key step towards stabilising the tsunami-hit nuclear site.

The effort to recycle water back into reactors at the power plant had been delayed due to problems with the treatment system used to decontaminate liquid that has accumulated during the three-month effort to contain the crisis.

The new system, which uses French and US technology, decontaminates the water, which is then pumped into cool reactor cores to prevent dangerous rises in temperature. The water was pumped into reactors 1, 2 and 3 on Monday.

"At 16:20 (0720 GMT) we started the system to circulate (cleaned) water" at three reactors at the Fukushima plant, a spokesman at Tokyo Electric Power Co. said.

The development would help the company meet its goal of bringing the reactors to safe shutdowns by January 2012 at the latest, he said.

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, causing an uproar from 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.

Prime Minister Naoto Kan's nuclear aide Goshi Hosono said it had "taken some time" to stabilise the processing of waste water. "That has led to delays in using the recycling system for cooling," he said.

"Factoring in delays and troubles, we wanted to start this circulation cooling by the end of this month. So I am relieved to have been able to come to this."