Emergency crews at Japan's Fukushima nuclear plant Friday started decontaminating more than 100,000 tonnes of highly radioactive water from three months of reactor cooling operations.

Embattled operator Tokyo Electric Power Co. (TEPCO) said the system, which employs US and French technology, went fully operational at 8:00 pm (1100 GMT) after a water leak during a test run the previous night had been fixed.

The operation kicked off months after a 9.0 magnitude quake triggered a tsunami that smashed into the plant. The wave knocked out reactor cooling systems, which sparked meltdowns, explosions and radiation leaks.

The world's worst nuclear accident since Chernobyl in 1986 has caused radioactive material to spew into the air, ground and sea and forced the evacuation of 80,000 people in a 20-kilometre (12-mile) radius.

Japan's government has said it will soon evacuate more households near radiation "hot-spots" outside that zone, especially pregnant women and children, without specifying when and from where people would be asked to move.

Inside the charred plant, workers have pumped water into reactor cores and fuel rod pools, leaving more than 100,000 tonnes of contaminated water in basements, drains and ditches, some of which has leaked into the ocean.

With little space left for more water, and the summer rainy season threatening to add to the volume, TEPCO is racing against time to decontaminate the water and prevent new leaks or emergency dumps into the Pacific.

The system is designed to remove caesium and other radioactive materials as well as oil and sea salt from about 1,200 tonnes of water a day, using equipment from France's Areva group and Kurion Inc of the United States.

The water must be decontaminated before it can be stored or recycled back through the reactors. Pumping out the highly radioactive water would then allow workers to start longer-term repair work to the cooling systems.

The treatment process creates another challenge -- what to do with the radioactive sludge left over -- said TEPCO official Junichi Matsumoto.

"We expect 2,000 cubic metres of it," he said, nearly enough to fill an Olympic-size swimming pool.

"We have a temporary storage space. But as a medium to long-term issue, we are studying how we can legally store and manage it. We are yet to decide how exactly we will process the sludge."

The water spill on Thursday night was blamed on human error, not a technical defect, another TEPCO official told reporters earlier.

Six tonnes of contaminated water had leaked through pressure release valves into another part of the decontamination system but had later been recovered.

TEPCO said a month ago that it expected to significantly reduce radiation leaks by July, and to bring its reactors to stable "cold shutdown" by January at the latest, in line with initial plans announced on April 17.

The utility and government said at a follow-up briefing Friday that there were no major changes to that roadmap and its target dates.

In Japan's tsunami disaster zone, meanwhile, a three-day search involving some 4,400 military, police and coastguard personnel continued Friday for people still missing from the March 11 disaster.

Police officers in radiation suits and masks were searching for bodies in parts of Minamisoma town on the edge of the evacuation area.

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