Three months after Japan's massive quake and tsunami sparked the world's worst nuclear accident since Chernobyl, emergency crews are still months away from bringing damaged reactors under control.

They hope to start crucial water decontamination in mid-June, a step that would then allow the launch of longer-term work to install new reactor cooling systems at the explosion-charred coastal facility northeast of Tokyo.

The embattled operator, Tokyo Electric Power Company (TEPCO), says it hopes to bring all reactors to stable "cold shutdown" some time between October and January, although decommissioning the crippled plant will take years.

Japan this week said that radiation released into the air in the first six days was 770,000 terabecquerels, more than twice its initial estimate, and that fuel inside three reactors may have melted through the pressure vessels.

The disaster hit on March 11 when a magnitude-9.0 seabed quake rocked the plant and sent a 14-metre (46-foot) ocean wave crashing into it, knocking out its power supply, reactor cooling systems and backup diesel generators.

The UN nuclear watchdog, the International Atomic Energy Agency (IAEA), has since then criticised Japan for underestimating the tsunami hazard, although it also praised the country's emergency response and evacuation operations.

In the days after the quake, Japan moved tens of thousands of people out of a 20-kilometre (12-mile) zone, as a series of hydrogen explosions ripped through the plant, blowing off roofs and sparking global fears.

Emergency crews, braving high radiation and dire working conditions, have poured hundreds of tonnes of water a day into reactors and fuel rod pools, using fire engines and giant concrete boom pumps, to cool the nuclear fuel inside.

The hosing operation, initially using corrosive seawater, has created more than 100,000 tonnes of radioactive runoff. Spills and emergency releases have dumped contaminated water in the Pacific, angering Japan's neighbours.

The high radiation inside reactor buildings has mostly kept workers at bay and delayed the start of repair work to the cooling systems, while US military robots have taken video footage and readings inside.

Japan this week started testing decontamination equipment supplied by French and US companies to start cleaning the toxic water so it can be pumped back into the reactors, and to prevent further toxic spills into the sea.

According to the latest IAEA update, freshwater is being continuously pumped into reactors one, two and three, where temperatures and pressure have remained stable, and as needed to top up spent fuel rod pools at units one to four.

TEPCO started work last month to install a support structure for the floor of the unit four spent fuel pool to protect it against aftershocks, and started preparatory work to install a roof cover over the reactor one building.

The utility has also sprayed an industrial resin over much of the site, using mostly remote controlled equipment, to trap radioactive particles.

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