Six months after Japan's massive March 11 earthquake and tsunami sparked the world's worst nuclear accident since Chernobyl, emergency crews are struggling to stop radiation seeping out.

Embattled operator Tokyo Electric Power Company (TEPCO) says it hopes to bring all crippled reactors to stable "cold shutdown" by January, although decommissioning the 1970s-era plant will take decades.

-- The plant comprises six boiling water reactors. At the time of the quake, reactors five and six were in cold shutdown awaiting maintenance. Reactor four was undergoing maintenance and there are no rods in the core.

-- 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.

-- Reactors one and three suffered meltdowns and hydrogen explosions that damaged buildings housing them and that of nearby reactor two, which also suffered a meltdown as its fuel melted through its pressure vessel.

Fires and explosions also damaged the building housing reactor four.

-- Operations to hose down reactors initially used corrosive seawater, creating more than 100,000 tonnes of radioactive runoff. Spills and emergency releases dumped contaminated water in the Pacific, angering Japan's neighbours.

-- Power supplies have been reestablished. A decontamination facility using French and US technology has been operating since mid-June to cleanse the radioactive run-off water, which is then pumped back into reactors one, two and three as coolant in a closed circuit.

-- Units one and three are showing temperatures below 100 degrees Celsius, crucial to stabilising the site. Nitrogen has been injected into the units to prevent further explosions.

-- High radiation levels in some parts of the facility beyond the capabilities of measuring instruments continue to hamper work.

-- TEPCO has sprayed an industrial resin over much of the site, using mostly remote controlled equipment, in an effort to trap radioactive particles.

-- Tens of thousands of people remain evacuated from homes, farms and businesses in a 20 kilometre (12 mile) radius around the plant and in pockets beyond. TEPCO and the government face a massive compensation bill.

-- Many activists and scientists have said the evacuation zone is not wide enough and does not account for unpredictable spread of fallout. The government has warned some areas near the plant may be uninhabitable for years.

-- The government has imposed a ban on a range of vegetables and dairy produce from parts of Fukushima prefecture and several neighbouring regions and banned fishing in the vicinity of the plant.

-- Cases of contaminated water, beef, vegetables, tea and seafood have been detected, including green tea from Shizuoka, 360 kilometres from the plant.

-- Japan has said that 770,000 terabecquerels of radiation were released in the first six days of the disaster, more than twice the initial estimate.

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