As Japan readies to mark the first anniversary of the quake-tsunami that triggered meltdowns at the Fukushima nuclear plant, here are key developments in the worst atomic accident since Chernobyl in 1986.

- March 11, 2011: A 9.0-magnitude earthquake, the world's fourth-largest since 1900, strikes off Japan's northeast coast, causing a massive tsunami that destroys entire towns and villages along the Pacific coast and kills about 19,000 people.

The power supply and reactor cooling systems at the coastal Fukushima Daiichi nuclear power plant, about 220 kilometres (135 miles) northeast of Tokyo are damaged, causing fuel inside to overheat and melt down.

The government orders the evacuation of residents living within three kilometres of the plant.

- March 12: Workers open a vent of a reactor, releasing pressure and radioactive fumes from inside.

A hydrogen explosion rips through a building casing reactor number one, but the reactor itself remains intact.

Work crews begin pumping sea water to cool the crippled reactors after fresh water coolant runs out.

The government orders the evacuation of residents within a 20-kilometre radius of the plant.

- March 14: Second explosion hits the plant, now at a building housing reactor number three. The reactor remains intact.

- March 15: Third explosion at the plant, this one at a building for reactor number four. The reactor remains intact.

- March 16: Emperor Akihito makes an emergency television address in a bid to reassure a worried public.

- March 25: Huge amount of highly radioactive waste water found inside four buildings with troubled reactors, hindering work to cool the overheating nuclear fuel.

- April 4: Operations start to dump 11,500 tonnes of radioactive water into the Pacific amid continued emergency cooling operations.

- April 12: Japan upgrades its assessment of the severity of the nuclear emergency to a maximum seven on an international scale -- equal with Chernobyl, although less radiation was released.

- May 5: Nuclear plant workers enter a reactor building for the first time since the explosion.

- May 11: Emperor Akihito visits evacuees from the radiation zone.

- May 23: A team of IAEA experts arrives to survey the plant and study Japan's plan to contain the accident.

- June 6: Japan more than doubles its initial estimate of radiation released from the plant in the week after the tsunami.

- July 9: Prime Minister Naoto Kan says the decommissioning of Fukushima will take decades, in the first government announcement of a long-term timeframe for the clean-up.

- August 30: Kan's government resigns. Finance minister Yoshihiko Noda becomes Japan's sixth prime minister in five years.

- September 19: Nuclear disaster minister Goshi Hosono addresses the IAEA general conference and promises to bring the reactors to cold shutdown by year-end.

- October 3: A government commission probing the accident estimates dismantling the Fukushima reactors will cost 1.15 trillion yen ($14.3 billion).

- November 12: Journalists visit Fukushima for the first time.

- November 17 and 29: Japan announces bans on sales of rice produced in the Fukushima region after samples show radioactive contamination well above legal limits.

- December 16: Japan says it has finally tamed the leaking reactors with the declaration they are in a state of cold shutdown.

- December 21: A roadmap produced by the government and plant operator Tokyo Electric Power (TEPCO) says that decommissioning reactors at Fukushima could take as long as 40 years.

- December 27: TEPCO asks for an extra $8.5 billion in aid from a government-backed fund to help it compensate families affected by the crisis.

- February 22, 2012: TEPCO says it is to cover 73,000 square metres (785,000 square feet) of the floor of the Pacific near the battered reactors with cement in a bid to halt the spread of radiation.

- February 24: Authorities say that some areas surrounding Fukushima will likely remain permanently off-limits.