Japan's announcement that crippled reactors at its Fukushima nuclear plant have been brought to a state of cold shutdown marks the second step in the government's recovery plan and is the culmination of nine months of sometimes chaotic efforts to bring the reactors under control.

Authorities have long flagged the move as vital to clean-up efforts.

The disaster erupted on March 11, when waves triggered by a magnitude-9.0 earthquake swamped the reactors' cooling systems, sparking meltdowns, explosions and the release of radioactive material in the world's worst nuclear accident since Chernobyl in 1986.

An initial key challenge was how to inject water into the reactors to cool melting fuel inside and then how to deal with the massive amounts of contaminated liquid that accumulated as a result of those emergency efforts.

With no power supply to pump water in, military helicopters were used to dump water from above while firefighters sprayed water from high-pressure hoses.

Within weeks of the disaster, plant operator Tokyo Electric Power Co. (TEPCO) was forced to discharge more than 10,000 tonnes of low-level radioactive water into the Pacific from the plant, located some 220 kilometres (135 miles) northeast of Tokyo.

The release provoked outrage among fishermen as far afield as China, who fretted their catch would be contaminated.

TEPCO subsequently built massive water tanks at the plant to store the runoff and rented a "mega-float" ocean tanker from Mitsubishi Heavy Industries.

Another initial focus was on injecting nitrogen into the reactors in a bid to prevent a repeat of the hydrogen explosions that had torn reactor housing units apart and worsened the release of radiation into the atmosphere.

In mid-July, the government and TEPCO said they had met three-month goals, including installing systems that remove radioactive substances from the polluted water before recycling the decontaminated liquid to further cool reactors.

"A turning point was to have successfully established a circulation system for cooling water," said Kazuhiko Kudo, a nuclear reactor expert and professor at Kyushu University in Fukuoka.

"But the circulation system is not a proper one for nuclear reactors and is still a stop-gap measure," Kudo said.

"The system in Fukushima, which is placed outside the reactor and connected with long pipes, is quite unstable," he added. "It is necessary to further improve the system as quickly as possible."

With ongoing fears of high radiation levels, workers have since battled to maintain the cooling system so that reactor water temperatures remain below 100 degrees centigrade (212 Fahrenheit) -- a condition necessary for Friday's "cold shutdown" declaration.

One of the major challenges TEPCO will face in the longer term is the removal of spent fuel from containment vessels.

"Removing fuel is quite challenging as the status of the fuel rods has yet to be confirmed," Kudo said.

One option under consideration is the possible covering of reactors with outer shells, a measure technicians hope will prevent further radioactive release.

The construction of wave barriers to guard against the effects of another tsunami in geologically unstable Japan is also a key priority, as is the containment -- and eventual disposal -- of contaminated water.

"Cooling water itself is still seeping out through cracks into underground water, which is eventually leaking into the sea," said Tetsunari Iida, director of the Institute for Sustainable Energy Policies, an anti-nuclear body.

"We now need to worry about different isotopes, such as strontium, which are more water soluble and could contaminate the marine food chain," he said.