Tokyo Electric Power Co. unveiled its plan for dealing with the crisis at the Fukushima Daiichi nuclear power plant, the worst nuclear disaster since Chernobyl.

TEPCO said the radiation levels should drop over the next three months. It will take about six months for the reactors to achieve cold shutdown in which the temperature of the water inside the reactor is less than 100 degrees Celsius (212 F).

Four of the six reactors were damaged when a tsunami following the magnitude 9.0 earthquake on March 11 knocked out the generators that power the plant's water pumps. At least one reactor suffered a partial meltdown and it seems that the spent fuel rods in another were also damaged when the cooling systems failed.

TEPCO workers made desperate attempts to cool the reactors, finally flooding one with seawater - which essentially means that it has to be decommissioned, as seawater corrodes delicate machinery in the plant.

The current plan for cooling the reactors will mean injecting nitrogen into the reactor pressure vessel. All four damaged reactors experienced hydrogen explosions when water, heated by nuclear fuel, turned to steam and reacted with the zirconium alloy cladding of the fuel rods. Hydrogen, when exposed to oxygen, combusts. Nitrogen is an inert gas, so TEPCO hopes that it will prevent further explosions.

The other part of the plan involves flooding the primary containment vessel - the structure that surrounds the reactor core. That, TEPCO says, should help cool the reactor down.

TEPCO also plans to cover the reactor buildings (essentially building new ones around them) as the walls and roofs were destroyed by hydrogen explosions.

Circulating the water used in cooling back into the reactor core is also in the cards, so that no more contaminated water needs to be dumped into the Pacific.

All this depends on reliable sources of electricity for the pumping systems. TEPCO admits that more power losses are still a risk.

TEPCO also has plans to reduce the scattering of radioactive material, treat the soil around the plant and install water treatment systems to remove some contamination from that used in the cooling process. One of the big risks is to the workers at the plant; much of this work might require them to be exposed to high levels of radiation.

The IAEA says that thus far no workers have been exposed to doses higher than the 250 millisievert limit for a nuclear emergency. Dose levels of 1,000 millisieverts are enough to cause health problems. Exposure is usually measured over time, and is cumulative, so even relatively small exposures can add up.

Meanwhile the company says it is trying to reduce radiation exposures outside the plant so that evacuees can return home. The Japanese government asked everyone within a 20 kilometer radius of the Fukushima plant to leave. Chief Cabinet Secretary Yukio Edano, during a press conference on Monday, said it will still be some time before the evacuees can return.

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