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TOKYO, April 18 (Reuters) - Tokyo Electric Power Co (TEPCO), the operator of the Fukushima Daiichi nuclear power plant at the centre of Japan's crisis, said on Sunday it hopes to achieve a "cold shutdown" of its crippled plant within six to nine months.
The following summarises what TEPCO aims for and some of the challenges facing the operator as Japanese engineers scramble to deal with the worst nuclear crisis since the 1986 Chernobyl meltdown.
A cold shutdown is a state in which the water cooling the fuel rods is below 100 degrees Celsius and the reactors are considered stable. It means that the water used to cool the fuel rods is below boiling point.
Of the Daiichi plant's six reactors hit by the earthquake and tsunami, two are seen as safe, but the other four remain volatile.
Workers succeeded in halting the reactors after the quake hit, but they were unable to cool them in time before some of the fuel melted inside the reactor cores after the water cooling them evaporated.
This is the first time TEPCO announced a timetable for its operations since the disaster that struck more than five weeks ago.
Within the first three months, the operator said, it plans to cool the reactors and the spent fuel stored in some of them to a stable level and reduce the leakage of radiation. [ID:nL3E7FH03J]
TEPCO then hopes to bring the reactors to a cold shutdown in another three to six months.
The operator said it wants to fill the reactors with enough water so the fuel rods can be cooled for a cold shutdown.
TEPCO has been pouring water into the reactor vessels, which contain the nuclear fuel rods, since the disaster to cool the rods as an emergency measure.
In a step forward from this emergency measure, TEPCO wants to achieve a cold shutdown by filling the containment vessel, an outer shell of steel and concrete that houses the reactor vessel, with water in a step known as water entombment.
At the same time it will work to restore the reactors' cooling system, which functions like a radiator on an automobile. TEPCO said mounting a separate cooling system externally was also a possibility.
For reactors like the No.2 reactor with suspected damage to the containment vessel, TEPCO said it hopes to seal damaged sections with a type of cement to prevent water being pumped in from leaking out.
The aim to fill the reactors with enough water sounds simple but engineers have had to battle with a damaged cooling system since the earthquake.
Until the cooling systems are fixed or other alternatives are online to continually cool the reactors, workers will be forced to keep injecting new water to cool the fuel and create a steady flow of contaminated water.
Within its self-imposed time frame TEPCO said it will build storage tanks to store the contaminated water and attempt to treat some of it. It also aims to cover the reactors with large covers to prevent the spread of radioactive material into the atmosphere.
TEPCO said constant aftershocks, power outages, high levels of radiation and the threat of hydrogen explosions were also factors that could hamper its work.
Weather conditions, such as the approaching rainy season and typhoons and lightning during the summer, could also pose problems.
Banri Kaieda, Japan's trade minister, told reporters on Sunday: "The first juncture towards safety would be when the fuel rods are fully submerged in water and a cold shutdown is achieved.
"But true safety will not come until the fuel rods are removed from the reactors," he said, adding that he would like to assess in six to nine months whether residents evacuated from near the Daiichi plant can return home.
Hidehiko Nishiyama, a deputy director-general at Japan's nuclear safety agency, said: "Unlike a normal shutdown when large quantities of sea water can be used to cool down a reactor, the options are limited, so it may take a significant amount of time until a cold shutdown is reached." (Reporting by Shinichi Saoshiro; Editing by Chris Gallagher)