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Q+A - What's going on at Japan's damaged nuclear power plant?
TOKYO (Reuters) - Japanese engineers are trying to gain control of the Fukushima Daiichi nuclear power plant, 240 km (150 miles) north of Tokyo, which was crippled by the huge March 11 earthquake and tsunami.
Radiation continues to seep into the sea and the air, although at far lower levels than at the peak of the crisis in mid-March.
Four of the six reactors at the plant, operated by Tokyo Electric Power Co (Tepco), are considered volatile.
Following are some questions and answers about efforts to end the world's worst nuclear crisis since Chernobyl:
WHAT IS HAPPENING?
Nuclear fuel rods at the plant's No. 1, No. 2, No.3 reactors melted in the immediate aftermath of the earthquake and tsunami and Tepco is trying to bring the reactors to a cold shutdown, where the water cooling them is below 100 degrees Celsius.
Efforts to cool the reactors by pouring water into them have brought down temperatures and the rods are no longer melting but the No. 1 reactor continues to leak radiated water and the No.2 and No.3 reactors are also believed to be leaking.
To achieve a cold shutdown, Tepco initially planned to use "water entombment", in which the containment vessels -- an outer shell of steel and concrete that houses the reactor vessel -- would be filled with water.
But this option is likely to be ruled out for the No. 1 reactor and possibly for the other two, after new data and inspections showed that the No. 1 reactor vessel had been punctured when the rods melted, allowing water being pumped as a coolant to pool in the basement of the reactor.
Tepco is readying a fallback plan that will involve decontaminating the water already accumulated and then pumping it back to cool the reactors.
Officials are also concerned about the slow pace of cooling at the No. 3 reactor and the No. 4 reactor was so badly damaged by a hydrogen explosion that workers will have to try to shore it up with steel beams and concrete to prevent a collapse.
In an effort to limit the spread of radioactive dust, the No. 1, No. 3 and No. 4 reactors will be covered with giant tent-like polyester covers supported by steel beams.
WHAT IS HAMPERING TEPCO?
Water is a huge headache for the operator. It has pumped in tens of thousands of tonnes of it to cool the reactors and much of it has ended up as contaminated runoff, accumulating as huge pools at the reactor buildings.
Preventing the massive pools of runoff from seeping out into the environment remains a challenge and Tepco is running out of space to store the radioactive water.
It is building tanks and towing in a massive barge to secure extra storage and is looking to build plants to treat some of the water. The operator caused in international outcry in April when it was forced to dump thousands of tonnes of contaminated water into the Pacific.
Weather conditions, such as the approaching rainy season and typhoons and lightning during the summer, could also pose problems.
HOW LONG WILL THE CRISIS LAST?
In April, Tepco announced a timetable for its operations. Within the first three months 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.
The company then hopes to bring the reactors to a cold shutdown in another three to six months. That would take the initial phase of work to stabilise the plant to January.
But with the damage to the reactors being worse than initially thought some experts said the process could take longer. Tepco said constant aftershocks, power outages, high levels of radiation and the threat of hydrogen explosions were factors that could hamper its work.
Even after the plant is under control, recovery work at the site is expected to continue for years.
For reference, officials have cited the work to clean up Three Mile Island after that U.S. reactor suffered a partial meltdown in 1979.
The Three Mile Island cleanup involved over 1,000 workers and took 13 years. It took nearly six years before the fuel from the reactor could be safely removed.
(Editing by Edwina Gibbs)
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