Japanese engineers are struggling to gain control of the Fukushima Daiichi nuclear power plant, 240 km (150 miles) north of Tokyo, which was seriously damaged by the March 11 earthquake and tsunami.

Two of the six reactors at the plant, operated by Tokyo Electric Power Co (TEPCO), are considered stable but the other four are volatile.

Following are some questions and answers about efforts to end the world's worst nuclear crisis since the 1986 Chernobyl accident:

What Is Happening?
Workers are trying to fill the reactors with enough water to bring the nuclear fuel rods inside to a "cold shutdown," in which the water cooling them is below 100 degrees Celsius and the reactors are considered stable.

TEPCO has been pouring water into the reactor vessels containing the rods since the disaster to cool them as an emergency measure.
In a further step towards a cold shutdown, TEPCO is filling the containment vessel - an outer shell of steel and concrete that houses the reactor vessel - with water in a procedure called water entombment. It has started by increasing the amount of water being poured into the No.1 reactor. 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 No.2, which is suspected of having a damaged containment vessel, TEPCO said it hopes to seal the damaged sections with cement to prevent the water being pumped in from leaking out.

What Is Hampering Operations?
The large amounts of runoff from the water TEPCO has been pumping in to prevent overheating of fuel rods and a nuclear meltdown. The operator estimates the amount of contaminated water at the Daiichi plant at around 70,000 tons.

It has been transferring radioactive water that has accumulated at the reactor buildings into tanks and storage areas at the plant, but the process has been progressing very slowly. Many storage tanks on site were damaged by the tsunami and authorities earlier in April made a decision to pump contaminated water with lower levels of radiation back into the ocean to secure storage space. That has since stopped but could resume if they run out of storage space again.

In the meantime, radiation continues to seep out of TEPCO's nuclear complex into the sea and into the air, although at far lower levels than at the peak of the crisis in mid-March. To contain contamination, workers have tried pouring liquid glass to stop a leak and spraying the ground with sticky resin to capture radiated dust. They are also injecting nitrogen into reactors to prevent new hydrogen explosions which would spread highly radioactive material into the air.

How Long Might This Take?

On April 17 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. TEPCO then hopes to bring the reactors to a cold shutdown in another three to six months. But some experts said the process could take longer. TEPCO itself said constant aftershocks, power outages, high levels of radiation and the threat of hydrogen explosions were 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.

What Are The Risks?
The main risk is radiation continuing to seep, or burst, out each time a pipe leaks or rising pressure forces workers to vent steam. Leaking water from within the nuclear pressure vessels could find its way into soil and the ocean, while spikes in radiation could contaminate crops over a wide area.

The risk that the spent fuel pools could go into a chain reaction is low, as long as temperature indicators are accurate. But some more of the contaminated runoff may have to be dumped into the sea, if workers run out of space to store the water. There is also a small risk of a corium steam explosion, particularly in the No.1 reactor, which is the plant's oldest and which is believed to have a weak spot.

If workers are unable to continue hosing operations, and if the nuclear fuel manages to melt through the bottom of the reactor and fall into a water pool below, this would result in a burst of high temperature and a sudden release of a huge amount of hydrogen explosion that could breach the containment vessel.

Should either worst-case scenario happen, high levels of radiation up to 20 km (12 miles) around the site could be dispersed, making it impossible to bring the reactors to a cold shutdown without great sacrifice.

Will The Site Become A No-Man's Land?
Most likely, yes. Even after a cold shutdown there are tons of nuclear waste sitting at the site of the nuclear reactors. Entombing the reactors in concrete would make them safe to work and live a few kilometers away from the site, but is not a long-term solution for the disposal of spent fuel, which will decay and emit radiation over several thousand years.

The spent nuclear fuel in Fukushima has been damaged by sea water, so recycling it is probably not an option, while transporting it elsewhere is unlikely because of the opposition that proposal would bring.

Experts say the clean-up will take decades.