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 Company (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 restart cooling pumps, essential to quickly shutting down reactors damaged by the 9.0 magnitude earthquake and tsunami. But key basement areas remain inaccessible due to flooding by highly radioactive water.

Workers must keep injecting water into damaged reactors to prevent overheating of fuel rods and a nuclear meltdown, but that creates more contaminated water, which in turn hinders operations.

Transferring the radioactive water has been progressing a few centimeters at a time. 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, 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 ?

Nobody knows. The most likely scenario is a long, drawn-out fight, with incremental progress interrupted by aftershocks, a lack of an internal cooling system and spikes of radioactivity.

Once Tepco repairs the internal cooling system it would take only a couple days to bring the reactors to a cold shutdown.

Engineers are working in the dark, hampered by radioactive debris and flooded equipment. Lights have only recently gone on in the control rooms, but electrically-powered monitors and gauges -- the workers' eyes and ears inside the reactors -- are still off, as are the lights in the dangerous basements.

Tepco believes that three reactor cores went into a partial meltdown and that at least one of the containment structures, or the pipes leading out from them, has been damaged. Some of the leaks have been stopped, but radioactive byproducts are still escaping. That could mean that bringing all reactors to a cold shutdown could take months to minimize the exposure to workers.

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.