TOKYO (Reuters) - 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 a 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 struggling to restart the cooling pumps in four reactors damaged by the 9.0 magnitude earthquake and tsunami and later drenched from desperate hosing operations to keep the reactors cool.
The immediate challenge is to pump out radioactive water flooding the basements in reactors No.1, No.2 and No.3 and hampering the restoration of electricity that would enable workers to cool the reactors and the fuel pools.
The No.2 reactor has posed especially nasty risks, emitting high levels of radiation at more than 1,000 millisieverts an hour in both the water and air in the basement of the turbine building. That is the highest reading seen in the crisis and compares with a national safety standard of 250 millisieverts over a year.
In the No.1 reactor, workers have been able to start running a circulatory system, used to capture released steam, to begin to clear contaminated water. But after four days of pumping, they have yet to clear the basement. The same systems in reactors No.2 and No.3 are flooded, so they can not handle the contaminated water, and TEPCO has said it needs to start thinking out of the box to find alternatives.
HOW LONG MIGHT THIS TAKE?
Nobody knows. The most likely scenario is a long, drawn-out fight, with incremental progress interrupted by emergency cooling measures and spikes in radioactivity.
Once the pumps and the residual heat removal systems are running, it would take only a couple days to bring the reactors to a cold shutdown. But engineers are literally working in the dark in most cases. Lights have only recently gone on in the control room, but electrically powered monitors and gauges -- workers' eyes and ears inside the reactor -- are still off. Radiation readings outside the reactors are still taken via a moving car, because the monitoring posts are not powered. Temperature and pressure readings from backup systems are all that workers have to "see" what is going on in the reactors.
Workers remain hampered by broken pipes, debris, flooded equipment and a scarcity of replacement pumps. Work has also been interrupted by hosing operations to lower rising temperatures in the reactor cores and spent fuel pools and spikes in radioactivity, as well as an occasional fire and radiation injuries.
Some experts suspect damage to the containment structure around the No.2 reactor, and said it may take as long as a few months to bring that reactor to a cold shutdown.
WHAT ARE THE RISKS?
The main risk comes from the radiation that will continue 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 enter the soil and the ocean, while spikes in radiation could contaminate crops over a wide area.
The risk that the spent fuel pools could reach recriticality seems remote, as long as there are workers and firefighters willing to douse the reactors with water each time temperatures start to rise.
But some experts say there is a small, theoretical 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, should nuclear fuel melt through the bottom of the reactor and fall into a water pool below. That would result in a high temperature and a sudden release of a huge amount of hydrogen in the containment vessel.
Should that happen, it could disperse high levels of radiation up to 20 km (12 miles) around the site, 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 is the issue of tonnes of nuclear waste sitting at the site of the nuclear reactors. Enclosing the reactors by injecting lead and encasing them in concrete would make it 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 fission fragments 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 given the opposition that proposal would bring.
(Editing by Robert Birsel)