Two of the six reactors at the plant, operated by Tokyo Electric Power Co (TEPCO), are considered stable but the other four are volatile and once under control will be scrapped.
The chief executive of French nuclear reactor maker Areva has arrived in Japan, broadening out a French delegation that has flown out to help bring the nuclear plant under control.
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 have drenched fuel rods with seawater to cool the reactors. The plants have also been flooded by seawater when the tsunami hit.
The challenge is to pump out radioactive seawater flooding the basements in reactors No.1, No.2 and No.3. The water is preventing work to restore electricity to the cooling pumps so that they can operate continuously.
The dilemma is that while it is necessary to pump the water into the reactors to cool them, this increases the amount of radioactive waste water at the plant and there is a danger that the operator will run out of places to store it safely.
Radiation may now be flowing continuously from the plant in seawater. Contamination was now 4,000 times the legal limit in the sea near the plant's drains.
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. This most likely means that byproducts from a partial meltdown in the reactor core are leaking out into the water.
In the No.1 reactor, workers have been able to start running a circulatory steam condensing system to begin to clear contaminated water. But there is only slight progress.
The same systems in reactors No.2 and No.3 are flooded and so need to be emptied before they can handle the contaminated water. TEPCO has said it may need to think out of the box to clear the dangerous waters, while preventing further flows into the sea and soil.
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. 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 and water tanks. Work has also been interrupted by hosing operations to lower rising temperatures in the reactor cores and spent fuel pools, as well as by an occasional fire and radiation injuries.
Because of the high levels of radiation in the water, experts suspect damage to the containment structures around the No.2 reactor core. They 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 escape each time a pipe leaks or rising pressure forces workers to vent steam containing radiation. Leaking water from within the nuclear pressure vessels could find their way into 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.
The same could be said of a small, hypothetical 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 high temperature burst and a sudden release of a huge amount of hydrogen that could, in an unlikely "perfect storm" scenario, breach the containment vessel.
Should either worst-case scenarios 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.
WHAT RISK FROM PLUTONIUM?
Plutonium has been found in soil samples at the site, further evidence that fuel rods in at least one reactor may have melted down considerably before they were cooled, and that there is damage to the structures containing the nuclear core.
Only trace amounts of the toxic substance have been detected. The level of up to 0.54 becquerals per kg of soil is not considered harmful. Most people have some plutonium in their bodies from atmospheric and underwater nuclear tests and some pacemakers are powered by plutonium.
But the presence of the radioactive poison outside the reactors compounds worry for the workers there as long as authorities are not sure how the heaviest of primordial elements leaked out.
Plutonium-239, used most in reactors, has a half-life of 24,200 years. It is not readily absorbed by the body but what is absorbed, stays put, irradiates surrounding tissue and is carcinogenic.
WILL THE SITE BECOME A NO-MAN'S LAND?
Most likely, yes.
The plant operator TEPCO officially said the plant will be decommissioned -- something that had been assumed ever since engineers began pumping corrosive seawater into them. But even after a cold shutdown, scrapping the plant would take decades.
There are 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 Michael Perry)