Japanese authorities struggled on Sunday to prevent a disaster at two reactors at the quake-stricken Fukushima Daiichi plant a day after an explosion and radiation leak in the wake of a massive earthquake.

Operator Tokyo Electric Power Co (TEPCO) began using sea water and boric acid to cool its No.1 reactor after an explosion blew off the roof and upper walls of its external concrete building during an attempt to release pressure. The containment structure surrounding it, however, remained intact. In addition, air was being released from the No.3 reactor's container vessel to lower pressure inside and avoid damage.

Assessments of the danger varied. The critical issue is what happens to the radioactive reactor fuel.

"We don't know enough about what the status of the fuel is in the reactor core," nuclear expert Mark Hibbs of the Carnegie Endowment for International Peace said. "The issue is whether the core is uncovered, whether the fuel is breaking up or being damaged, or whether the fuel is melting."

What Happened?
An explosion occurred at the 40-year-old Daichi 1 reactor as TEPCO tried to reduce pressure in the core after the total loss of power needed to keep water circulating to prevent it from overheating.

This led to fears of a disastrous meltdown at the plant, which shut down automatically after Friday's quake.

The government later said radiation levels were low because the explosion had not affected the reactor core container, although it had severely damaged the main building.

On Sunday, TEPCO began filling the container structure with seawater which will be mixed with boric acid to prevent any elevation in nuclear reaction, the government said.

"The most probable (cause of the blast) is that the coolant, particularly if it's water, can overheat and turn to steam more rapidly than it was designed to," said nuclear fuel technology professor Timothy Abram at Manchester University.

The cause and exact location of the blast still needs to be established, said nuclear physics professor Paddy Regan at Surrey University. "So far it looks like it's not the reactor core that's affected, which would be good news."

The World Nuclear Association, a London-based industry body, said the blast was probably due to hydrogen igniting and that this was unlikely to cause a big accident by itself.

"It is obviously a hydrogen explosion," communications director Ian Hore-Lacy said. "If the hydrogen has ignited, then it is gone, it doesn't pose any further threat."

Will The Seawater Work?
David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project, said the seawater is a sign of trouble but may be effective.

"The use of seawater means they have run out of options. If they had any other water they would have used it. It likely means the power for their pumps is gone. They must be pumping the seawater in." However he did say "So long as they keep the seawater over the fuel, the situation should be OK - so long as the fuel was not damaged.  

"The low levels of radiation reported could be a sign that the fuel is not damaged - damaged fuel would release much larger amounts of radiation than we have heard so far. As the lesson of Three Mile Island demonstrates, if cooling to the reactor core is restored, a meltdown can be stopped. Keeping the reactor fuel cool is key," he said.

How Serious Could It Be?
Views differ. Stratfor, a risk consultancy, initially said there appeared to be a reactor meltdown, but others disagreed, dismissing any comparisons with the 1986 Chernobyl nuclear disaster in Ukraine.

In an updated analysis, Stratfor said new developments "may suggest positive signs for authorities' efforts to contain the problem." But "many dangers and risks remain," it added.

Abram, the Manchester professor, said it was unlikely it would develop into anything more serious, though this would depend on the integrity of the fuel. He believed it "pretty unlikely" that the fuel had been significantly damaged.

"If the fuel is substantially intact, then there'll be a much, much lower release of radioactivity and the explosion that's happened might be just due to a buildup of steam in the reactor circuit," he said.

Apparently backing this view, the government said the plant's concrete building collapsed in the blast, but the reactor container inside did not explode.

The top government spokesman said TEPCO, the operator, planned to fill the leaking reactor with sea water to cool it down and reduce pressure.

Carnegie's Hibbs said: "If they are suggesting that the reactor vessel is intact and that they have a way to get cold water into the core of the reactor to cool that core down, that is very good news indeed."

It is too early to say that a "catastrophe has been averted," Stratfor said.

A nuclear technology expert who declined to be identified said the situation was still "very serious" as the cause of the explosion had yet to be determined. He blamed the accident on rising pressure inside the reactor.