Q&A: Dangers posed by Japan's quake-hit atom plant

VIENNA (Reuters) - Radiation leaked from Japan's quake-stricken Fukushima Daiichi nuclear plant on Saturday after a blast blew its roof off. 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 plant operator Tokyo Electric Power Co (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.

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

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 build-up 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 named 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.

(Editing by Tim Pearce)