Japanese scientists battle to prevent third nuclear incident as

A 20km (12m) exclusion zone has been set up around the plant, and more than 180,000 people have been evacuated from the area.

Radioactive fuel rods at the plant's Unit 2 reactor have been exposed, leading to concerns over a possible meltdown.

The plant was rocked by a second explosion this morning, sending a massive cloud of smoke into the air and injuring 11 workers.

The blast, in reactor three, was felt 25 miles away, but the plant's operator said the radiation levels at the affected unit were still within legal limits.

Workers have been trying to cool reactor three with sea water after a system failure in the wake of the earthquake and tsunami.

It triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.

Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessel - and potentially lead to an explosion - but felt they had no choice if they wanted to avoid a complete meltdown.

In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast.

The inner containment shell surrounding the Unit 3 reactor was intact, Mr Edano said, allaying some fears of the risk to the environment and public. But the outer building around the reactor appeared to have been devastated, with only a skeletal frame remaining.

Tokyo Electric Power Company, which operates the plant, said radiation at Unit 3 was well under the level where a nuclear operator must file a report to the government.

A similar explosion occurred on Saturday at the plant's Unit 1, injuring four workers, causing mass evacuations and destroying much of the outer building.

Shortly after today's explosion, Tokyo Electric warned it had lost the ability to cool Unit 2. Hours later, the company admitted that the fuel rods had been fully exposed.

It was trying to channel sea water into the reactor to cover the rods, cool them down and prevent another explosion.

States of emergency have been declared at six Fukushima reactors, where the main cooling systems and backup generators have been knocked out.

Three are at Daiichi and three at the nearby Fukushima Daini complex.

Most attention, though, has been focused on Daiichi units 1 and 3, where operators have been funnelling in sea water in a last-ditch measure to cool the reactors. A complete meltdown - the melting of the radioactive core - could release radioactive contaminants into the environment and pose major, widespread health risks.

Mr Edano said no Fukushima reactor was near that point, and he was confident of escaping the worst scenarios.

International scientists say there are serious dangers but little risk of a Chernobyl-style catastrophe. Chernobyl had no containment shell around the reactor.

"The likelihood there will be a huge fire like at Chernobyl or a major environmental release like at Chernobyl, I think that's basically impossible," said James Stubbins, a nuclear energy professor at the University of Illinois.

And, some analysts noted, the length of time since the nuclear crisis began indicated that the chemical reactions inside the reactor were not moving quickly toward a complete meltdown.

BOSSES SAID FACILITY WAS 'EARTHQUAKE PROOF'

An expert in energy engineering who visited the Tokyo nuclear power station in 1994 told how plant chiefs had boasted that the facility was "earthquake proof".

Professor Tariq Muneer, of Edinburgh Napier University, said: "I was invited to the plant in 1995 with the Royal Academy of Engineering Engineers and was given a complete tour on which they told me how it was basically earthquake proof. Clearly that is not the case if what is being reported is right then that is not the case. Nuclear power stations are vulnerable to natural disasters and prone to accidents - they say they are error proof which is not true."

Meanwhile, Stephen McDonald, who is leading Save the Children's frontline response to the disaster, warned the psychological effects on orphaned and injured children would last for generations.

Mr McDonald, from Midlothian, said: "It doesn't fell a lot like other disasters because Japan is a developed country. However, when you see the full extent of the damage, it's clear that it will have the same effect on the children as any other disaster.

"They won't die of exposure or malnutrition, but the psychological effects will last for generations - thousands of children have lost everything."