Japanese authorities say that the Fukushima reactor is still safe and the radiation leak has been low – but the crisis is far from over as the exclusion zone widens around the nuclear plant.
The explosion at the Fukushima nuclear plant earlier triggered fears of a nuclear meltdown.
The radiation leak in Japan recalls memories of accidents at the Chernobyl and Three Mile Island power stations, and how it unfolds will be a critical test for international acceptance of nuclear energy.
There are direct comparisons with the 1979 disaster at Three Mile Island in the United States — in both cases a cooling fault led to a build up of pressure in the radioactive core and resulted in a relatively small radiation leak.
Both use water to control the temperature as uranium degrades in a nuclear chain reaction at the reactors’ core, creating steam which drives a turbine to generate electricity.
The stricken Japanese reactor north of Tokyo has little parallel, however, with the Soviet plant at Chernobyl, where fundamental design faults led to a deadly serious of explosions in 1986, causing hundreds of deaths among emergency workers and contamination across Ukraine and beyond throughout Europe.
Japan’s nuclear agency said the problem at Fukushima rated a 4 on a seven-point scale of gravity, less severe than Three Mile Island, which was a 5, and well short of Chernobyl, a full 7.
The 8.9-magnitude earthquake triggered a series of events at two Tokyo Electric Power Company plants (TEPCO) that led to conditions for the radioactive leak because there was no electrical power to circulate vital cooling water over the superheated uranium fuel rods.
The electric plants at Daiichi and Daini are some 40-miles from the epicentre of the earthquake.
Explosion at Reactor Number 1 - Science Correspondent Tom Clarke analyses what happened:
The Fukushima explosion occurred at the number one reactor, the oldest of six at the plant.
A square reactor building contains a concrete containment dome. Which in turn contains the reactor housed in a steel pressure vessel.
A water cooling system prevents the uranium fuel in the reactor from becoming too hot.
It's understood that when the cooling system failed some of the fuel became exposed - heat and pressure in the reactor became too much.
To control the pressure steam was released into the containment dome. As pressure built up there it was announced last night steam would be released to the outside world to prevent the containment dome from failing.
One explanation for the explosion is that reactive metal surrounding the fuel in the reactor split hydrogen from some of the superheated steam.
When that escaped into the dome. The hydrogen, mixed with air detonated.
Jon Snow blogs from Japan:
Have flown to Shanghai and on to Osaka and I'm now on bullet train to Tokyo.
We shall then proceed by car to Sendai which is the worst affected city (One million souls).
The death toll is still clarifying. Amazing arriving in the south of Japan to find sublime normality yet people obsessively reading newspapers, listening to the radio and watching television on their mobile phones.
And yet and awareness of what we have seen on our own mobiles of devastating scenes and devastating power.
Trying to work out whether one of us should go to the nuclear plant. We hope to transmit tonight from Sendai.
The contrast with Haiti where infrastructure was so crude and here where it is so sophisticated, is acute. The death toll will reflect it.
The nuclear industry has long maintained that nuclear reactors are immune from the effects of natural disasters but then came the announcement from the Tokyo Electric Company that it had lost the ability to control pressure at several reactors and that it was struggling with a valve that would allow the reactor pressures to be relieved.
A major evacuation was activated and an exclusion zone has been thrown up around the Fukushima plants with police roadblocks stopping people from entering the zone from up to 60-kilometers away. The plant is about 150-miles 240 km north of Tokyo.
“An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel,” risk information service Stratfor said in a report before the explosion. “This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor.”
But Sue Ion, former chief technology officer at British Nuclear Fuels said that a major radioactive disaster was not likely: “I wouldn’t expect there to be a radiation emergency ultimately, they may have something to fix but it’s a precaution more than anything else.”
Industry experts said the precautions taken at Fukushima showed that enhanced security at nuclear power plants should prevent any disaster. But green groups said the threatened leak showed that the risks were still too high.
Altogether, some 11 Japanese reactors shut down after the earthquake and tsunami.
Environmental groups said the threat of a radiation leak underlines the risks from atomic energy.
Greenpeace’s Sven Teske said: “We’ve opposed nuclear power for decades, and this is another proof that it can’t be safe.”
A leading US scientist group said the incident highlighted the grave risk of inadequate back-up power to cooling systems at U.S. facilities.
New interest from governments and investors in nuclear power follows the development of more advanced plants, and a new focus on security of energy supply and moves to reduce carbon emissions.
Nuclear plants generate low-carbon power in contrast to fossil fuels and can produce constantly unlike wind and some other clean energy sources.
The Vienna-based International Atomic Energy Agency (IAEA) estimated last month that about 10 countries have decided to introduce nuclear power and started preparatory infrastructure work, up from four in 2008.
In Pictures – Japan tsunami and earthquake photo gallery