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LONDON (Reuters) - The radiation leak in Japan immediately 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.
The Fukushima incident, brought on by the biggest earthquake ever recorded in Japan, took a turn for the worse on Saturday after a blast blew the roof off the facility.
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.
In Japan, the earthquake and possibly the following tsunami overwhelmed mains and back-up power to the coolant pumps.
At Three Mile Island in Pennsylvania human and technical error caused a confused response to a similar failure, leading to over-heating, a meltdown of the nuclear core and a write-off of the reactor. But, unlike at Chernobyl, there was no breach of a pressure vessel, the shell which insulates the heart of a reactor, nor any major radiation leak.
Comparing the problems at Fukushima with Three Mile Island, Robin Grimes, director of the Center of Nuclear Engineering at Imperial College London, said: "There are parallel situations in terms of some of the processes that have occurred, but not parallel because in one case it was due to the safety processes in place and in this case it was natural disaster."
In addition, he said, there was no evidence that the core at Fukushima had melted, as happened at Three Mile Island.
That would, if it happened, lead to a greater build-up of radiation within the pressure vessel. Nor was there any sign yet that inner protective shields were damaged, Grimes said.
"It could be that we've had a breach of a fuel pin, a core melt. We don't know yet," he said, referring to the uranium fuel rods which lie at the heart of nuclear power generation.
He said that an increase in radiation in the surrounding area of eight times the normal, natural level was an indication that damage to the reactor was so far limited:
"That you're only getting eight times background radiation outside the exploded structure suggests to me the containment vessel and the nuclear reactor pressure vessel both remain intact," he said.
The plants at Fukushima and Three Mile Island used water cooling technology in common with most nuclear reactors. Their use of water is slightly different, however.
The Japanese reactor applies heat from the nuclear reactions in the fuel rods directly to water, creating steam which drives an electricity-generating turbine. At Three Mile Island, water under pressure transfers heat to a separate system in a less direct process for producing steam.
At Chernobyl, in a safety check gone wrong, the operators deliberately prevented a shutdown of the reactor in a rapidly escalating fiasco. Runaway nuclear fission reactions created a build-up of pressure and a massive and deadly radiation leakage as explosions ripped through a too-feeble reactor shell.
Three Mile Island did not result in a serious leak but was still damaging at the time for the reputation of nuclear power.
The Fukushima reactor dates to the early 1970s, older than many still in use, and the industry has since developed "even safer" reactors better equipped to cool naturally, said Grimes.
Opponents of nuclear power have been swift to use the problems at Fukushima to reinforce arguments that the hazards of atomic energy outweigh the benefits. Supporters say that the ability of Japan's dozens of reactors to survive frequent earthquakes shows their worth, particularly to countries like Japan which lack their own reserves of oil, gas and coal.
Paddy Regan, Professor of Nuclear Physics at the University of Surrey near London, said: "We must remember that there are 55 reactors in Japan and this was a huge earthquake.
"As a test of the resilience and robustness of nuclear plants, it seems they have withstood the effects very well."
Additional reporting by Daniel Fineren and Ikuko Kurahone in London