The Tokaimura nuclear plant was designed by the French company SGN (Societe Generale des Techniques Nouvelles) and has been operating since the early 1980s, according to Mr Mycle Schneider of the World Information Service on Energy in Paris.
He said the highly enriched (15 to 20 per cent) uranium involved in the runaway nuclear reaction is rare.
Fifty-nine "criticality" accidents - the point at which a nuclear chain reaction becomes self-sustaining - have occurred in nuclear installations around the world since 1945, said the French Institute for Nuclear Protection and Safety. Most happened in the 1960s.
The US holds the record for 33 uncontrolled reactions, with 19 in the former Soviet Union. Two-thirds of the accidents happened in research centres, the rest in nuclear fuel plants.
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