Reactor explosion `could never happen again'

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Defence Correspondent

The Chernobyl nuclear disaster could not be repeated, either at Chernobyl itself or elsewhere in the former Soviet bloc, the Chairman of the World Association of Nuclear Operators (Wano) said yesterday in a statement marking the tenth anniversary of the world's worst nuclear accident. Technical improvements to the installations and the growth of a Western- style "safety culture" meant that the accident "as it happened in 1986, cannot happen again".

But the Chernobyl explosion has received less attention than other nuclear sites in the former Soviet Union because of the economic situation in Ukraine. Similar reactors - RBMKs - in Russia itself had received more modifications. Although the European Union had put about pounds 5m into improving nuclear installations in eastern Europe and the former Soviet Union, the same amount again was needed.

Remy Carle, the chairman of the voluntary association which unites 126 operators of nuclear power plants around the world, was addressing a meeting of the association in London yesterday, the last before the anniversary of the accident on 26 April. He said the cost of redeploying workers, closing the plant and replacing it, estimated at $1bn - had discouraged the idea.

"The main causes of the accident have disappeared - the deficiencies have been corrected," Mr Carle said.

He said the main design shortcomings of the RBMK reactor, conceived in the 1960s and built in the 1970s, were the design of the control rods and the "void coefficient". Before the accident, the control rods could cause a sharp increase in radioactivity when they were lowered into a reactor operating at low power. This can no longer happen.

The "void coefficient" is the change in reactivity - the intensity of the nuclear chain reaction - which accompanies a change in the density of the liquid circulating round the reactor core - the "primary coolant". When water turned to steam, reactivity would increase as the density of the coolant decreased. This could create a positive void coefficient, which could cause the reactor to become unstable. Steps taken meant this was now impossible. Attention had also been paid to fire-retardant measures.