It was in a University of Chicago squash court that Enrico Fermi set off the first controlled nuclear chain reaction in 1942.

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In Japan in 1999, an uncontrolled nuclear chain reaction at a uranium-reprocessing plant killed two employees and spewed radioactive neutrons over the countryside.

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Ouchi and his colleagues had just set off the one thing that nuclear workers fear most - a spontaneous, uncontained nuclear chain reaction.

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This meant likely degradation of their zirconium alloy cladding, the possible release of hydrogen, and - by Tepco's admission - the risk that a nuclear chain reaction could begin.

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It says that the simulations focused on how shock waves from conventional explosives could compress the spherical fuel at the core of a nuclear device, which starts the chain reaction that ends in an explosion.

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