fission

It is produced by nuclear fission for use in medical gauges and devices, but can also be a byproduct of nuclear reactors and weapons testing, the agency said.

But because no nuclear reaction is taking place at the enrichment sites - and a blast from a bomb would not trigger one - these dangerous radioactive "fission products" would not be present, he said.

The team in Bristol have come up with a whole new approach, using nuclear fusion rather than fission.

It will contain radioactive uranium split by nuclear fission, in a chain reaction which generates a lot of heat.

The reaction creates helium and not the long-lived radioactive waste of the fission process which is used in existing nuclear power stations.