By Jeremy Lovell

LONDON, Sept 21 (Reuters) - Britain has amassed a stockpile of more than 100 tonnes of plutonium -- enough for 17,000 bombs of the size that flattened Japan's Nagasaki in 1945, a report from the country's top science institution said on Friday.

The toxic stockpile, which has doubled in the last decade, comes mainly from reprocessing of spent uranium fuel from the country's nuclear power plants, so to stop it growing the practice must end, the Royal Society said.

"There should be no more separation of plutonium once current contracts have been fulfilled," said the report "Strategy options for the UK's separated plutonium".

Plutonium, one of the most radiotoxic materials known, is produced when spent uranium fuel from power stations is reprocessed to retrieve reusable uranium.

It can be processed into mixed oxide (MOX) fuel but it can also be used in nuclear weapons and so poses a security threat.

"Just over six kilogrammes of plutonium was used in the bomb that devastated Nagasaki," said Geoffrey Boulton, the report's lead author. "We must take measures to ensure that this very dangerous material does not fall into the wrong hands."

Paradoxically, the Royal Society said the safest option was to leave spent fuel as it was when it came out of the reactor because it was so radioactive that it was far harder to handle.

The second best was to produce and burn MOX pellets and then leave them unreprocessed.

"Spent fuel is more radioactive and therefore harder to handle than plutonium -- and more difficult to use in nuclear weapons because it would need to be reprocessed first," the report said.

PUBLIC CONSULTATION

The report comes as the government is in the middle of a public consultation process on whether new nuclear power stations should be built to replace the ageing existing stations which provide 20 percent of the country's electricity.

All but one of the stations will be closed within 15 years due to old age.

The government has provisionally said new stations are needed on the grounds of energy security and in the fight against climate change because nuclear power emits little of the carbon dioxide that is blamed for global warming.

Environmental campaigners have complained that the consultation is a sham with questions and information presentations heavily loaded in favour of new nuclear stations, and threatened new court action against the process.

Some academics too have expressed disquiet over the "form and function" of the process.

The government was forced to embark on a new consultation process by a court ruling in February that described the original public consultation as seriously flawed.

Many questions remain over the role and safety of nuclear power, although public opinion has moved grudgingly in favour particularly when cast in the light of climate change.

Not least of these is disposal of nuclear waste.

Last year CoRWM, the Committee on Radioactive Waste Management, recommended burying the waste unrecoverably.

But the government now has to find a site that meets the combined criteria of being accessible for disposal, very difficult for illicit retrieval, geologically stable and acceptable to the local community.

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