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LONDON, March 6 (Reuters) - Britain said on Thursday it was planning an unscheduled re-fuelling of one of its nuclear submarines following an investigation into a problem discovered over two years ago at a prototype reactor based permanently in Scotland.
Defence Secretary Philip Hammond said the decision to re-fuel HMS Vanguard, the longest serving submarine in Britain's nuclear fleet, was being taken as a precautionary measure, and that the submarine's own reactor had not suffered any problems.
Hammond said that a reactor based at Douneray in Scotland, which is identical to those used on board Britain's nuclear fleet and used to mimic on-board conditions, had been shut down in 2012 after a problem was detected.
"In January 2012, low levels of radioactivity were detected in the cooling water surrounding the prototype core," he said in a statement to parliament, adding that Scottish authorities and nuclear regulators had been notified at the time.
"Clearly the fact that low levels of radioactivity have been detected in the coolant water means that the reactor is not operating exactly as planned," he said.
Britain is one of only five officially recognised nuclear-armed countries. Four submarines equipped to carry nuclear missiles - the Vanguard, Victorious, Vigilant and Vengeance - are based in deep-water lochs along the west coast of Scotland. At least one of the four is always at sea.
"This occurrence does not present any safety risk. It does, however potentially present additional risks to future submarine availability," Hammond said.
"This is the responsible option, replacing the core on a precautionary basis at the next arising opportunity rather than waiting to see if the core needs to be replaced at a later date."
The replacement of HMS Vanguard's nuclear core would be carried out in 2015 during other scheduled maintenance and would cost an estimated 120 million pounds ($200 million), he said. (Reporting by William James; editing by Stephen Addison)