One of the UK’s nuclear submarines will undergo an unscheduled re-fuelling after a potential problem was uncovered by a test reactor.
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.
The decision comes after low levels of radioactivity were detected in a prototype core, which is identical to those used on board Britain's nuclear fleet and used to mimic on-board conditions, that has been running at the Naval Reactor Test Establishment at Dounreay in Scotland since 2002.
"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."
Hammond said the reactor, designed to assess how reactor cores in submarines will perform over time, had been shut down in 2012 after a problem was detected.
The Ministry of Defence (MoD) said the core had been run for significantly longer periods and at a significantly higher intensity than those in submarines to allow the MoD to identify early any potential age- or use-related issues that may arise later in the lives of the operational reactor cores.
Vanguard is one of the UK’s four submarines equipped to carry nuclear missiles, the others being the Victorious, Vigilant and Vengeance. They are based in deep-water lochs along the west coast of Scotland and 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 will be carried out in 2015 during other scheduled maintenance and will cost an estimated £120m, he said.
The MoD said radiation exposure for workers and discharges from the site have remained well inside the strictly prescribed limits set by the regulators and the issue has been classed as ‘Level 0’ against the International Atomic Energy Agency’s measurement scale for nuclear-related events, a rating described as “below scale – no safety significance”.