Defense Secretary Philip Hammond, speaking in the House of Commons, said the decision to refuel the vessel was made after low levels of radioactivity were detected in a prototype core that has been running at the Naval Reactor Test Establishment at Dounreay in Scotland since 2002.
The prototype is to help assess how the reactor cores within submarines will perform over time to allow the Ministry of Defense to identify use-related issues that could develop over time.
"The safety of the UK's naval nuclear reactor at the test establishment at Dounreay and on our submarines is of critical importance to us, as is the maintenance of continuous at-sea deterrence," Hammond said. "That is why I have taken the decision to apply the precautionary principle, even though there is no evidence at this stage that the problem detected with the test reactor is likely to present in the operational reactors.
"The refueling will increase our confidence that Vanguard will be able to operate effectively and safely until the planned fleet of Successor submarines begins to be delivered from 2028.
"The refueling will be conducted within the currently planned dry dock maintenance period for Vanguard, which starts in late 2015 and will last for around 3 1/2 years, and is therefore expected to have no impact on deterrent operations," he said.
The low levels of radiation detected at the facility where the prototype core operates are well inside the prescribed limits set by the regulators and present no risk to workers and the local community, the Ministry of Defense said.