HMS Vengeance nuclear sub returns home after power loss
A nuclear-powered submarine is returning to port after a mechanical problem caused loss of power while it was on a training exercise.
HMS Vengeance is returning to Faslane under its own power on the sea surface despite a reduction in propulsion.
The incident happened on Thursday night while the vessel, which is one of four nuclear-powered Vanguard class submarines, was in the North Atlantic.
An MoD spokesman said the incident was "not nuclear related".
He said: "Vengeance has suffered a mechanical defect resulting in a reduction in propulsion.
"She is returning to Faslane under her own power."
John Large, a consultant on nuclear safety, said the most likely cause of the failure would be discarded fishing nets or heavy mooring cables.
He said the submarines were designed to disengage their engines and "dump" power in the event of a propeller unit becoming jammed.
The boat carries up to 48 nuclear warheads on up to 16 Trident missiles, which weigh 60 tonnes and have a range of 4,000 nautical miles.
The submarine also carries conventional Spearfish torpedoes.
One of the four submarines, which together comprise Britain's nuclear deterrence, is always on patrol as an "insurance policy" known as Continuous At Sea Deterrence.
A nuclear-powered submarine ran aground off the Isle of Skye in October.
Two months later, on its first day back at sea, HMS Astute broke down.
According to reports, a fault had affected the propulsion and desalination system that makes sea water drinkable.