In the 1998 Strategic Defence Review we announced that we will have only one submarine on patrol at a time, carrying a reduced load of 48 warheads. Before the Strategic Defence Review, the announced ceiling was 96.
Of the four Vanguard class submarines, two have already completed long overhaul periods (refuelling) (LOP(R)). The LOP(R) for HMS Vanguard started in February 2002 and was completed after three years and six months; the LOP(R) for HMS Victorious was started in January 2005 and was completed after three years and eight months.
All Vanguard class submarines have been, or will be, fitted with long life reactor cores that will last for the remainder of their operational life without the need for refuelling. These cores are being fitted as part of the submarines' long overhaul periods (refuelling) (LOP(R)s).
Vanguard class submarines with long life cores will subsequently undergo long overhaul periods (LOPs). Work is still under way to determine the composition of the work package for a LOP; it is therefore too early to-estimate their duration.
[holding answer 26 March 2009]: It is our intention that both the future UK and US submarines will share a common missile compartment within which the missiles will be carried. In the event that the US decides to develop a successor to the Trident D5 missile, there is no risk that it will be incompatible with this common compartment and hence with the future UK submarine.
Since the adoption by the UK of a submarine-based nuclear deterrent, the UK has conformed to a common design for the missile launch tubes for both the Polaris and Trident systems. Missile compartments for both systems conform to US specifications necessary to incorporate the US-supplied weapon system. There are, however, agreed variations to reflect UK-specific requirements.