Since 1998, the system has also provided the Government with the option of a lower-yield, 'sub-strategic' nuclear strike capability. The oldest Vanguard-class submarine had been expected to remain in service until 2019 without a refit.

The majority of this system is based in Scotland at HMNB Clyde (HMS Neptune), which includes the Faslane home of the Vanguard submarines, and at RNAD Coulport on Loch Long. It is collectively known as the Trident system. The SDR considered this was the minimum number of warheads adequate for deterrence. Under the continuous at sea deterrence policy, at least one Vanguard-class SSBN is kept on patrol with up to 16 Trident missiles sharing up to 48 warheads from the stockpile at any given time. Since the Strategic Defence Review (SDR), the UK has maintained a stockpile of around 215 warheads, with around 120 active (usable). It is intended to deter a potential enemy because they cannot ensure eliminating the entire stockpile in a first strike if a ballistic missile submarine remains undetected. Since the retirement of the last Royal Air Force WE.177 nuclear bomb in 1998, the British nuclear arsenal has been wholly submarine-based. Main article: Trident (UK nuclear programme) The next three boats will be called Valiant, Warspite and King George VI. Provisionally named 'Successor' (being the successor to the Vanguard class SSBNs), it was officially announced in 2016 that the first of class would be named Dreadnought, and that the class would be the Dreadnought class. Their replacement is necessary if the Royal Navy is to maintain a continuous at-sea deterrent (CASD), the principle of operation behind the Trident system. The Vanguard submarines entered service in the United Kingdom in the 1990s with an intended service life of 25 years. Like their predecessors they will carry Trident II D-5 missiles. The Dreadnought class is the future replacement for the Vanguard class of ballistic missile submarines.

8 to 12 Lockheed Trident II D5 SLBMs (carrying up to 8 warheads each).

4 × 21 inch (533 mm) torpedo tubes for: Spearfish heavyweight torpedoes.Limited only by food and mechanical components Rolls-Royce PWR3 nuclear reactor, turbo-electric drive, pump-jet Nuclear-powered ballistic missile submarineġ7,200 t (16,900 long tons 19,000 short tons) £31 billion (2016) lifetime cost of total programme (est.).Artist's rendering of Dreadnought-class submarine

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