The Rolls-Royce pressurised water reactor (PWR) series has powered British nuclear submarines since the Valiant class, commissioned in 1966. The first British nuclear submarine, HMS Dreadnought, was powered by a Westinghouse S5W reactor.
The first British naval reactor, the PWR1, utilising a core and reactor assembly of purely British design went critical in 1965. Technology transfers under the 1958 US-UK Mutual Defence Agreement eventually made Rolls-Royce entirely self-sufficient in reactor design in exchange for a "considerable amount" of information regarding submarine design and quietening techniques being passed on to the United States.
Rolls Royce and Associates at Derby became the centre for design and manufacture of the reactors. The Ministry of Defence's Vulcan Naval Reactor Test Establishment, at Dounreay, tested each reactor prior to its installation in nuclear submarines.
- Core A
- Core B
- Core Z
PWR2 is the latest nuclear reactor designed to power the Royal Navy's submarines. The PWR2 was developed for the Vanguard-class Trident missile submarines and is a development of the PWR1. The first PWR2 reactor was completed in 1985 with testing beginning in August 1987 at the Vulcan Naval Reactor Test Establishment.
The latest design of the PWR2 is the "Core H", which removes the need for refuelling, allowing a submarine to avoid two reactor refits in its service life. HMS Vanguard will be fitted with the new core during its refit, followed by her three sister boats. The Astute-class submarines will have this full-life core installed. As they were developed for SSBNs, the reactors are considerably larger than those of current British fleet submarines. The diameter of Astute-class hulls have therefore been increased to accommodate the PWR2.
A safety assessment of the PWR2 design by the Defence Nuclear Safety Regulator in November 2009, was released under a Freedom of Information request in March 2011. The regulator identified two major areas where UK practice fell significantly short of comparable good practice, loss-of-coolant accident and control of submarine depth following emergency reactor shutdown. The regulator concluded that PWR2 was "potentially vulnerable to a structural failure of the primary circuit", which was a failure mode with significant safety hazards to crew and the public.
In January 2012 radiation was detected in the PWR2 test reactor's coolant water, caused by a microscopic breach in fuel cladding. This discovery led to HMS Vanguard being scheduled to be refueled early and contingency measures being applied to other Vanguard and Astute-class submarines, at a cost of £270 million. This was not revealed to the public until 2014.
- Core H
Three propulsion options were considered for the successor to the UK Trident system - PWR2, PWR2b (a PWR2 derivative with improved performance), and PWR3. PWR3 is a new system based on a US design but using UK reactor technology. Both PWR2b and PWR2 would cost roughly the same but PWR3 is a simpler and safer design with a longer life and lower maintenance requirements than the PWR2 variants.
In March 2011 Defence Secretary Liam Fox indicated PWR3 was the preferred option "because those reactors give us a better safety outlook" and two months later the Ministry of Defence announced that PWR3 had been selected for Successor at a cost of about £3 billion. PWR3 had a whole life cost of about £50m/boat more than PWR2 designs over a 25-year life, but its lower maintenance costs makes it cheaper over the longer service life that is possible with the PWR3 design.
A confusing press release suggested that the seventh Astute-class submarine, HMS Ajax, would have the PWR3 reactor but in October 2012 it was confirmed that she would have a PWR2 like the rest of the Astute class.
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