Select Committee on Defence Eighth Report

2  The UK's Strategic Nuclear Deterrent

Components of the UK's Strategic Nuclear Deterrent

13. The Trident weapons system is the UK's third generation strategic nuclear deterrent and was developed during the final decade of the Cold War. It was introduced into service over a six year period beginning in December 1994 and is currently the UK's sole nuclear weapons system.

14. The UK's Trident system has three key technical components: the platform; the missile; and the warhead.


15. The platform for the UK's current strategic nuclear deterrent is the Vanguard-class nuclear-powered submarine (SSBN). The UK has four of these submarines—HMS VANGUARD, HMS VICTORIOUS, HMS VIGILANT, HMS VENGEANCE. These entered service in December 1994, December 1995, June 1998 and February 2001 respectively.[14]

16. All four submarines were designed and built in the UK by Vickers Shipbuilding and Engineering Ltd. (VSEL), now BAE Systems, in Barrow-in-Furness, Cumbria.[15]

17. Each submarine weighs approximately 16,000 tonnes, is 150 metres in length, is powered by a Rolls Royce PWR2 nuclear reactor, and has 16 independently-controlled missile tubes which house the Trident II D5 missiles.[16]

18. Each of the Vanguard-class submarines has a projected service life of up to 30 years.[17]


19. The Trident II D5 submarine-launched ballistic missile (SLBM) carried on the UK's Vanguard-class submarines is a three-stage solid-fuel inertially guided rocket. Each missile is approximately 13 metres in length, nearly 2 metres in diameter, and weighs 60 tonnes. It has a range of between 6,500 kilometres and 12,000 kilometres, dependent on payload, and is accurate to within a few metres.[18]

20. Each missile is capable of carrying 12 warheads, which means that each Vanguard-class submarine is capable of carrying up to 192 warheads. Following the 1998 Strategic Defence Review, the number of warheads per Trident II D5 missile was limited to 3 warheads (and 48 warheads in total per submarine). Each missile has a MIRV (multiple independently-targetable re-entry vehicle) capability which enables each Trident missile to engage multiple targets simultaneously.[19]

21. The Trident II D5 missile was designed and manufactured in the United States by Lockheed Martin. Under the Polaris Sales Agreement (modified for Trident), the UK has title to 58 missiles. Aside from those currently deployed, the missiles are held in a communal pool at the US Strategic Weapons facility at King's Bay, Georgia, USA. Maintenance and in-service support of the missiles is undertaken at periodic intervals at King's Bay, normally after a submarine has been through refit.[20]


22. The nuclear warhead fitted to the tip of the Trident II D5 missile was designed and manufactured in the UK at the Atomic Weapons Establishment at Aldermaston, Berkshire. Although public information is limited, the nuclear warhead on UK's Trident II D5 missile is reported to be closely related to the American W76 warhead, a thermonuclear warhead with a yield of around 100 kilotons.[21]

23. During our visit to the United States in May 2006, we heard that the US and UK collaborated closely on nuclear weapons and that there was a rich flow of nuclear ideas between the US and the UK. We were also told that the fiftieth anniversary of the 1958 Mutual Defence Agreement, which formalised this cooperation, would be a cause for both pride and celebration.


24. The UK's Trident system is underpinned by a range of supporting industrial and manufacturing infrastructure.

25. The submarine basing infrastructure: The Naval Base at Faslane, Strathclyde, is home to the UK's Trident submarine force. It has a staff of over 7,000 and is also home to conventionally-armed submarines. The nuclear warheads carried onboard the Vanguard-class SSBN submarines are stored and fitted to the UK's Trident II D5 missiles at the Royal Naval Armaments Depot at Coulport, near Faslane.

26. The onshore submarine construction and maintenance infrastructure: This comprises the building yard at Barrow-in-Furness, Cumbria, owned by BAE Systems, and the operational and refit and support site at Devonport, Plymouth, owed by DML (a consortium of which fifty-one per cent is owned by the US firm Halliburton). This part of the defence industrial base is characterised by its need for a highly specialised and skilled workforce and large-scale purpose-built physical infrastructure. Together, these requirements are present at all stages of the nuclear-powered submarine's life, from concept design through to operation, maintenance and disposal and carry significant levels of fixed cost that have to be incurred if key capabilities are to be retained. Once lost, these capabilities are likely to be very difficult and potentially expensive to recreate.[22]

27. The warhead research and manufacturing infrastructure: The UK's expertise in nuclear weapons design is concentrated at the Atomic Weapons Establishment at Aldermaston, Berkshire. AWE is a 'Government Owned Contractor Operated' (GOCO) facility. It is managed by a consortium, in which a third of the shares are held by the US firm Lockheed Martin. The role of AWE is to build, maintain and certify the existing weapons stockpile, as well as to ensure good stewardship of nuclear weapons knowledge. Prior to the MoD's current investment programme at Aldermaston (announced in July 2005), the AWE's workforce was around one-third of its peak Cold War levels. The MoD has stated that this funding is designed to ensure the UK skills base and manufacturing infrastructure in nuclear weapons is maintained until a decision on the future of the nuclear deterrent is taken.[23]

The UK's nuclear posture

The UK's strategic nuclear arsenal 1952 to 1991

28. The UK first tested a nuclear device in October 1952. It deployed an operational nuclear weapons capability the following year. Initially, the UK's nuclear deterrent rested on the 10 kiloton Blue Danube free-fall bomb, carried by the V bombers of the Royal Air Force's strategic bomber force. The UK tested a thermonuclear device in 1957, and an operational thermonuclear weapon entered service in 1961.[24]

29. In 1958 the UK and USA concluded the "Mutual Agreement for Co-operation on the Uses of Atomic Energy for Mutual Defence Purposes". The Agreement, which has long been regarded as the cornerstone of the UK's nuclear weapons programme, enables exchanges of technical information and allows the UK to draw on US warhead designs, although final responsibility for building and maintaining the warheads remains with the Atomic Weapons Establishment.

30. During the 1950s, the UK and USA were also involved in a joint project to develop the Skybolt air-launched nuclear missile, which the UK regarded as the central component of its future nuclear force. In 1962, the US Kennedy Administration cancelled the project. Later that year, the UK agreed to procure the Polaris submarine-launched missile system which entered service in the late 1960s.

31. The Polaris system comprised four Resolution-class SSBN submarines, each armed with 16 Polaris missiles. Like the current Vanguard-boats, the submarines were designed and built in the UK, albeit with initial assistance from the US in designing the nuclear propulsion system. The missiles themselves, like the current Trident II D5 missiles, were purchased from the United States. The warheads were designed and built in the UK with US collaboration. The UK subsequently developed a new version of Polaris, known as Polaris Chevaline, which could better penetrate Soviet defences.

32. By the final decade of the Cold War, the UK's strategic nuclear deterrent had three main elements: strategic, sub-strategic and tactical. Polaris Chevaline served in the strategic role for use against multiple targets in the adversary's homeland. The sub-strategic role for a more limited strike against individual targets on enemy territory was fulfilled by the WE 177 free-fall bomb carried by the RAF's Vulcan and Tornado aircraft. Lower yield WE 177 devices served in the tactical role for use against enemy troops and equipment on the battlefield. American tactical nuclear warheads were deployed on heavy artillery and short-range Lance missiles under a US-UK dual-key arrangement.[25]


33. Following the end of the Cold War and the collapse of the Soviet Union in 1991, the UK Government withdrew from service a range of nuclear weapons, including the US tactical nuclear warheads mounted on heavy artillery and the Lance system and the Royal Air Force's sub-strategic air-launched nuclear weapons (the WE 177 free-fall nuclear bombs).

34. Completed in 1998, these reductions left Trident, which replaced Polaris Chevaline in 1994, as the UK's sole nuclear weapons system. The total warhead stockpile was reduced by 20 per cent and the number of operationally available warheads fell from around 400 during the 1980s to under 300 with the result that the explosive power of the UK's nuclear deterrent fell by an estimated 40 per cent of the megatonnage available during the 1970s.[26]


35. The 1998 Strategic Defence Review (SDR) promised "a rigorous re-examination of [the UK's] deterrence requirements". It sought to define a deterrence posture based upon:

36. As a result of the SDR, the number of operationally available warheads was reduced by one third, from around 300 to under 200, whilst the number of warheads carried on each Trident submarine was reduced by half, from 96 to 48. As a result of these reductions, the Government estimated that the explosive power of the UK's nuclear deterrent would be 70% less than that of the operationally available warheads held during the 1970s.[28]

37. The SDR also defined the 'operating posture' of the UK's strategic nuclear deterrent, clarifying both the technical specification and policy baseline for that deterrent.

38. A Minimum Nuclear Deterrent Force: The SDR prescribed further limitations on the maximum number of warheads to be deployed on each of the UK's Vanguard-class submarines. Although each Trident II D5 missile is capable of carrying up to 12 warheads, the SDR stipulated that no more than 3 warheads would be fitted to each missile.[29] Some missiles are believed to carry a single warhead and the warheads themselves are believed to be of variable yields. The precise number of warheads carried on patrol at any given time remains classified information.[30]

39. The Continuous-at-Sea Deterrent Cycle (CASD): The SDR stated that the UK would maintain continuous at-sea deterrent patrols. This meant that one of the four Vanguard-class Trident submarines would continue to be on patrol at any given time. The SDR stated that the purpose of CASD was "to avoid misunderstanding or escalation if a Trident submarine were to sail during a period of crisis".[31] By keeping one submarine on patrol at all times, the UK avoids the risk of sending incorrect or misleading signals to a potential adversary at times of heightened alert.

40. 'De-targeting' and 'State of Readiness': The SDR stated that the Trident missiles aboard the Vanguard-class submarines would not be targeted and would normally be at several days 'notice to fire'. However, the SDR also noted that "we will… ensure that we can restore a higher state of alert should this become necessary at any time".[32] In the course of our inquiry, we were told that targeting the missiles does not take very long. Although some sub-systems aboard the submarines, such as the navigation sub-system, might take time to reach their accuracy levels, Commodore Tim Hare, a former Director of Nuclear Policy at the MoD, told us that "political rather than technical issues" explained the extended notice-to-fire of the UK's Trident system.[33] Dr Rebecca Johnson, of the Acronym Institute for Disarmament Diplomacy, argued that both de-targeting and the reduced state of readiness were essentially meaningless since they could be could be easily overridden.[34]

41. A sub-strategic role: The SDR also defined a sub-strategic role for the Trident nuclear deterrent alongside its principal strategic function. It stated:

    The credibility of deterrence also depends upon retaining an option for a limited strike that would not automatically lead to a full scale nuclear exchange. Unlike Polaris and Chevaline, Trident must also be capable of performing this 'sub-strategic' role.[35]

In comparison with a strategic strike, which would involve a full-scale attack against an adversary in which all or a significant part of the available Trident force would be launched, a sub-strategic strike would involve the launch of one or a limited number of missiles against an adversary as a means of conveying a political message, warning or demonstration of resolve. Commodore Hare told us that this sub-strategic role "offers the Government of the day an extra option in the escalatory process before it goes for an all-out strategic strike which would deliver unacceptable damage to a potential adversary".[36] Although the Government has revealed little information about the precise number and yield of UK warheads, it is widely believed that Trident missiles intended for this sub-strategic role carry only a single warhead, potentially with a significantly reduced yield.

42. It is important not to confuse this sub-strategic role with a tactical role. Trident is not designed or intended to fulfil a tactical role on the battlefield.[37]

43. During our visit to the United States, we heard that the US was considering modifying its Trident system to allow its submarines to carry conventional weapons. In early 2006, the Pentagon proposed a $503 million Conventional Trident Modernisation Programme in order to diversify its strategic options. This programme, currently under consideration in the Congress, has generated much controversy because of concern that the launch of a conventionally-armed Trident missile could be mistaken for a nuclear attack. We know of no plans for the UK to follow the US lead in developing a conventional role for its Trident force.

44. The UK's nuclear forces are formally committed to NATO's nuclear posture.[38] During the Cold War they were part of the United States's Single Integrated Operational Plan which included all NATO nuclear forces, other than those of France, and provided continuous, integrated targeting for all such forces. Since UK nuclear forces were formally de-targeted, this no longer applies in the same way, but the presumption remains that UK forces would cover NATO designated targets. The right and the capacity to fire the UK's missiles independently at targets designated by the UK Government is a derogation from the default setting that the UK's nuclear forces remain at the service of NATO.

The UK's Strategic Nuclear Deterrent in a comparative international context

45. The UK's strategic nuclear arsenal is small in comparison with the other established nuclear powers. Its total stockpile of approximately 185 nuclear warheads represents 1.4% of the total number (13,470) in the world and only just 1.5% of the world total of strategic nuclear warheads (12,193). The United States, Russia, China, France and Israel each possess more warheads than the UK. Only India, Pakistan and North Korea have significantly fewer.[39]

46. The USA is believed to have approximately 4,216 strategic and 680 non-strategic nuclear warheads in its active inventory, with a further 5,454 additional warheads held in reserve or in inactive stockpiles, some of which will be dismantled in the coming years. By 2012, it is estimated that the total US stockpile will number approximately 5,945 warheads. Russia is estimated to have around 16,000 intact warheads, down from around 35,000 at the end of the Soviet era in 1991. Of these 16,000, 7,360 are operational, with 3,980 being strategic warheads and 3,380 non-strategic. The remainder of the stockpile may be officially retired and awaiting disassembly, or in short- or long-term storage. Dismantlement of Russian warheads is believed to be proceeding at the rate of 1,000 to 2,000 a year. China is estimated to hold around 400 nuclear warheads, of which 282 are thought to be strategic warheads and 120 non-strategic. France is estimated to have 348 warheads, all of which are believed to be strategic warheads.[40]

47. The UK has abandoned the concept of the nuclear triad, where weapons are deployed by air, land and sea. In contrast, the United States, Russia and China all have powerful systems in all three areas. So too does Israel. India and Pakistan have two legs of a potential triad; air-based and land-based systems. India is expected to have a full triad by around 2007-08.[41]

The purpose of the UK's Strategic Nuclear Deterrent

Original purpose

48. The UK's current strategic nuclear deterrent was developed in the international political and strategic context of the Cold War. Its central purpose, at the time of its procurement in the early 1980s, was to discourage aggression against the UK, its allies and its interests from the Soviet Union and the Warsaw Pact which "had both the perceived capability and assumed intention to expand into Western Europe and elsewhere".[42] To contemporary decision-makers in the UK, the Cold War offered a compelling justification for the possession of a strategic nuclear deterrent to counter both the actual and developing Soviet military and political threat.

49. To discourage Soviet aggression, the UK's strategic nuclear deterrent had to be able to fulfil what was known as the 'Moscow criteria'—"the ability to threaten to inflict sufficient damage on Moscow and a number of other Soviet cities at any time of the day, 365 days of the year" and to be able to inflict such damage even after a surprise Soviet nuclear attack against the UK. It was assumed that the UK's nuclear deterrent could also help to compensate for the Warsaw Pact's large superiority in conventional forces.[43]

50. Commentators have also suggested an additional rationale for the UK's strategic nuclear deterrent during the Cold War; that it was a means of influencing the foreign policy and military decision-making of its principal ally, the United States. We were told that "though retaining the ability to act, in extremis, alone, Britain's policy was to influence the ultimate guarantor of the country's political independence and physical survival".[44]


51. The end of the Cold War and the collapse of the Soviet Union has radically altered the international political and strategic environment within which the UK's nuclear deterrent operates and has changed the nature of, and requirements for, strategic deterrence.

52. This raises the issue of what deterrence means, in practice, in the post-Cold War era. The Oxford English Dictionary defines the term deterrence as "discouragement by fear". This is arguably what the UK's strategic nuclear deterrent does: Trident is intended to discourage aggression by the fear of retaliation.

53. But Trident is not the UK's only means of deterrence. Deterrence is "an extremely broad concept" and refers to "a whole range of instruments for the prevention of war, or the discouragement of aggression, some of which may not even be military".[45] Deterrence can be exercised by a spectrum of options ranging from economic sanctions and robust diplomatic pressure to conventional military options and the threat of strategic nuclear retaliation.

54. Strategic nuclear deterrence is not intended as a means of countering all threats to the security of the UK. Rather, it is "on the right hand of the deterrence equation to be used in extremis when the survival of the nation state is at stake".[46]

55. In considering the future of the strategic nuclear deterrent, the UK will need to examine whether the concept of nuclear deterrence remains useful in the current strategic environment and in the context of the existing and emerging threats to the security of the country. We will have to consider whether those states and non-state actors posing such threats can, in reality, be deterred from instigating acts of aggression by either existing or new approaches to nuclear deterrence. We will also have to consider how the UK's nuclear capability should be adjusted to meet new strategic realities. Trident was developed during the final decade of the Cold War, and was designed to counter the threat posed by the size and technical capabilities of the Soviet strategic nuclear arsenal: we need to consider whether the form of the UK's current nuclear deterrent is best suited to today's and tomorrow's strategic challenges.

56. We believe that it is essential that, before making any decisions on the future of the strategic nuclear deterrent, the MoD should explain its understanding of the purpose and continuing relevance of nuclear deterrence now and over the lifetime of any potential Trident successor system.

14   Stockholm International Peace Research Institute, SIPRI Yearbook 2000, (Oxford 2000), p 486 Back

15   Michael Clarke, "Does my Bomb look big in this?", International Affairs, vol 80, no 1, (2004), p 50  Back

16   Ibid. Back

17   Ministry of Defence, The Strategic Defence Review, Cm 3999, July 1998, p 17 Back

18   Stockholm International Peace Research Institute, SIPRI Yearbook 2005, (Oxford 2005) p 589 Back

19   Stockholm International Peace Research Institute, SIPRI Yearbook 2005, (Oxford 2005) p 589 Back

20   Ibid. Back

21   Michael Clarke, "Does my Bomb look big in this?", International Affairs, vol 80, no. 1, (2004) p 37 Back

22   Ev 143 Back

23   HC (2005-06) 835, Annex C Back

24   Library Standard Note, SN/1A/3706, House of Commons Library, April 2006, p 4 Back

25   Library Standard Note, SN/1A/3706, House of Commons Library, April 2006, p 4 Back

26   Ministry of Defence, The Strategic Defence Review, Cm 3999, July 1998, p 18 Back

27   Ibid. Back

28   Ibid. Back

29   Ministry of Defence, The Strategic Defence Review, Cm 3999, July 1998, p 19 Back

30   Ibid. Back

31   Ibid. Back

32   Ibid. Back

33   Q 148 Back

34   Q 6 Back

35   Cm 3999, July 1998, p 18 Back

36   Q 149 Back

37   Ibid. Back

38   Ev 133 Back

39   International Institute of Strategic Studies, The Military Balance 2003-2004 (Oxford 2003) p 228 Back

40   Stockholm International Peace Research Institute, SIPRI Yearbook 2005, (Oxford 2005) p 579 Back

41   Michael Clarke, "Does my Bomb look big in this?", International Affairs, vol 80, no. 1, p 52 Back

42   Ev 74 Back

43   Ev 90 Back

44   Ev 101 Back

45   Q 13 Back

46   Q 149 Back

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