British Submarines to Receive Upgraded US Nuclear Warhead

The Royal Navy plans to “integrate” the US W76-1/Mk4A warhead onto British SSBNs.

By Hans M. Kristensen

Sea-launched ballistic missiles on British ballistic missile submarines will be armed with the upgraded W76-1 nuclear warhead currently in production in the United States, according to a report from Sandia National Laboratories.

According to the Labs Accomplishments from March 2011, “the first W76-1 United Kingdom trials test was performed at WETL [Weapon Evaluation Test Laboratory], providing qualification data critical to the UK implementation of the W76-1.”

Rumors have long existed that the nuclear warhead currently used on British sea-launched ballistic missiles (SLBMs) is very similar to the U.S. W76 warhead deployed on American SLBMs. Official sources normally don’t give U.S. warhead designations for the British warhead but use another name. A declassified document published by the National Nuclear Security Administration (NNSA) in 1999, which I published in 2008, described maintenance work on the W76 warhead but called the British version the “UK Trident System.”

Labs Accomplishments from Sandia National Laboratories describes “the first W76-1 United Kingdom trial test” and “UK implementation of the W76-1.”
Download the publication here.

The Sandia report, on the contrary, explicitly uses the U.S. warhead designation for the warhead on British SLBMs: W76-1.

Approximately 1,200 W76-1s are current in production at the Pantex Plant in Texas. The W76-1 is an upgraded version of the W76-0 (or simply W76) produced between 1978 and 1987. In its full configuration, the upgraded weapon is known as the W76-1/Mk4A, where the Mk4A is the designation for the cone-shaped reentry body that contains the W76-1 warhead.

After first denying it, the British government has since confirmed that the Mk4A reentry body is being integrated onto the British SLBMs but it has been unclear which warhead it will carry: the existing warhead or the new W76-1. The Sandia report appears to show that the United Kingdom will get the full package: W76-1/Mk4A.

HMS Vanguard launches US-supplied Trident II D5 SLBM off Florida in October 2005. In the future, missiles on British submarines will carry US-supplied W76-1/Mk4A nuclear warheads.

The upgrade extends the service life by another 30 years to arm U.S. and British nuclear-powered ballistic missile submarines (SSBNs) through the 2040s. That timeline fits the 2010 British Strategic Defence Review finding that “a replacement warhead is not required until at least the 2030s.”

HMS Vanguard launches US-supplied Trident II D5 SLBM off Florida in October 2005. In the future, missiles on British submarines will carry US-supplied W76-1/Mk4A nuclear warheads.

The transfer of W76-1/Mk4A warheads to the United Kingdom further erodes British claims about having an “independent” deterrent. The missiles on the SSBNs are leased from the U.S. Navy, the missile compartment on the next-generation SSBN will be supplied by the United States, and new reactor cores that last the life of future submarines hint of substantial U.S. nuclear assistance.

The transfer of nuclear technology from the United States to Britain is authorized by the 1958 US-UK Mutual Defense Agreement, which was most recently updated in 2004 and extended through 2014 to permit the “transfer of nonnuclear parts, source, byproduct, special nuclear materials, and other material and technology for nuclear weapons and military reactors” between the two countries. The will text is secret but a reconstructed version is here (see note 3).

Acknowledgements: Thanks to Nick Ritchie at Bradford University (“US-UK Special Relationship”) and John Ainslie at Scottish CND.

This publication was made possible by a grant from Carnegie Corporation of New York and Ploughshares Fund. The statements made and views expressed are solely the responsibility of the author.



5 thoughts on “British Submarines to Receive Upgraded US Nuclear Warhead

  1. How much is this upgrade costing – or this this included in the £2billion a year running cost for Trident?

    Reply: That is entirely unknown at this point. Even the costs of the Mk4A upgrade have not been released. HK

  2. A very interesting blog posting which sheds further light upon US – UK co-operation on nuclear weapons. Thanks for publishing this.

    My reading of the Sandia National Laboratories ‘Labs Accomplishments’ report is a little more nuanced that your article suggests, Hans. The SNL report doesn’t state that complete warheads will be supplied to the UK, and I think the UK will continue to build and maintain its own warheads at the Atomic Weapons Establishment. However, we know that SNL are providing the UK with the Arming, Fusing, and Firing system for the Mk4A warhead upgrade. The Lab Accomplishments report indicates that warhead components and / or possibly even complete warheads are being transported from the UK to SNL for qualification testing, presumably after having been fitted with the AF&F upgrade and other new features.

    As you rightly say, this provides further evidence to show that claims that the UK’s nuclear weapons are “independent” are pretty thin.

  3. How does this work out with the selectable yield for the British warheads? Will the UK loose that option with the upgraded warhead or will the US get selectable yields as a new capability?

    Reply: Who says the British warheads have “selectable yield”? There are many rumors. “Selectable yield” is a specific capability incorporated into gravity bombs and cruise missiles that enables the ground or air crew to chose among a limited number of specific pre-set selectable yields. Warheads used in reentry vehicles on ballistic missiles were designed with single yield. But all warheads have some basic characteristics that give limited options for lowering the yield: the secondary can be “turned off” so only the lower-yield primary detonates, or the tritium boosting can be reduced to create a less efficient burn of the secondary. This type of “grooming” of the warhead must be done at the factory before deployment. This is probably what British officials mean when they stated in the past that Britain has “some flexibility in the choice of yield” for the warheads on the Trident force. HK

  4. How many warheads on one missile?

    Reply: The current estimate is 1-3 warheads per missile. The UK government has stated “up to 48” per deployed sub. But the Trident II D5 SLBM has the capacity to carry many more (it was test flown with 14). Under the new defense plan published in 2010, the SSBN fleet will transition to a posture of eight “operational” missiles on each sub with a total of 40 warheads. That is a odd posture and one unknown is what the status of the “non-operational” missiles will be. Will they be armed yet non-operational? If not, then each of the operational missiles would have to carry an average of five warheads, possibly more given that some of the missiles have “sub-strategic” roles with fewer warheads. Therefore it seems more likely that the non-operational missiles will carry a portion of the 40 warheads but require longer to ready for launch. The British government might want to be a little more clear about what “non-operational” means. HK

  5. Every country is busy in signing treaties with one another but, no one seems to stop the use of nuclear weapons or the manufacture of it. I understand it is required to defend oneself but how far would this go.

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