Will America Help Britain Build a New Nuclear Warhead?


The future of the United Kingdom’s nuclear deterrent depends, in part, on decisions being made right now in the U.S. Congress. At stake are Britain’s plans to build a replacement for its current nuclear warhead. According to the U.K. defense secretary and senior U.S. officials, the United Kingdom’s program is reliant on the United States pursuing its own new warhead program of record, the W93. But the Donald Trump administration’s Fiscal Year 2021 request for funds for the W93 was first nixed by House appropriators and then excluded from the stopgap continuing resolution. It is neither clear whether the W93 program will eventually make it into the budget proper, nor whether it would be taken up immediately by a potential incoming Joe Biden administration.

The United Kingdom’s new warhead will be housed in the U.S. Navy’s proposed new Mk7 aeroshell, and is intended to be developed in parallel with the W93 warhead, sharing key design parameters and using some common non-nuclear components. In April, U.K. Defence Secretary Ben Wallace wrote to members of Congress on relevant committees, claiming that their “support to the W93 program in this budget cycle is critical to the success of our replacement warhead program and to the long-term viability of the U.K.’s nuclear deterrent.” Senior Trump administration officials have also repeatedly told Congress that a failure to fund the W93 will prevent the United States from supporting the British program. The future of a nuclear deterrent that the United Kingdom calls its “ultimate insurance policy as a nation” and a contribution to the “ultimate guarantee of collective Euro-Atlantic security” is being called into question.



Most media discussion of Wallace’s letter focused on the propriety of the defense secretary lobbying Congress. But Wallace’s letter was remarkable on grounds of substance, not just process. At face value, his letter made very serious claims, suggesting that the fate of the United Kingdom as a nuclear power is in the hands not of members of Parliament, but of congressional appropriators. These claims deserve close interrogation — not least by Parliament, which has so far failed in its duty of scrutiny. British legislators should be asking why, exactly, a new warhead is needed; what the backup plan in case U.S. assistance is disrupted; what military and technical requirements are being set for the new warhead; whether the United Kingdom’s fraying infrastructure can deliver what is asked of it; and how much this endeavor will cost. Buried in these questions are significant risks and long-term strategic choices for the United Kingdom, the future of its nuclear deterrent, and Anglo-American defense ties.

Dependent Deterrent

The United Kingdom’s nuclear force is strongly dependent, in material and programmatic terms, on the United States. The Royal Navy deploys four nuclear-armed submarines equipped with the U.S.-built and maintained Trident II D-5 submarine-launched ballistic missile, drawing from a common missile pool at King’s Bay, Georgia. The essence of this cooperative relationship on delivery systems has been in place since the conclusion of the Polaris Sales Agreement in 1963, signed after Prime Minister Harold Macmillan persuaded President John F. Kennedy to sell the United Kingdom the Polaris submarine-launched ballistic missile. Bilateral cooperation on warhead-relevant matters, including the transfer to the United Kingdom of special nuclear material and non-nuclear weapon components, is authorized under the 1958 U.K.-U.S. Mutual Defense Agreement, which freed London from Washington’s postwar prohibition on nuclear cooperation under the 1946 McMahon Act.

The United Kingdom is currently working on a successor submarine, the Dreadnought class, which will replace the currently deployed Vanguard class in the 2030s, and which will share a common missile compartment with America’s own successor class, the Columbia. In 2006, Prime Minister Tony Blair secured from President George W. Bush an agreement that the United Kingdom would participate in a missile life extension program so that the Dreadnought class could continue to carry the Trident II D-5. In addition, London would be invited to participate in any program to replace or further life extend the D-5. A program is now underway to develop a successor missile once the D-5 leaves service in 2042, currently designated the D-5 Life Extension 2.

The basic parameters of the British warhead are thus set by the need for it to be certified for use with an American missile system and housed in an American aeroshell. The United Kingdom’s current nuclear warhead, the Holbrook, is sometimes referred to as an “Anglicized” version of the U.S. W76. It is certainly a similar design, and is referred to as such by U.S. national laboratories, although the degree of similarity is not publicly known. Several non-nuclear components of the warhead are known to be procured from the United States, including the arming, fusing, and firing system; neutron initiator; and the gas transfer system. The United Kingdom has made some updates to the Holbrook while the United States has conducted a life extension of the W76, now designated the W76-1, including at least the incorporation of the Mk4a arming, fusing, and firing system. However, it has not been publicly disclosed whether the United Kingdom has conducted an equally extensive life extension program of its own.

There is little reason to believe that the United Kingdom wishes to depart significantly from this model when it comes to building the next warhead. The structural incentives to hew closely to U.S. plans are strong. Procurement of non-nuclear components from the United States is seen as an obvious cost-saving measure, and reliance on U.S. facilities and information-sharing gives Britain a hedge against technological risks in design and certification. An explicit goal of the United Kingdom’s program to modernize its nuclear infrastructure has in recent years been to “increase engagement with the United States to align capabilities and requirements for any future warhead decision.”

Unless the United Kingdom wants to diverge significantly from the United States, then its new warhead program needs a parallel U.S. program against which to align. Enter the W93. The announcement of a new program of record was good news for the U.K. nuclear establishment, which had been in a holding pattern during several years of U.S. deliberations and interservice wrangling. But the United Kingdom fields only one warhead in its nuclear arsenal, and so has considerably less margin for error than the United States, which already has two warhead types delivered by submarine-launched ballistic missiles (the W76-1 and W88, plus a lower-yield W76-2 variant), as well as the redundancy of two alternative delivery vehicles (land-based intercontinental ballistic missiles and strategic bombers), each with their own warhead types. The United Kingdom also appears to be working toward a rather pressing deadline. According to official U.K. statements, a replacement warhead could be needed as early as the late 2030s, and it has previously been estimated to take 17 years from a procurement decision to the eventual production of the warhead.

This sense of urgency leaves Britain in an uncomfortable position, because many in Congress do not appear convinced that the United States truly needs the W93 program to start right away. Funding for the W93 program was not anticipated to be required for two more years, and the timing of the administration’s request has provoked pushback from Democratic legislators. One possible theory is that Trump administration officials want the W93 on the books before a potential Biden administration enters office. Another more concrete explanation is that U.S. defense officials might not trust the National Nuclear Security Administration — the semi-autonomous agency within the Department of Energy which manufactures the United States’ nuclear warheads — to deliver the W93 on schedule, and would like a more generous margin of error. The National Nuclear Security Administration request is only for the first of seven phases of warhead development, and calls for the relatively modest sum of $53 million, alongside a Pentagon request for $32 million for the Navy Mk7 program. The latter request, unlike that of the National Nuclear Security Administration, was originally approved by House appropriators.

Even if a potential incoming Biden administration agreed with the requirement for a new warhead for its submarine-launched ballistic missiles, it might still choose to delay the program. This could be a symbolic gesture toward reducing emphasis on nuclear weapons, or a practical recognition both that National Nuclear Security Administration already has four life extension or modification programs to execute and a new sea-launched cruise missile warhead to produce, and that the U.S. nuclear modernization program is shaping up to be spectacularly expensive at a time of COVID-19 induced budget pressure. Even without a conscious decision to delay the program, it might still be pushed back if a Biden administration wished to consider the W93 in the context of a Nuclear Posture Review, which would take time to complete.

Unanswered Questions

The United Kingdom, in other words, is in quite a bind. The defense secretary has stated, in writing, that the viability of the British deterrent depends on a program which the U.S. Congress might be about to stymie. It could be the case that, if work on the Mk7 aeroshell can start even in the absence of National Nuclear Security Administration funding for the W93, the United Kingdom could begin work on its own program. But the longer that Britain proceeds without a parallel U.S. warhead program in place, the greater the risks it would be incurring. Vanishingly little is publicly known about the decision-making process which has led to this point, which makes the precise degree of risk facing the United Kingdom very difficult to judge.

Congress has now had four public hearings at which the W93 was discussed, including several references to the program’s importance to the United Kingdom, and the administration has given briefings on the rationale behind the program. Yet in the United Kingdom, where the stakes are allegedly much higher, the sum total of the government’s public output on the warhead is a vaguely written statement to Parliament, and confirmation that the warhead will use the Mk7 aeroshell. This leaves open several key questions.

Why Has the United Kingdom Decided to Build a New Warhead, Rather Than Seeking to Further Refurbish or to Remanufacture Holbrook?

The fact that the United Kingdom has decided to build a new warhead at all, rather than seeking to further prolong the life of the Holbrook, is something that needs justifying. Has the United Kingdom made an independent judgment that a replacement warhead is essential, or is this decision simply the combination of a U.S. decision to proceed with W93 and the United Kingdom’s preference for alignment? Ever since the 2006 White Paper in which the decision to renew the ballistic missile submarine force was taken, the U.K. government has repeatedly told Parliament that at some point a decision on whether to build a new warhead would be need to be made. That decision has now been made, and Parliament and the public do not know why.

It is not clear that the United States sees a critical need to replace the W76-1, which has recently been life-extended, providing for additional decades of use. Some discussions of the W93’s role suggest that it could exist in parallel with both the W76-1 and the W88 as one of America’s three submarine-launched ballistic missile warheads. Were the United States to decide that the W76-1 could be further life-extended, or remanufactured from scratch, the U.K. government has not yet provided any public reasoning why the same could not be done for the Holbrook. Such reasons can certainly be imagined: there might be materials used in the original U.K. design that have now aged, and cannot for technical, legal, or safety reasons be remanufactured other than at disproportionate cost. There is also a case to be made that further life-extending old warheads introduces a degree of risk of technical failure that is unacceptably high for a country that depends on a single design, and that changes in the security environment, such as developments in missile defenses, could set future military requirements that the current warhead cannot meet. These arguments, however, have not yet been publicly made by the U.K. government.

What Are the Critical U.S. Activities on Which the U.K. Replacement Warhead Will Depend in the Next Few Years, and What Is the United Kingdom’s Backup Plan if These Activities Are Not Funded?

The U.K. government has emphasized the need for its warhead to be “compatible” with America’s Trident system. What this probably means in practice is that the United Kingdom needs to know key parameters of the Mk7 aeroshell which will define the size, shape, mass distribution, and other aspects of the British warhead which will fit inside it. Beyond that, in order for the United Kingdom to be able to cooperate closely with the United States on key scientific and engineering aspects of the warhead’s design, manufacturing, and certification, the United Kingdom will need to know U.S. intentions for various design choices. Until the United States starts work on the W93 program, the United Kingdom will either have to delay its own choices or make assumptions about likely U.S. decisions in order to begin necessary work. Either path could involve increased costs and technical risk.

Beyond a small delay, more serious disruption to the W93 program raises very challenging questions: would the United Kingdom pursue indigenous production of components that would otherwise have been procured from the United States, and if so, at what risk and cost? If the United Kingdom still wishes to remain aligned with the United States, are there alternatives to a replacement warhead based on the W93/Mk7? More broadly, Parliament might ask whether such close alignment to the United States is truly worth the accompanying loss of sovereignty. It is often assumed that the United Kingdom has simply no other option, and a more independent program would certainly involve taking a greater share of technical risk and would very likely incur greater financial costs. Nevertheless, close alignment with the United States has downsides as well as upsides, including greater vulnerability to disruption or delayed supply of materials, components, and expertise, and less discretion in setting military and other requirements for the warhead. This is a strategically important choice which has not yet been fully debated in public.

What Are the Likely Requirements for the W93, and How Do These Relate to the U.K. Program?

Assuming that the W93 program does go ahead, the first phases of its development will involve, among other things, the drafting of military characteristics and a stockpile-to-target sequence. Taken together, these will define the performance requirements and physical characteristics of the weapon, as well as the environments and threats it will be exposed to that must be taken into account in its design. This will require deciding, for example, what explosive yield the warhead will have, what defenses it must defeat — including nuclear, hit-to-kill, and (potentially) directed energy weapons — and what kind of hardening and countermeasures will be necessary. Choices will also be made regarding surety requirements, such as whether to use insensitive high explosives, which could mean a relative increase in mass and volume.

Requirements set in the United States during this process are likely to determine or strongly influence several characteristics of the British warhead. Embedded in those requirements are important implications for U.K. policy and strategy, and although London is likely to have a voice in U.S. discussions over such questions, it will not have a deciding vote. Briefings by U.S. officials suggest, for example, that the W93 is intended to be of higher yield than the W76-1. If the United Kingdom were to follow this path, the overall explosive yield of its operational stockpile could increase for the first time since it began deploying Trident in 1994. The emphasis placed by U.S. officials on the W93’s “flexibility” implies variable yield, which would be somewhat consistent with the existing lower-yield warhead variant the U.K. reportedly deploys at present, but would leave the United Kingdom vulnerable to accusations that it was reinforcing a global trend toward the development of supposedly more “usable” nuclear options. Any potential improvement to the warhead’s ability to strike hard targets might also draw criticism from those opposed to enhancements in nuclear weapon capabilities, as was the case when the U.K. began introducing the Mk4a arming, fusing and firing system. Lastly, U.K. defense planners might be thinking about future deterrence requirements for countries other than Russia, the traditional driver of U.K. warhead needs. This might have an impact on the requirements for a new warhead, such as on the question of the new warhead’s weight, which helps determine the maximum range that missiles can reach. This is a potentially relevant factor when considering the risk that future developments in anti-submarine warfare might complicate U.K. operations.

Can the United Kingdom Successfully Execute a Warhead Replacement Program?

The state of the United Kingdom’s nuclear weapons infrastructure suggests that the country will face significant challenges in producing its next warhead, even if cooperation with the United States runs entirely smoothly. By now, at least three key U.K. facilities should have been up and running: Pegasus (to handle enriched uranium components), Mensa (to assemble and disassemble warheads), and Hydrus (to conduct hydrodynamic tests). None is fully operational. Pegasus has been suspended after initial designs were judged too expensive and unwieldy, Mensa is now being built at least six years late and at more than twice the original cost, and Hydrus has been replaced by a joint U.K.-French hydrodynamic facility in France not scheduled to be fully operational for the United Kingdom at least until 2022.

More fundamentally, this will be the first warhead the United Kingdom has designed for some thirty years, and the first ever without explosive nuclear testing. It will also be designed just as the last generation of Atomic Weapons Establishment employees with firsthand experience designing new warheads are retiring. With civil nuclear projects also planned for the coming decades, the Atomic Weapons Establishment will be facing considerable workforce recruitment, training, and retention challenges.

How Much Will the United Kingdom’s New Warhead Cost?

In the context of the COVID-19 crisis, the forecast hit to the U.K. economy caused by Brexit, and an ongoing strategic review, there is likely to be financial pressure on the warhead program even if it is shielded from immediate cuts. Yet the government is staying remarkably coy about the projected costs of the new warhead. The 2006 White Paper estimated the cost to be 2 to 3 billion pounds ($2.6 to $3.9 billion). This estimate was confirmed in government documents as late as 2013. Since then, however, no official estimate has been provided, although the 2013 Trident Alternatives Review, a government-published document — though not a statement of policy — estimated the cost of a new submarine-launched ballistic missile warhead to be 4 billion pounds in 2012 prices.

The Nuclear Warhead Capability Sustainment Program, under which much of the supporting infrastructure for building a new warhead was supposed to be built, has an overall budget of 21 billion pounds spread out from 2005 to 2025, and is subject to the scrutiny requirements applied to major projects. The government has said that the program to build the new warhead will also be subject to those requirements, but is not giving a specific cost estimate, citing national security concerns. Likewise, it has not said which parts of the Nuclear Warhead Capability Sustainment Program will be subsumed under the new warhead program. It is certainly tricky to separate the costs of the new warhead from the overall costs of having nuclear infrastructure capable of maintaining the existing warhead. And yet not only has the U.K. government done so in the past — making it difficult to imagine national security grounds for withholding the information now — the U.S. government has also provided estimates of how much the W93 might cost: $14.4 billion, according to the Nuclear Security Administration’s last published assessment.

Take Back Control

The United Kingdom’s replacement nuclear warhead program is a long-term, complex, and expensive endeavor. It deserves proper scrutiny. And while many of the technical details of the U.K. warhead must remain classified, the broad parameters of the decision the government has made — and the risks the program faces — are fair game for public debate. The British public learned of the decision to replace the Holbrook warhead not because the government decided to announce it, but because U.S. officials told Congress and reporters in February. In several respects, the transparency of the U.S. government, and the persistence of Congress in extracting answers, is throwing the opacity of the United Kingdom’s nuclear warhead program into stark relief. It is time for Parliament to take back control.



Matthew Harries (@harries_matthew) is a senior research fellow at the Royal United Services Institute for Defence and Security Studies (RUSI). He previously worked on the staff of the U.K. House of Commons Foreign Affairs Committee. Prior to that, he was managing editor of Survival and a senior fellow at the International Institute for Strategic Studies (IISS). This article is drawn from a research project supported by the John D. and Catherine T. MacArthur Foundation.

Image: U.S. Navy (Photo by John Kowalski)