One state not party—the United States—engaged in conduct in 2022 that was not compatible with the TPNW’s prohibition on transfer of nuclear weapons, by virtue of its export of key components to the United Kingdom’s nuclear arsenal.
The United States has effectively transferred nuclear weapons to the United Kingdom, because the United Kingdom’s nuclear-weapon system in very large measure is imported from the United States: the UK leases its Trident II (D5) submarine-launched ballistic missiles (SLBMs) from the United States’ stockpile; the design for the UK’s Holbrook nuclear warhead for its Trident missiles is based on the US W76 design; key components for the Holbrook warhead are imported from US nuclear-weapons laboratories; the Mk4A reentry vehicle for the warhead, and key components of the UK’s ballistic missile submarines (SSBN)s are imported from the United States (the Trident SLBM fire control system and missile compartments); and the submarines’ reactors were developed from a US design.
These transfers of key components also violate the corresponding prohibition on transfer by nuclear-weapon states in Article 1 of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).
UK officials have reportedly lobbied the US Congress to expedite the development of a new warhead, the W93, on which a replacement for the Holbrooke warhead would be based. One commentator has suggested that over the course of 25 years of studies, engineering, and production, the W93 programme may cost up to US$14 billion, with production of the first warheads expected between 2034 and 2036. But, Shane Ward observes, the United States’ need for the W93 ‘is not as urgent as the timeline suggests.’ Rather, the programme’s urgency ‘seems attributable to the United Kingdom’s nuclear modernization efforts’. In 2020, a senior civil servant at the UK Ministry of Defence told the Parliament's Select Committee of Defence that there is 'a close realignment' between the US W93 warhead and the new British warhead. He further explained that ‘[i]t’s not exactly the same warhead but…there is a very close connection in design terms and production terms. So we are intimately involved in that.’ In January 2022, a US Department of Energy fact sheet on the W93 programme stated that it was ‘vital for continuing the United States’ longstanding support to the United Kingdom’. These statements imply that the UK’s replacement nuclear warhead is inextricably linked to the status of the United States’ W93 programme, and that the degree of technical information-sharing will amount to indirect transfer under Article 1(1)(b) of the TPNW as well as under Article 1 of the NPT.
As discussed under the section below on the parallel prohibition under the TPNW on receiving transfer or control of nuclear weapons, another potential compatibility issue concerns the US B61 nuclear bombs that are stored in Europe but remain under the command and control of the United States. If, in a future war, full control over any of the bombs should be transferred by the United States to any of the host states Belgium, Germany, Italy, or the Netherlands, for loading and use in their dual-capable NATO-designated aircraft, this would contravene the prohibition on transfer in Article 1(1)(b) of the TPNW, and also the prohibition on transfer in Article 1 of the NPT.