Sources of HEU in the United Kingdom

The UK effort to re-establish the production of HEU for miitary purposes appears to be related to the uncertainty about the future supply of HEU from the United States.

The United Kingdom was producing military HEU between 1955 and 1962 at the gaseous diffusion plant at Capenhurst. In addition, it was receiving HEU from the United States under the terms of the 1958 Mutual Defense Agreement (see Global Fissile Material Report 2010, pp. 72-73). After the domestic production was stopped in 1962, the United States became the only source of HEU for the United Kingdom. The transfer of the material has been regulated by the Mutual Defense Agreement, which was concluded in 1958 and has been regularly amended since then, with the most recent amendment signed in 2024. The agreement explicitly permits the United States to transfer nuclear material "for research on, development of, or use in atomic weapons" (Article IIIbis.A.3), and since a 1984 amendment also allows the transfer of enriched uranium and enrichment services "for military purposes" more broadly (Article IIIbis.C). Transfer of HEU for naval fuel is separately permitted (Article III.B). Transfers of material between the two countries are a regular occurrence.

The United States stopped the production of HEU for weapons in 1964 and ended all HEU production in 1992. After the shutdown and dismantlement of its gaseous diffusion plants, the United States has no enrichment facilities that can produce HEU. The United States has been relying on HEU declared excess to produce naval fuel since the 1990s. In 2018, however, it started exploring the option of building a new enrichment facility to produce HEU for military purposes (see also the discussion in 2020). After an exploratory study launched in 2024, BWXT was awarded a contract to build the pilot enrichment plant. This indicates that the United States is not planning to rely solely on its reserve of excess HEU.

Urenco treaties

The United Kingdom has an operating commercial enrichment plant at Capenhurst, which is owned and operated by the multinational consortium Urenco. The United Kingdom holds one third of its shares, with the rest held by German utilities and the Netherlands. The consortium operates under the framework established by the Treaty of Almelo in 1970. Urenco plants use gas centrifuges that are currently supplied by the Enrichment Technology Company (ETC), which uses the technology largely developed at Urenco. Today, ETC is owned in equal parts by Urenco and the French company Orano. The legal framework for the operations of ETC on issues related to the development and production of gas centrifuges was established by the Treaty of Cardiff, concluded in 2005.

The treaties that regulate the Urenco and ETC activities impose certain restrictions on the use of its facilities and technology for military-related HEU production. They do not, however, make it legally impossible.

The Treaty of Almelo explicitly prohibits Urenco enterprises from the production of "weapons grade uranium for the manufacture of nuclear weapons or other nuclear explosive devices" (Article VI.2). This provision, however, does not prohibit the use of the Capenhurst plant for the production of HEU for non-weapon military applications, such as naval reactor fuel.

Article VII(1) of the Almelo treaty also mandates the application of "appropriate safeguards procedures" to all facilities of the consortium. While this provision appears to limit the use of the Capenhurst plant for military purposes, in practice it probably does not. For the United Kingdom, the "appropriate safeguards procedures" would be those included in its Voluntary Offer Agreement (VOA) with the IAEA, INFCIRC/951. (At the time the Almelo treaty was signed these also included Euratom safeguards, but the United Kingdom has left Euratom as part of the Brexit process.) In practice, the Almelo Treaty condition will prevent the removal of the Capenhurst plant or any of its cascades from the list of facilities that are subject to IAEA safeguards. However, should the plant produce some military material, the United Kingdom could legally withdraw it from IAEA safeguards. Urenco's corporate structure is likely to be a more serious obstacle here, since it is unlikely that the consortium partners would approve this arrangement. Since Urenco was created specifically to develop commercial applications of the enrichment technology, operating a facility that produces material for military purposes would be incompatible with the foundational principles of the consortium.

In any event, a more practical option for military HEU production would be the construction of a separate enrichment facility dedicated to this task. The Almelo Treaty prohibits the parties from "the commercial exploitation of the gas centrifuge process" outside of the consortium (Article III(1)(a)). It does not, however, explicitly limit the use of the technology for non-commercial purposes, which the production of military HEU would represent. This dedicated facility could operate outside of the Urenco corporate structure and would not be subject to the Article VII(1) safeguard requirements. Moreover, since the United Kingdom is a nuclear weapon state, the Almelo Treaty alone would not constrain the use of the information or equipment developed by the cooperation for military purposes (including nuclear weapons). The treaty applies this constraint either to non-nuclear-weapon states (Article VI(1)) or to "joint industrial enterprises" (Article VI(2)).

The Treaty of Cardiff does not add any restrictions on the use of the centrifuge technology supplied by ETC for military applications in the United Kingdom. Like the Treaty of Almelo, it prevents parties from using the centrifuge technology outside of the partnership, but it applies only to commercial activities. Article IV of the treaty, which covers peaceful uses, only imposes limits on non-nuclear-weapon states (Article IV(1)) and on France, requiring that facilities built on its territory do not produce weapon-grade uranium for nuclear weapons (Article IV(2)). Neither does the Cardiff Treaty impose additional safeguards obligations on the United Kingdom.

Potential limits on UK enrichment

The way the Treaty of Almelo and the Treaty of Cardiff are written does not give parties the legal power to prevent the construction of a dedicated military enrichment facility by the United Kingdom. Neither the Joint Committee created by Almelo nor Cardiff's Quadripartite Committee has the explicit power to deny the supply of centrifuges to one of the parties or to attach additional conditions to this supply (this power exists for external parties and was used in the case of the United States via separate treaties). It should be noted, though, that the supply guarantee included in the Cardiff Treaty (Article II(6)) would not apply to a potential UK military plant.

Although the treaties are silent on the issue of a dedicated military facility in the United Kingdom, the parties have a procedural way of blocking the supply of centrifuges for this purpose through ETC's corporate structure. Since ETC is owned in equal parts by Urenco and Orano, any UK partner--the Netherlands, Germany, or France--can instruct its representatives to vote accordingly. However, they can also collectively decide to permit the military use of ETC technology.

While these decisions are likely to be made at the level of governments, they would technically be an internal corporate matter rather than something that touches on any specific treaty obligation. It is therefore unclear to what extent such decisions would be open to public scrutiny and discussion.

The reaction to the request from the Netherlands suggests that Urenco partners are likely to approve the UK request. In a 29 June 2026 letter to the parliament, the Minister of Foreign Affairs stated that at the time Urenco was established "the United Kingdom reserved for itself the possibility of using Urenco's technology for national defence purposes" (translated from the Dutch). It is not clear if this reservation was documented at the time, but the Almelo Treaty was structured in a way that indeed leaves that possibility.

The letter further stated that "Urenco is obliged to cooperate with [the UK] request" and offered what appears to be an approval of the UK request for assistance:

The cabinet is of the opinion that the ambition of the British government contributes to the collective security of the Netherlands and its allies, and makes no further statements regarding the nuclear defence programme of the United Kingdom.

The memo that accompanied the letter stated that the request is compatible with the NPT and the Almelo Treaty and that "[f]urther details cannot be made public given the nature of this subject." The government did not indicate that it intends to seek an approval from the parliament.

Germany, the third party to the Almelo Treaty, has not issued any public statement regarding the UK request yet. This may signal that the government is still developing its position, but it is also possible that Germany has chosen to avoid public discussion of the issue. The absence of an official reaction can also be attributed to the German government's position, which is different from the Dutch one. The German stake in Urenco is held by two utilities, RWE and E.ON, rather than by the federal government, so the government faces less pressure to comment publicly.

France, a party to the Cardiff Treaty that has a say in decisions regarding ETC technology, is extremely unlikely to object to a decision to provide ETC centrifuges to a UK military-oriented plant. The reason is that France's only enrichment plant, Georges Besse II, which uses ETC centrifuges, is a source of enriched uranium (LEU in France's case) for naval reactors.

All this suggests that the UK project is likely to proceed. The most practical path for the program would be the construction of a dedicated facility that will use ETC centrifuges to produce weapon-grade uranium, as required by the UK submarine reactors.

A return to weapon-grade uranium production

The resumption of the production of weapon-grade uranium in one of the NPT nuclear weapon states would be a significant development that could complicate the efforts to curb the production of fissile materials for weapons. Today, four of the five nuclear weapon states--France, Russia, the United Kingdom, and the United States--maintain a voluntary moratorium on the production of fissile materials for weapons. China is believed to have stopped the production as well but resists committing to a moratorium. Even though France and Russia continue producing enriched uranium for their naval reactors, neither country needs weapon-grade HEU for this purpose. France uses LEU and Russia is believed to be using HEU with lower than weapon-grade enrichments.

What sets the UK project apart is the fact that UK naval reactors use weapon-grade HEU in their fuel. This means that if the United Kingdom wants to maintain the credibility of its moratorium, it would have to implement measures to prove that the material it produces is not diverted to weapons. The United States will face this problem as well once it resumes the production of HEU for its naval reactors.