International Panel on Fissile Materials

United Kingdom is a nuclear weapon state member of the Nuclear Non-proliferation Treaty. In 2010 it made a commitment to reduce the nuclear arsenal to "no more than 180 warheads." In 2015 the United Kingdom officially declared that its nuclear arsenal includes fewer than 120 "operationally available" nuclear warheads. In addition, about 60 warheads appeared to be kept in reserve and further 15-20 were awaiting dismantlement. The goal of 180 warheads was expected to be met in mid-2020s. In 2021, however, the United Kingdom reconsidered its previous commitment and announced that it "will move to an overall nuclear weapon stockpile of no more than 260 warheads."

The U.K. stock of fissile materials is estimated to be 119.6 tonnes of separated plutonium and 23 tonnes of unirradiated HEU. Most of UK's separated plutonium - 116.4 tonnes - is the material separated from spent fuel of power reactors.

Highly enriched uranium

The United Kingdom produced its military HEU at the Capenhurst Gaseous Diffusion Plant. The plant was producing HEU between 1952 and 1962. After that it continued to operate until 1982, producing LEU for power reactors. In addition to domestic production, the United Kingdom received about 13 tons of uranium-235 in HEU from the United States.

According to the official HEU balance, released in 2006, the total audited stock of military HEU was reported to be 21.86 tonnes as of March 31, 2002. The average enrichment of the material was not given.

The United Kingdom reported having 691 kg of civilian HEU as of the end of 2022. Of this amount, 554 kg is unirradiated HEU.

The United Kingdom operates a gaseous centrifuge uranium enrichment plant in Capenhurst. This facility is part of the URENCO consortium, which has an obligation to produce only material for civilian purposes.

Military plutonium

The main production site for UK military plutonium was the Sellafield complex, which hosted a total of six production reactors: the two Windscale Piles and the four Calder Hall reactors, and all reprocessing operations. The United Kingdom operated four additional dual-use reactors at Chapelcross, whose fuel was also sent to Sellafield for reprocessing. The first discharges of spent fuel from five nominally civilian Magnox reactors was also put towards the military stockpile.The two air-cooled graphite-moderated "Windscale Piles" were shut down after a graphite fire in 1957. The Calder Hall reactors were used to produce military plutonium until 1989. The Chapelcross reactors were used to produce plutonium until 1964 and tritium thereafter. Both sets of reactors were dual-purpose, that is, they also produced electric power. The two groups of reactors were shut down in 2003 and 2004 respectively.

In addition to its own production, the United Kingdom also exchanged some plutonium with the United States.

In 1998, the United Kingdom declared a stockpile of 7.6 tonnes of military plutonium. Subsequently, 4.1 tonnes of non-weapon grade plutonium and 0.3 tonnes of weapon-grade plutonium were declared as excess to military requirements and to civilian custody. This leaves 3.2 tonnes of weapon-grade plutonium that is either in weapons or available to weapons.

Civilian plutonium

The United Kingdom has separated large quantities of reactor-grade plutonium produced both by its commercial power plants and its fleet of dual-use reactors (Calder Hall and Chapelcross) when they were operated in civilian-mode. It also took ownership of some foreign-origin plutonium stored in the country. As of December 31, 2022, the amount of separated reactor-grade plutonium owned by the United Kingdom was 116.4 tonnes. This amount includes 4.4 tonnes of military-origin plutonium declared excess and moved to the civilan stock. As the UK has left the European Union, this material is not under EURATOM safeguards anymore.

In addition to its own plutonium, the United Kingdom stores 24.1 tons of foreign separated plutonium belonging to non-nuclear weapon states (21.757 tons of this material is owned by Japan).

The United Kingdom has operated two reprocessing plants, located at Sellafield. The THORP plant was closed down in 2018, the B-205 plant, which reprocessed metal natural-uranium fuel from first-generation "Magnox" reactors, was shut down in 2022.