Countries: United Kingdom


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 3.2 tonnes of military plutonium and 21.9 tonnes of military HEU. The HEU stock also includes 734 kg of HEU that is reported as civilian in an annual declaration submitted to IAEA. Based on the IAEA declaration, the United Kingdom owned 115.8 tonnes of separated reactor-grade plutonium as of December 31, 2019.

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 734 kg of civilian HEU as of the end of 2019. Of this amount, 598 kg is unirradiated HEU.

The United Kingdom operates a gaseous centrifuge uranium enrichment plant in Capenhurst. This facility is part of the URENCO consortium and has been placed under Euratom safeguards.

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, 2019, the amount of separated reactor-grade plutonium owned by the United Kingdom was 115.8 tonnes. This amount includes 4.4 tonnes of military-origin plutonium declared excess and moved to the civilan stock.

The United Kingdom currently has one reprocessing plant, located at Sellafield. The B-205 plant reprocesses metal natural-uranium fuel from first-generation "Magnox" reactors, the last of which was shut down in 2015. It is expected to be shut down in 2021.