Chapelcross nuclear power station is a former Magnox nuclear power station undergoing decommissioning. It is located in Annan in Dumfries and Galloway in southwest Scotland, and was in operation from 1959 to 2004. It was the sister plant to the Calder Hall nuclear power station plant in Cumbria, England; both were commissioned and originally operated by the United Kingdom Atomic Energy Authority. The primary purpose of both plants was to produce weapons-grade plutonium for the UK's nuclear weapons programme, but they also generated electrical power for the National Grid. Later in the reactors' lifecycle, as the UK slowed the development of the nuclear deterrent as the cold war came to a close, power production became the primary goal of reactor operation.

Magnox is a type of nuclear power / production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The name comes from the magnesium-aluminium alloy, used to clad the fuel rods inside the reactor. Like most other "Generation I nuclear reactors", the Magnox was designed with the dual purpose of producing electrical power and plutonium-239 for the nascent nuclear weapons programme in Britain. The name refers specifically to the United Kingdom design but is sometimes used generically to refer to any similar reactor.