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.

The reactors are spent-fuel free and are currently undergoing dismantlement of primary loop equipment such as heat exchangers and hot gas ducts.

Both Chapelcross and Calderhall were the only nuclear power stations built as part of the UK's gas reactor fleet to use cooling towers as a heat sink as opposed to using a body of water.

It was initially owned and operated by the Production Group of the United Kingdom Atomic Energy Authority (UKAEA) until the creation of British Nuclear Fuels Limited (BNFL) in 1971 by an act of Parliament.

[2] Ownership of all of the site's assets and liabilities was transferred to the Nuclear Decommissioning Authority (NDA), a new regulatory body created by the Energy Act 2004.

Several significant events in 2001 persuaded BNFL to upgrade the fuel routes of both Calder Hall and Chapelcross to near modern standards at a cost of tens of millions of pounds, to guarantee that a License Instrument would be granted by the Nuclear Installations Inspectorate (NII) to permit final defuelling: the engineering work was carried out by BNS Nuclear Services (formally Alstec).

The Health and Safety Executive (HSE) granted consent to carry out decommissioning projects at Chapelcross under the regulations to Magnox Electric Ltd on 26 September 2005.

The first visible sign of decommissioning was controlled demolition at 09:00 BST on 20 May 2007 of the four natural-draught concrete cooling towers, which were of the same hyperboloid design as conventional inland power stations such as Didcot, Drax, Ferrybridge and Fiddlers Ferry.

A large part of the shell of tower 1 managed to resist the explosives despite having a visible bulge that resulted from a construction anomaly.

Two of the reactors were used to produce tritium for the UK's nuclear weapons and required enriched uranium fuel to offset the neutron absorbing effect of the lithium target material.

This was achieved by neutron bombardment of lithium target material, with the tritium gas extracted in the Chapelcross Processing Plant (CXPP).

[11] A small amount of Magnox depleted uranium (MDU) leaked from some corroded mild steel drums due to rainwater ingress and leaching.

MDU is a dense yellow powder that is less radiologically toxic than naturally occurring uranium but chemotoxic in a similar manner to lead.

Remote TV camera inspections revealed that twelve of the elements had fallen just over 80 ft (24 m) down the discharge well into a water-filled transport flask at the bottom.

[13] Because of known shrinkage of the graphite moderator bricks in the core due to in-service irradiation effects, some of the steel charge pans on top of them had become dislocated from their design position in the interstitial channel and were suspended from the Burst Can Detection (BCD) pipework.

BNFL were unable to make an adequate safety case or effect an economic repair and therefore, Reactor 1 did not return to power from its annual outage in August 2001.