Calder Hall was principally a source of weapons grade fissile material. The fact that it manged to keep a few light bulbs illuminated was a convenient spin-off!
Plutonium for the nuclear weapons programme was produced at Windscale until 1957 and reprocessed on site before being shipped to Aldermaston. Plutonium production at Calder Hall (on the Sellafield site) for the nuclear weapons programme began in 1956, and at Chapelcross in 1958, with reprocessing at Sellafield by the same facility used to reprocess spent fuel from the civil programme, before being shipped to Aldermaston. Both Calder Hall and Chapelcross were used to produce electricity for the national grid in addition to supplying material for the defence nuclear programme. (from
http://www.mod.uk/publications/nucle...accounting.htm - yes, really! Honest!!)