It seems that there were, after all, factories at both Drigg and Sellafield.
According to a book I'm reading, building of the Drigg factory was started in Jan 1939 and completed in Feb 1941. The Sellafield Roayal Ordnance Factory opened in May 1943.
As my grandad died in Jan 1942, he must have worked at Drigg. His death announcement in the Whitehaven News said "of 79 Scotch Street, late of Lowca" so I guess he left the pit in 1941 to start at Drigg.