International Nuclear Event Scale
Windscale Pile No. 1 became operational in October 1950, just over three years from the start of construction, and Pile No. 2 became operational in June 1951. Later in 1957 Pile 1 suffered a major accident; the Windscale fire, the worst nuclear accident in UK history, ranked in severity at level 5 out of a possible 7 on the International Nuclear Event Scale.