The windscale reactors the UK used for Pu production were open cycle air cooled. Pull in air from the outside, run it through the core, and out via the chimney.

The windscale reactors the UK used for Pu production were open cycle air cooled. Pull in air from the outside, run it through the core, and out via the chimney. One morning in 1957 operators coming to work noticed thick black smoke coming from the chimney and thought "this can't be good" (it wasn't).