Windscale

In 1957, the graphite moderator of one of the air-cooled plutonium production reactors at Windscale (now Sellafield), had a fire which resulted in the first significant release of radioactive material from a reactor. The reactor served a second purpose at the time - production of Po-210 (polonium) from bismuth. Po-210 was also released. These gas cooled reactors were operated by the British government at the time.