Chelyabinsk-65 (in the past Chelyabinsk-40), currently Ozersk, was established in 1948 to produce plutonium for nuclear weapons. The construction of the Combine 817, currently the Production Association Mayak, began in 1947. The first production reactor (Reactor A) went into operation in June 1948 and the first batch of plutonium was produced at the radiochemical plant (Plant B) on February 26, 1948. This plutonium was fabricated into nuclear device components at the plutonium finishing plant (Plant V). The Chelyabinsk-65 nuclear complex continued to expand during the next four decades and is currently one of the largest and most diverse nuclear facilities in Russia.
The industrial area bordering the southeastern shore of the lake Kyzyltash, where the plutonium production complex is located, is approximately 90 km2.
The production of plutonium in Chelyabinsk-65 took place in five uranium-graphite reactors (A, IR-AI, AV-1, AV-2, and AV-3) commissioned between 1948 and 1955. The uranium-graphite reactors were shut down between 1987 and 1990.
http://www.slate.com/articles/health_and_science/science/2012/08/mars_rover_curiosity_its_plutonium_power_comes_courtesy_of_soviet_nukes_.html