Lavrentiy Beria led the Soviet atomic bomb project. He directed the construction of the Mayak Plutonium plant in the Southern Urals between 1945–48, in a great hurry and in secrecy as part of the Soviet Union's atomic bomb project. Over 40,000 Gulag prisoners and POWs built the factory and the closed nuclear city of Ozersk, called at the time by its classified postal code "Forty". Five (today closed) nuclear reactors were built to produce plutonium which was refined and machined for weapons. Later the plant came to specialise in reprocessing spent nuclear fuel from nuclear reactors and plutonium from decommissioned weapons.