The Norwegian heavy water sabotage were a series of actions taken by Norwegian saboteurs during World War II to prevent the Germans from acquiring heavy water which could be used to produce nuclear weapons. The Vemork hydroelectric plant, site of the heavy water production in World War II, Nazi Germany investigated the possibility of building an atomic bomb. As with all nuclear weapon development, the main problem was securing enough "weapons grade" material, particular isotopes of either uranium or plutonium. In order to produce these materials, found only in tiny quantities in nature, one must either extract the uranium from natural ore, or "breed" plutonium in a nuclear reactor. The German scientists decided to use plutonium, as the critical mass was smaller, and the bomb itself theoretically much easier to construct. Unable to perfect a graphite-moderated reactor for plutonium production, they instead explored a heavy water based design. This could have been used to do bomb research, and, ultimately, to breed plutonium from which a bomb could be constructed. At the time, Europe's major supply of heavy water came from the Norwegian Vemork hydroelectric plant, run by Norsk Hydro, near Rjukan in the Telemark region. Between 1942 and 1944 a sequence of sabotage actions by the Norwegian resistance movement, as well as Allied bombing, ensured the destruction of the plant and the loss of the heavy water produced. These operations—codenamed Freshman, Grouse and Gunnerside—finally managed to knock the plant out of production in early 1943, basically ending the German research.