The Manhattan Project had been a program to produce a nuclear weapon. Along the way, promising ideas had been set aside. Norris Bradbury, who replaced J. Robert Oppenheimer as director of the Los Alamos National Laboratory in late 1945, revived such projects in order to entice scientists to remain at, or return to, Los Alamos. One of these projects was the "Super", a nuclear weapon using nuclear fusion, which Edward Teller's F-1 group had worked on under Fermi's direction. The technical problem was figuring out a way to get a fusion reaction to initiate and propagate, which required temperatures attainable only with a fission bomb. The hydrodynamic calculations involved were daunting, and ENIAC was used to run a computer simulation of the Super in December 1945 and January 1946.