 The prospect of more-efficient nuclear weapons impressed Oppenheimer, Teller, and Hans Bethe, but they decided that an expert on explosives would be required.Kistiakowsky's name was immediately suggested, and Kistiakowsky was brought into the project as a consultant in October 1943.The implosion project remained a backup until April 1944, when experiments by Emilio G. Segrè and his P-5 Group at Los Alamos on the newly reactor-produced plutonium from the X-10 Graphite Reactor at Oak Ridge and the B Reactor at the Hanford site showed that it contained impurities in the form of the isotope plutonium-240.