 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 . This has a far higher spontaneous fission rate and radioactivity than plutonium @-@ 239 . The cyclotron @-@ produced isotopes , on which the original measurements had been made , held much lower traces of plutonium @-@ 240 . Its inclusion in reactor @-@ bred plutonium appeared unavoidable . This meant that the spontaneous fission rate of the reactor plutonium was so high that it would be highly likely that it would predetonate and blow itself apart during the initial formation of a critical mass . The distance required to accelerate the plutonium to speeds where predetonation would be less likely would need a gun barrel too long for any existing or planned bomber . The only way to use plutonium in a workable bomb was therefore implosion . 
