 Segrè's group measured it at 1.6 million fissions per gram per hour, compared with just 40 per gram per hour for plutonium-239.This meant that reactor-bred plutonium was unsuitable for use in a gun-type weapon.The plutonium-240 would start the chain reaction too quickly, causing a predetonation that would release enough energy to disperse the critical mass before enough plutonium reacted.