The critical mass of a bare mass sphere of 90% enriched uranium-235 is 52 kg. Correspondingly, the critical mass of a bare mass sphere of plutonium-239 is 8–10 kg. The bomb that destroyed Hiroshima used 60 kg of U-235 while the Nagasaki Pu bomb used only 6 kg of Pu-239. Since all Pakistani bomb designs are implosion-type weapons, they will typically use between 15–25 kg of U-235 for their cores. Reducing the amount of U-235 in cores from 60 kg in gun-type devices to 25 kg in implosion devices is only possible by using good neutron reflector/tamper material such as beryllium metal, which increases the weight of the bomb. And the uranium, like plutonium, is only usable in the core of a bomb in metallic form.