Natural uranium is 0.711 wt% U-235 and the balance U-238. It is the U-235 that readily fissions. U-238 can fission with higher neutron energies but most often will readily absorb a neutron in a nuclear reactor and become Pu-239, Pu-240, Pu-241, and Pu-242. Pu-239 and Pu-241 are readily fissionable with thermal (low energy neutrons). In US light water reactors the uranium is processed as UF6 (uranium hexafluoride) and enriched to not more than 5.0 wt% U-235 including tolerances. In a bomb it would be enriched to more than 90 wt% U-235 and it would be pure metal in its final form. In a light water reactor the UF6 is enriched and then converted to U3O8 and then to UO2. U3O8 is known as yellow cake. Plutonium is more sensitive to neutron fissioning by the fast and thermal neutron energy spectrum. As a result it is a better bomb material for the classical atom bomb. In a hydrogen or thermonuclear bomb a fission trigger is used to create a very high temperature using gamma rays from fission to fuse the hydrogen isotopes of deuterium and tritium. Fusion is the mechanism the sun uses to produce energy. Uranium 238 is the most massive naturally occuring isotope on the planet. It has a half life of 1.2 billion years so it is still around. Plutonium 239 only has a half life of 23400 years approximately so it has all decayed away. That is why plutonium and higher mass elements are considered “man made” because they are only produced in a nuclear reactor. More than you want to know I am sure. I B A Nuclear Engineer of 34 years in reactor design and fuel management.