(1) U-235 Explanation: U235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope U238 it is fissile, i.e., it can sustain fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature. If at least one neutron from U235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustpa itself, it is said to be critical, and the mass of U required to produce the critical condition is said to be a critical mass.