U235 can decay after absorbing a very low energy but U238 needs a large energy to decay, so that a chain reaction used U235, right??
other questions, why U238 needs a large energy to decay but U235 only needs < 1eV. The stability of U235 is lower than U238 ?? or other reasons ~?
and a whole chain reaction only used U235? any other isotopes or elements and how can they decay?(absorbing the energy that produced by U235?)