Why do different nuclei need different neutron speed to achieve fission?? My understanding is that U-235 requires a thermal neutron in order to be likely to cause a fission reaction, however Pu-239 requires the additional energy of a fast neutron in order to cause the nucleus to undergo fission rather than just absorbing the neutron and becoming Pu-240. I also think that a U-235 nucleus is unlikely to absorb a fast neutron. (is this true?) Is it true that the strong nuclear force in the vicinity of the nucleons is responsible for the "capture" of a passing neutron? If so why should U-235 and Pu-239 be so different in terms of the likelihood of them capturing fast neutrons? Also, if a U-235 nucleus did absorb a fast neutron, would it be likely to split? Thanks for any help.