WHy does U-238 undergoes fission with a fast moving neutrons and not slow moving neutron?
Answer I’ll try to answer these in order: 1) U-235 can fission with a fast neutron, it just fissions far more efficiently with a slow neutron. The probability is there, but the capture probability for slow neutrons is radically higher because the quantum wavelength of slower-moving particles is much larger. Therefore the probability for a neutron to interact is much higher. 2) U-238 is generally stable when interacting with a slow neutron, because the most likely outcome is for it to capture that neutron and form U-239. It’s the inherent stability of the product nucleus that matters in this case. A fast neutron, however, can add enough energy to excite the system’s vibrational modes and separate into two fission products. 3) I think you meant that U-238 is more stable than U-235. It turns out that heavier nuclei have increasing numbers of neutrons relative to protons to remain stable, so it’s not quite as simple as more neutrons making a nucleus less stable. Both nuclei, for example, h
