From 'B/A' against A curve, we know that Binding Energy per Nucleon in Uranium region is about 7MeV, which is almost equal to height of potential barrier opposing fission.

Now,U-238.in when a Neutron is added to Nucleus of ₉₂U²³⁵, we get ₉₂U²³⁶ i.e. an Even-Even nuclide. The pairing term in mass formula for Uranium is about 0.6MeV. Thus, we see that Thermal Neutron capture in U-235 results in total energy release of 7.6 MeV and since to cross potential barrier, only about 7 MeV is required. So U-235 can undergo fission by Thermal Neutron capture.

On the other hand, U-238 nucleus, after Neutron capture becomes a U-239 nucleus which is an Even-Odd Nuclide. Due to absence of pairing term for U-239, the fission barrier cannot be crossed. So Thermal Neutrons will not induce fission.

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