science

Why is uranium-235 fissionable but uranium-238 not directly?

šŸ’” The Answer

  • U-235 can undergo fission when absorbing a slow (thermal) neutron.
  • U-238 requires fast neutrons with energy above 1 MeV to fission.
  • The difference arises from the odd number of neutrons in U-235.
  • U-238 often captures neutrons to become plutonium-239, which is fissionable.

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