Fissile and Fissionable Fuel

• Thus when a neutron is absorbed in U²³³, the neutron deposits its binding energy in the
  compound nucleus, which is more than what is required to make U²³⁴ to fission (refer to table shown on the previous page).
• Those elements which can be fissioned by zero energy neutrons are called fissile.
• Thus U²³³, U²³⁵ and Pu²³⁹ are fissile.
• On the other hand, Th²³² and U²³⁸ need neutrons with 1.4 MeV and 0.6 MeV respectively for
  inducing fission. Hence these are called fissionable nuclei.
• Natural Uranium has 99.3% of U²³⁸ and 0.7% of U²³⁵.