The fission of a nucleus basically requires the supply of a minimum energy. If a neutron is attached to a nucleus an energy is released consisting of the kinetic energy of the neutron and the binding energy of this neutron to the nucleus. If this energy is greater than the excitation energy for a fission of this nucleus, the nucleus may fission. For U-235 the excitation energy for fission amounts to 5.7 MeV, the binding energy of the attached neutron is 6.5 MeV, so that even neutrons with very low kinetic energies (e.g. thermal neutrons with a kinetic energy of only 0.025 eV) may trigger the fission. The ratios of excitation energy and binding energy are similar for the nuclei of U-233, Pu-239 and Pu-241. For U-238 and Th-232, on the other hand, the excitation energy required for fission is 6.5 MeV and thus much higher than the binding energy of the attached neutron of 4.8 MeV, so that fission of the nucleus is possible only if the neutron has a kinetic energy of at least 1.7 MeV. Spontaneous fission is also possible in the case of some very heavy nuclei.

nucleus of an atom Excitation energy for fission MeV binding energy of the last neutron MeV

Th-232

6.5

4.8

U-233

6.2

6.8

U-235

5.7

6.5

U-238

6.5

4.8

Pu-239

5.8

6.5

Pu-240

6.2

5.2

Pu-241

5.6

6.3

Excitation energy for fission