- 2012 Springer Science+Business Media, LLC. All rights reserved. The common approach of representing energy dependence of neutron-nucleus interactions consists of discretizing the energy dependence in a number of energy groups. During the fission reactions neutrons are emitted with an average energy of approximately 2 MeV. Neutron numbers are negligibly small above 10-15 MeV, making the maximum energy of interest in most nuclear fission reactors to be of about 15 MeV. Neutrons are then slowed down to much lower energies by collisions with the reactor components.