Why thermal reactors?
The question is pertinent since the neutrons at birth in fission are very fast with an average kinetic energy of about 2 MeV, and the neutrons have to be slowed down artificially, to have a thermal reactor. The answer lies in the neutron-nuclear reaction properties. A reactor works mainly based on fission chain reaction. Assume that every fission gives one neutron to continue the chain, the excess if any being lost somehow. Let s denote probability of a neutron reaction with one nuclide, with a suffix indicating the reaction, and the superscript indicating the nuclide. Let us analyze sfU-235/scU-238 curve vs. energy given in Fig.1. For the fissions to be more than captures, NU-235 sfU-235/NU-238scU-238 must well exceed unity. N represents the isotopic concentration. In the case of natural uranium NU-235/NU-238 is about 0.7/99.3 (~0.00705) and so sfU-235/scU-238 must be more than 142. This condition is satisfied only in the slow region leading to the concept of thermal reactors. In the
