Neutron Interaction with Matter

Abstract

The design and operation of a nuclear reactor requires the ability to determine how neutrons are distributed within a reactor core. This can be achieved by deriving and solving “neutron balance” equations which include all the relevant neutron loss and gain rates. The first step in such a derivation must be the development of a method by which the rates of neutron interaction with matter can be calculated. That is the subject of this chapter. It is a broad and quite complicated subject. One of the main complications is that it is not realy appropriate to speak of the neutrons that will be present in a reactor as if they were a single entity. As shown in Chap. 2 during the discussion of fission events, the neutrons generated by fission events have a wide distribution of energies. The neutron energies will become even more varied as the neutrons interact with reactor materials. The kinetic energy that a neutron posesses has a very large influence upon the rate at which it interacts with other nuclei. Thus the calculation of a single interaction rate which represents all neutrons in a core requires carefull averaging procedures and even then has limited usefulness. As you will see, it turns out to be more appropriate to evaluate several reaction rates which apply to neutrons of various energies. Such “multi-energy” group cross sections will be considered in Chap. 5. In this chapter the basic definitions and concepts which are required to evaluate neutron-neuclei interaction rates are presented.