The Combined Slowing-down and Diffusion Equation

The purpose of the present chapter is to develop a more general model for describing the neutron population in a reactor, a model which combines the principal features of the two already presented. That this is indeed a prerequisite for treating the general reactor problem may be 

The mass number of the moderator is sensibly larger than unity 

Consider a small volume dr of material in a medium supporting a multivelocity neutron flux at steady state, and let us confine attention to those neutrons whose lethargies lie in du about u. It is possible to write an expression for the neutron balance 

By calculating each of the above quantities on the basis of events per unit time, we may write the balance equation as follows  

w) du = number of neutrons with lethargies in du about u which escape per unit time from unit volume around r 

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