This factor is a measure of how well the design uti­lises the thermal neutrons in the moderator in getting as many as possible to where it matters, i. e., into the fuel.

_ number of thermal neutrons absorbed in the fuel total, number of thermal neutrons absorbed

In Fig 1.13 it is supposed that at some instant there are ni thermal neutrons in the moderator. A fraction of the ni neutrons is absorbed in the fuel thus giving fni thermal neutrons in the fuel [870 thermal neutrons for f = 0.87 say]. There will be (1 — f)m neutrons lost by capture in reactor materials other than U-238 and U-235.

Clearly, the more fuel there is in the reactor rela­tive to the moderator the higher the value of f; ultimately f = 1 for 100% fuel. Conversely the more moderator there is the smaller the value of f; ulti­mately f = 0 for 100% moderator.

Hence we may conclude that the value of f decreases progressively from unity towards zero as more and more moderator is added to a given amount of fuel.