Prompt neutron lifetime in a strongly heterogeneous lattice

The prompt neutron lifetime I[sub p] is an important input parameter in the solution of the point reactor kinetics equations. Over the years, several methods have been developed to calculate the prompt neutron lifetime, or approximations to it. Many of these approximations, such as the thermal inverse velocity method, rely on conventional widsom, which dictates that for thermal reactors, the prompt lifetime is dominated by thermal neutrons, and consequently, little error accures from the use of the thermal neutron lifetime as an approximation. This is not necessarily true in strongly heterogeneous lattices such as those used at the Savannah River Site (SRS). Explicit calculations of the prompt neutron lifetime have shown that calculation of the thermal neutron lifetime can result in the overprediction of the prompt lifetime by as much as 15% in a typical SRS lattice. Therefore, a new method for calculating the prompt neutron lifetime has been developed. The alpha search method (ASM) utilizes the results of an alpha (time eigenvalue) search performed by the GLASS lattice physics code to compute the full-spectrum lifetime. This method has been found not to succumb to the same overprediction as the methods using the thermal neutron lifetime. The inverse velocity methodmore » and the problems associated with its use in strongly heterogeneous lattices as well as the formation of the new ASM are presented. The eigenvalue method has been chosen as a further comparison method, and results of these three methods for several different media are shown and discussed.« less