A Two Overlapping-Group Transport Computational Method for Thermal Neutron Problems

This paper describes a method for solving the energy-dependent thermal neutron transport equation in X - Y geometry. Two variants of this method have been developed. In the first variant the flux over the whole thermal energy range is treated by the Buslik overlapping group technique. The trial function is of a form first introduced by Lancefield, involving two space-angle functions each multiplied by a trial spectrum. The space-angle functions are computed by solving two coupled transport equations, using S_N or P_N methods. Numerical experiments show that this first approach is not always adequate and that a more complicated, second variant must, sometimes, be used instead. In this second variant the thermal range is split into two bands. The upper band is treated as one neutron group, while the two-overlapping group method is applied in the lower band. Experience indicates that, even when the first method is inadequate, the second is accurate enough for most analysis work.