Implementation of the direct S(α,β) method in the KENO Monte Carlo code

Abstract

The Monte Carlo code KENO contains thermal scattering data for a wide variety of thermal moderators. These data are processed from Evaluated Nuclear Data Files (ENDF) by AMPX and stored as double differential probability distribution functions. The method examined in this paper uses S(α, β) probability distribution functions derived from the ENDF data files directly instead of being converted to double differential cross sections. This allows the size of the cross section data on the disk to be reduced substantially amount. KENO has also been updated to allow interpolation in temperature on these data so that problems can be run at any temperature. Results are shown for several simplified problems for a variety of moderators. In addition, benchmark models based on the KRITZ reactor in Sweden were run, and the results are compared with the previous versions of KENO without the Direct S(α, β) method. Results from the Direct S(α, β) method compare favorably with the original results obtained using the double differential cross sections. Sampling the data increases the run-time of the Monte Carlo calculation, but memory usage is decreased substantially.

Treatments of Thermal Neutron Scattering Data and Their Effect on Neutronics Calculations

“…Although several efforts have been made in Monte Carlo codes to directly use the TSL data (Čerba et al, 2013;Liu et al, 2016;Hart and Maldonado, 2017), for most cases, the TSL data should be first processed to calculate total scattering cross section and double-differential cross section and then converted to a specific format required by neutronics codes. For Monte Carlo codes, the obtained cross sections are converted to tabular data representing the energy and angle distributions of the secondary neutrons and stored in the ACE (A Compact ENDF) (Conlin and Romano, 2019) format library, whereas for deterministic-based codes adopting the multigroup approximation, the tabular data are further converted into multigroup cross sections and scattering matrices.…”

Rejection-based sampling of inelastic neutron scattering

“…On the other hand, in neutronics applications, e.g., [3], scattered neutron states are traditional sampled from the pre-processed double differential cross sections in either continuous or discrete forms for incident neutrons up to a few electronvolt. However, in recent years, attracted by its small memory footprint and the absence of geometrical limits to its validity, the method of directly sampling from scattering kernels [4] has been implemented in a few neutronic codes as well [5,6,7].…”