ON-THE-FLY DOPPLER BROADENING OF UNRESOLVED RESONANCE REGION CROSS SECTIONS VIA PROBABILITY BAND INTERPOLATION

摘要

A procedure for computing temperature-dependent unresolved resonance region cross sections in Monte Carlo neutron transport simulations is presented. This approach relies on the generation of equiprobable cross section magnitude bands on an energy-temperature mesh. Within a simulation, the bands are sampled and interpolated in energy and temperature to obtain cross section values on-the-fly. This is in contrast to the typical process of pre-generating probability tables at all discrete temperatures present in a model. As part of this work, a flexible probability table generation capability is integrated into a continuous-energy neutron transport code and tested in both differential and integral code-to-code comparisons with NJOY. Using the new implementation, integral benchmark simulations of the Big Ten critical assembly show that, for a system mat is highly-sensitive to the unresolved resonance region, interpolation intervals of ~300 K, spaced throughout the range of temperatures relevant to reactor physics modeling, are sufficient to reproduce the k_(eff) values that are obtained with probability tables generated exactly at a specific reference temperature to within ~10 pcm. These interpolation intervals are also shown to be sufficient to reproduce reference neutron flux energy spectra, tallied on an energy group structure that is commonly used in fast reactor simulations, to within ~0.25%. Regarding the computational efficiency of the procedure, ~10% reductions in particle simulation rates are observed and the necessary cross section data consume ~100 kB of computer memory per nuclide.