Optimization of neutron energy-group structure in thermal lattices using ultrafine bilinear adjoint function

To solve neutron transport equation in multigroup approach, in addition to weighting function and number of energy groups, proper selection of the group boundaries have high importance for the accuracy of the calculations. In the current paper, the bilinear combination of forward and adjoint neutron spectra is used for the optimization of 69 energy group structure of WIMSD5 lattice physics code. To remedy the energy self-shielding effect, homogeneous adjoint and forward BN equations on an ultrafine energy group structure have been solved to obtain the ultrafine forward and adjoint spectra. The coarse group intervals are selected to have equal values of bilinear function in each energy interval. In order to verify the validity of the optimized structure, infinite multiplication factor and fission rate values are calculated using WIMSD5 code for two main types of thermal critical lattices: Mixed Oxide (MOX) and Uranium Oxide (UOX). A Comparison of the results with MCNP-4C code shows that the optimized 69 group structure improves the infinite multiplication factor and fission rate in the mentioned thermal lattices. •A new approach is introduced to optimize thermal lattices energy group structure using contribution theory.•The ultrafine forward and adjoint B1 equations are solved to construct ultrafine adjoint bilinear function.•Required ultrafine data are extracted from ENDF/B-VI.8 using NJOY data processing system.•The optimized structure gives more precise results comparing with the original structures.