Sensitivity of 238U Resonance Absorption to Library Multigroup Structure as Calculated by WIMS-AECL

Abstract

In simulations of the TRX-1 experimental lattice, WIMS-AECL overpredicts, relative to MCNP, resonance absorption in neutron-energy groups containing the three large, low-lying resonances of 238U when a standard ENDFIB-V-based library is used. A total excess in these groups of 4.0 neutron captures by 238U per thousand fission neutrons has been observed. Similar comparisons are made in this work for the MIT-4 experimental lattice and simplified CANDU lattice cells containing 37-element fuel, with and without heavy-water coolant. Eleven different 89-group cross-section libraries were constructed for WIMS-AECL from ENDFIB-V data: only the neutron-energy-group boundaries used in generating multigroup cross sections and the Goldstein-Cohen correction factors differ from one library to the next. The first library uses the original 89-group structure, and the other ten involve energy groups of varying widths centred on the three large, low-lying resonances of 238U. For TRX-1, some reduction in total discrepancy in 238U capture can be achieved by using a new structure, although the improvement is small. The discrepancies in 238~ capture are of the same order for the MIT-4 case as those observed for TRX-1 for both the original group structure and the ten new structures. The WIMS-AECL calculation of 238U resonance absorption in the same ranges of energy for the simplified CANDU 37-element lattice are in better agreement with MCNP than they are for TRX-1 and MIT4: when the original structure is used, WIMS-AECL underpredicts total capture rate by 238U in the energy range of interest by only 0.56 per thousand fission neutrons (coolant present) and 0.88 per thousand fission neutrons (voided coolant channel). The discrepancies are reduced when some of the new structures are used. For almost all of the cases considered here-TRX-1, MIT4 and CANDU with coolant-better group by-group agreement of 238U capture around the 6.67-eV resonance is achieved by using a new library structure. One particular group structure is recommended for future library development.