Improvement on multi-group scattering matrix in thermal energy range generated by NJOY

This paper describes an improvement pertaining to the multi-group scattering matrix in thermal energy range, which is obtained by the NJOY code. By using the multi-group cross-section obtained by the original NJOY code, neutron spectrum in the thermal energy range shows considerable discrepancy from that of the continuous energy Monte–Carlo code, MCNPX. Extensive investigation of this issue reveals that multi-group scattering matrix generated by the NJOY code does not have enough accuracy in thermal energy range. Root cause is as follows. The scattering matrix in thermal energy range is tabulated at specific incident energy points that are implemented in the THERMR module of the NJOY code. Since the implemented energy grid is not sufficiently fine, numerical integration in the GROUPR module to obtain the multi-group scattering matrix causes considerable error. By increasing the number of incident energy points from 59 (original) to 349 (improved), the accuracy of the multi-group scattering matrix is improved. Consequently, the discrepancy of neutron spectra in thermal energy range between MCNPX and that obtained by the NJOY multi-group cross-section is resolved. The above issue has considerable impact on thermal reactor analyses using multi-group cross-sections generated by the NJOY code and incorporation of the present improvement is recommended.