Extension of SARAX code system for reactors with intermediate spectrum

•The fast reactor code system SARAX was extended for the intermediate spectrum calculation.•The energy-partitioned resonance self-shielding treatment was used in the fast and intermediate energy range.•The pointwise energy-collapsing super-homogenization technique for group condensation was proposed. The fast reactor code system SARAX, which was developed at Xi’an Jiaotong University, has recently been extended for wider energy range in order to meet the requirements of new reactor designs with intermediate spectrum. The extension efforts mainly included three parts. First, the new pointwise and multigroup cross-section libraries considering the thermal scattering law were generated by the Atlas code and GESALIB interface. Second, the energy-partitioned resonance self-shielding treatment was used to consider the distinct resonance phenomena in the fast and intermediate energy range. The one-dimensional hyperfine group method was integrated for wide resonance self-shielding in the intermediate energy range, while the Tone’s method with narrow resonance approximation was used for resonance self-shielding in the fast energy range. Third, the pointwise energy-collapsing super-homogenization technique was proposed to obtain the reaction rate conservation in the pointwise-multigroup equivalence. The new SARAX system was verified against several problems with different neutron-moderating materials. Results showed that for the heterogeneous subassembly calculation, the eigenvalue error was less than 100 pcm, and the relative error of multigroup self-shielded cross-sections of major actinides was less than 3.1% compared to the results of Monte-Carlo code OpenMC. For the lead-cooled small modular reactor with beryllium reflector, the results of eigenvalue, Doppler constant, power distribution all agreed well with the OpenMC results.