Improvements in Transport Calculations by the Energy-Dependent Fission Spectra and Subgroup Method for Mutual Self-Shielding

This paper shows two ways to improve the accuracy of the transport calculations. These improvements, implemented in the APOLLO2 code, concern the fission source calculation and the self-shielding models. The calculation of the fission source was generalized to fission spectra including an incident neutron energy dependence. The subgroup self-shielding model was updated for a mixture of resonant nuclides. Some tests on fast neutron systems like a critical sphere without reflector, a sodium-cooled cell, and a helium-cooled cell show that the use of four optimized incident macro groups for fission spectra guarantees a correct representation of the fission source.

The tests on a critical sphere with a thick steel reflector and on a water-moderated mixed oxide cell prove that the subgroup self-shielding, accounting for the mutual shielding of several resonant nuclides, allows us to improve the accuracy of the neutron transport solution.