Performance Enhancement of Anisotropic Scattering Treatment in MOC Calculation by Angular Flux Storage

In order to incorporate anisotropic scattering in the Method of Characteristics (MOC) transport calculations, explicit anisotropy treatment schemes that employ angular flux moments associated with spherical harmonics were implemented in numerous direct whole calculation codes such as nTRACER and its effective has been examined with various benchmark problems. In this method, region averaged anglur flux moments such as scalar flux and current are calculated during the ray tracing calculations. In this on-the-fly moment calculation, proper angle dependent weight terms are necessary that are derived using spherical harmonics. Although higher order expansions would give more accurate solutions, it turned out that normally second order expansion could provide quite accurate results in PWR analyses. In two dimensional (2D) MOC calculations, additional five angular flux moment terms are required for the second order expansion and the calculation of these terms can induce significant computational overhead during the ray tracing calculation. The angular flux storage scheme (AFSS) for the MOC calculations with the scattering anisotropic treatment with spherical harmonic expansion was implemented successfully in the nTRACER direct whole core calculation code. It was noted that although the AFSS shows slightly worse performance for the case of P₀ MOC calculations, it renders about 50% computing time saving in the case of P₂ MOC calculations