Verification of PHITS components and estimation of their numerical-implementation uncertainty for neutron and Photon transport below 20 MeV

We verified components of the Particle and Heavy Ion Transport code System (PHITS) for neutron and photon transport below 20 MeV, an energy range selected to cover reactor-related shielding applications and the conventional low-energy neutron nuclear-data regime. The mathematical models and computational algorithms implemented in these components were systematically described, and the numerical uncertainties associated with their implementation were evaluated. The code uncertainty associated with the numerical implementation of the verified components was conservatively estimated to be below 0.2%. This estimate excludes Monte Carlo statistical uncertainty and application-dependent systematic uncertainties arising from physics-model selection, evaluated nuclear data, user-selected approximations, and input parameters. In addition, this study provides a traceable verification procedure and a transparent code uncertainty budget for interpreting PHITS simulation results, while presenting a framework applicable to other Monte Carlo radiation transport codes.