Multi-physics analysis of a supercritical water reactor with improved MCNP modeling

• An example SCWR assembly was analyzed with MCNP and a custom-developed single-pin-cell code.
• Thermal scattering effects and MAKXSF-derived cross-sections were included in the reactor physics analysis.
• A significant reactivity effect ($2) was noted between this detailed treatment and conventional (Doppler broadening-only) analysis.
• Neutron flux profiles are also changed significantly based on a more detailed reactor physics treatment.

Supercritical water reactors (SCWR) are proposed Generation IV light water reactors where supercritical water acts as the working fluid. Due to the large density gradients in the coolant, evaluation of reactor designs requires a detailed multi-physics analysis. In this work, MCNP is coupled to a single channel analysis tool to analyze a fuel assembly design whose neutronics performance has been experimentally validated. Accurate modeling of the reactor physics in a SCWR requires a detailed temperature treatment, including the effects of Doppler broadening (TMP cards in MCNP), thermal scattering (MT cards), and fully temperature-dependent cross-section sets for fuel and coolant (using the MAKXSF code). There is a $2 difference in keff between this highest-fidelity approach and a conventional treatment that addresses only Doppler broadening. An analysis is performed to determine the size of the temperature intervals for custom-developed cross-section sets, which indicated the necessity of 50 K bins in the fuel and 10 K bins in the coolant.