Neutronic and thermal-mechanical coupling analyses in a solid-state reactor using Monte Carlo and finite element methods

Abstract

Improved computer capabilities and parallel algorithms have enabled high fidelity coupled analyses of classical nuclear power plant designs, which can also be applied to analyze the new heat pipe cooled reactor design. The neutronic/thermal coupling is a key part of multi-physics, high-fidelity reactor simulations. In a heat pipe cooled reactor, the solid-state core can reach temperatures over 1000 K which causes significant thermal expansion. The expansion then affects the neutronic and thermal conditions. Therefore, the mechanical conditions should also be carefully considered in the highly coupled solid-state system. The Monte Carlo program RMC and the commercial finite element program ANSYS Mechanical were used to develop coupled solutions of the neutronic (N), thermal (T), and mechanical (M) effects in a heat pipe cooled reactor with the ANSYS APDL interface used to link the two programs. The neutronic (N) and thermal–mechanical (T-M) coupled strategy was based on an iterative solution of the nonlinear governing equations. The method was used to study the reactivity changes occurring during startup of the MegaPower heat pipe cooled reactor. The most important effect of the temperature reactivity feedback during startup was the fuel temperature Doppler effect, while the monolith and reflector Doppler effects were much smaller. The radial expansion of the monolith and the reflector and the axial expansion of the fuel pellets have the most significant effects on the mechanical reactivity feedback. The results further show that the thermal–mechanical feedback in the solid-state reactor provides a negative reactivity feedback (-1500 pcm) and yet deteriorates the heat transfer due to the expansion. The peak fuel temperature increases 11 K when the thermal/mechanical feedback is considered. Therefore, the N/T-M coupling must be considered to produce conservative designs.