Noble metal mass transport model for molten salt reactor analysis in VERA-CS

Molten salt reactor (MSR) designs require robust multiphysics modelling capabilities to further the development of this next generation technology. Due to the unique liquid fuel design, traditional reactor physics analysis codes do not capture all of the important physics of these reactors. This work builds upon a general species transport model implemented into the multiphysics core simulator suite VERA-CS, by adding source and sink models for insoluble fission products, specifically the noble metals. Noble metal (NM) fission products are born in the fuel-salt, but do not form stable fluorides, and instead plate-out on various surfaces in the reactor fuel loop. A concentration gradient advection-diffusion based mass transfer model is implemented into the general species transport model within CTF – the subchannel thermal hydraulics code in VERA-CS – and verified using convergence tests in space and time. Two noble metal decay chains are analyzed, and their steady state bulk liquid densities and surface concentrations are calculated for a simple flow loop in CTF that is roughly representative of the Oak Ridge National Laboratory Molten Salt Reactor Experiment (MSRE) [1]. Insoluble species ⁹⁹Mo decaying to ⁹⁹Tc captures wall accumulation of decaying species, and insoluble ¹³¹Sb to semi-soluble ¹³¹Te to soluble ¹³¹I captures wall loss of decaying species. Results are compared with MSRE data and the possible multiphysics effects of this phenomena are identified. Future work to enhance the robustness of the physical model and couple it with neutronics to evaluate the effects of NM mass transport is discussed.