Coupled neutronics-fuel behavior calculations in steady state using the Serpent 2 Monte Carlo code

This paper describes the recent developments in the multi-physics capabilities of the Serpent Monte Carlo code concerning coupled multi-physics calculations with fuel behavior feedback. We cover the structure and operation of the fuel behavior interface in Serpent 2 as well as the coupled calculation routines implemented for steady state multi-physics calculations with any internally or externally coupled solver. The intended solution flow and code-to-code communication in internally and externally coupled multi-physics simulations is described alongside with the stochastic approximation based solution relaxation methods implemented in Serpent. The two-level multi-physics coupling scheme in Serpent 2 is demonstrated by obtaining a coupled solution for the neutronics-fuel behavior problem using first the internally coupled FINIX fuel behavior module and then the externally coupled ENIGMA fuel performance code in a 3D assembly geometry. Parameters such as maximum pellet centerline temperatures can be evaluated from the coupled solution. The temperature fields obtained from the coupled solution are also used to estimate the effect of the detailed radial representation of the fuel temperature distribution compared to various radially averaged effective fuel temperature representations. The convergence of the coupled solution is investigated alongside the possibility to speed up the convergence by using the Uniform Fission Sites method.