Multiphysics simulation of fuel relocation for a single fuel pin during startup

Analyzing light water reactor (LWR) nuclear fuel performance requires complex interlinked multi-physics, especially during nuclear reactor startup The fuel deforms under irradiation and heating, affecting the fuel temperature, which impacts neutron interactions within the fuel During nuclear reactor startup, initial heating of nuclear fuel causes a temperature gradient across the pellet, inducing stress in the ceramic that causes the fuel to crack in a phenomena known as fuel relocation As the fuel relocates, the fuel-cladding gap decreases due to the increased fuel diameter, increasing heat transfer Traditional fuel performance codes use empirical models to model neutronics, whereas MAMMOTH, a multi-physics reactor analysis tool, couples the fuel performance code BISON with the radiation transport application Rattlesnake to more precisely model the interactions Put forth in this paper is a method for obtaining a linear heating rate from the Rattlesnake's power density distribution, which is then transferred to BISON from two different finite element fuel pin meshes to model fuel relocation during startup Furthermore, this paper makes a comparison between a standalone BISON model using traditional empirical models and a loosely coupled BISON and Rattlesnake model using MAMMOTH for both a two-dimensional (2D) axisymmetric and full three-dimensional (3D) quarter fuel pin assembly Both models agreed closely for the linear heat rate and pin power density with relocation occurring within the fuel rod during the first three hours of startup with similar displacements (authors)