2018 | AFC ACCOMPLISHMENTS 164 3.4 FUEL PERFORMANCE MODELING Modeling Porosity Migration in LWR and Fast Reactor MOX Fuel Using the Finite Element Method Principal Investigator: Stephen Novascone Collaborators: Pavel Medvedev, John W.Peterson,Yongfeng Zhang, Jason Hales An engineering-scale finite element simulation of pore migration in oxide fuel has been developed.The porosity field is governed by an advection-diffusion equation which is coupled to the fuel temperature and stress fields through the thermal conductivity and volumetric heat source term.The engineering-scale porosity equation models the microscopic process of vapor transport of fuel across pores, taking into account thermal and vapor pressure gradients within the fuel. In the simulations, the porosity is initialized to a constant value at every point in the domain, and as the temperature gradient is increased by application of a heat source, the pores move up the thermal gradient and accumulate at the center of the fuel in a time frame that is consistent with experimental observations. Results from representative simulations are provided to demonstrate the new capability, and we show that a sufficiently high power ramp rate limits restructuring and leads to a corresponding increase in fuel temperature. Results from multidimensional simulations are also presented. Project Description: After start-up and attainment of full power and nominal operating temperature, ceramic, or mixed oxide fuel (UO2 and MOX) experiences micro- and engineering-scale restructuring, which has a profound effect on the bulk properties of the fuel. Restructuring mostly affects MOX fuel in fast reactors due to the higher temperatures, but can be relevant to light water reactors if temperatures go outside of normal operating ranges, such as during a loss of coolant accident. As such, the objective of this study is to consider oxide fuel in general, using models from a variety of sources. Here, restructuring means that the fuel pellet develops distinct regions from fuel center to fuel surface. Going from the center of the fuel outward in sequence, these regions are characterized by a central void, a region of increased density, and then a region with radially aligned grains and un-restructured grains.These regions, which depend on both the temperature value and temperature gradient, form radial isosurfaces within the fuel.The salient feature