Multi-scale Computational Model of Fission Product Transport in Silicon Carbide

Sarah Khalil, David Shrader, Izabela  Szlufarska, Dane Morgan, Materials Science Program

     Silicon Carbide has high chemical and thermal stability, as well as a low cross-section for neutron capture, and thus is used as a primary fission product barrier in the TRISO-coated (tristructural isotropic) fuels used in high-temperature gas-cooled nuclear reactors.  The fuel particle consists of a kernel of uranium dioxide embedded in a porous graphite layer and coated with a TRISO layer, which is a spherical composite of about 1mm in diameter.  The TRISO coating is comprised of a very dense pyrolytic carbon (PyC) layer for structure integrity, a SiC coating, and then another PyC layer. The SiC layer provides the main barrier for diffusion of fission products into the coolant.

     While SiC retains most of the fission products very well, it has been found that some elements (such as Ag) diffuse very fast through the TRISO-coating.  In order to address the problem of a quick release of fission products under normal and accident conditions, a thorough understanding of the mechanisms responsible for transport of these products through the SiC layer is essential.  We will build a multi-scale computational model to understand and predict the transport of select fission products through SiC.  The transport is modeled by solving the diffusion equation in complex microstructures, where the diffusion constants are derived from first principles and molecular dynamics (MD) simulations.  This project has three parts: molecular dynamics (MD) simulations of SiC will provide detailed structural information about relevant grain boundaries, quantum mechanical and MD calculations will be used to determine diffusion constants in these structures, and this information will be built into continuum level diffusion model of transport through digitized experimental microstructures.

Coated fule particle picture taken from http://httr.jaea.go.jp/eng/outline/fuel.html

     This project is done in a close collaboration with the experimental group of Prof. Todd Allen from the Department of Engineering Physics at UW-Madison.  Dr. Lizen Tan and Tyler Gerczak are performing experiments to establish microstructures and using diffusion couples to test predictions of our MD and quantum mechanical calculations.

     We gratefully acknowledge financial support from the Department of Energy, Nuclear Engineering Research Initiative (NERI), award number DE-PS07-06ID14762, and the US Nuclear Regulatory Commission.