Development of an improved model for predicting fission product release from high temperature gas cooled reactors

Gas Turbine-Modular High Temperature Gas Reactors have the capacity to supply low cost electric power and high temperature process heat for industrial applications. The important factor in keeping the cost of the reactor system low is the high degree of inherent safety of the reactor core under normal operating and extreme accident conditions. The reactor fuel has a enormous capacity to retain fission products in the fuel compact. This work has developed an improved fuel performance model for predicting fission product retention under normal operating and extreme accident conditions. While the model has been developed for Pebble Bed Reactor (PBR) fuel, it also provides information pertinent to the performance of U.S. Modular High Temperature Reactor Fuel.

PBR fuel particles consist of a sphere of Uranium Oxide (radius=250µm) coated by successive layers of pyrocarbon, silicon carbide and pyrocarbon. These particles are embedded in a sphere of matrix graphite. The improved fuel performance model predicts fission product release by treating diffusion of the fission products out of the fuel kernel, through the various coated particle layers and out of the fuel element. It also includes release due to contamination in the matrix graphite, contamination of the outer PyC layer, and possible fracture of coated particle layers. The emphasis here is on the diffusive release of fission products under normal operation and accident conditions. In general, the primary mechanism for the release of fission products from the fuel is diffusion. Overall, this work has developed a phenomenally sound model for the release of fission products by numerically solving the Fickian Diffusion Equation under several different conditions. Good agreement was obtained between calculated and measured data from experimental fuel results for PBR fuel consistent with both normal operation and accident conditions.