TRISO-ATOPS: A Mechanistic Source Term Model for Gas-Cooled Reactors

Source term modeling for TRistructural ISOtropic (TRISO) fuel has been performed for previous reactor designs, but few are available in the open literature. Thus, there is a need to develop a simple, versatile, and mechanistic model of fission product release and transport in gas reactor cores that can be applied to a variety of reactors through user inputs and reactor-specific radionuclide inventories. To meet this need, the TRISO Analysis Tool for Predictive Source terms was developed. This model calculates the release of the key safety-important fission products by diffusion through the kernel, silicon carbide, and graphite based on fuel and graphite temperatures in the reactor under normal operation. These releases from the fuel enter the coolant where some can plate out on cooler surfaces. A cleanup model is included for designs with a coolant purification system for removing fission gases. This initial distribution of fission products in the reactor serves as an initial condition for potential releases under postulated accident conditions. From this initial condition, the model calculates the fission product release for any transient temperature profile, and the fission product releases can then be used to assess radiological dose to the workers and the public using conventional radiological dose tools. Data on the diffusion of fission products is based on historic German TRISO experiments and the more current Department of Energy advanced gas reactor TRISO fuel development program. The example cases in this work demonstrate the flexibility of the model.