Modelling Silver: Evaluation of German Experience

For direct cycle gas cooled high temperature reactor designs, operating conditions may be limited as a result of excessive maintenance dose rates caused by the Ag-110m source term on the turbine. It is therefore important to accurately predict silver release from fuel during reactor operation. Traditionally diffusion models were used to derive transport parameters from limited irradiation testing of fuel materials and components. Best estimates for all applicable German fuel irradiation tests with defendable uncertainty ranges were never derived. However, diffusion theory and current parameters cannot account for all irradiation and heat-up test results, and for some tests, it appears unacceptably conservative. Other transport mechanisms have been suggested, and alternative calculation models are being considered. In this paper the applicable German irradiation test results are evaluated with a classic diffusion model as well as an alternative model called the Molecular Vapour transport Release (MVR) model. New transport models and parameters for silver in fuel materials are suggested and compared.