Development of a mechanistic fission gas release model for LWR UO2 fuel under steady-state conditions

A mechanistic model has been developed to predict the release behavior of fission gas during steady-state irradiation of LWR UO2 fuel. Under the assumption that UO2 grain surface is composed of fourteen identical circular faces and grain edge bubble can be represented by a triangulated tube around the circumference of three circular grain faces, it introduces the concept of continuous formation of open grain edges tunnels that is proportional to grain edge swelling. In addition, it takes into account the interaction between the gas release from matrix to grain boundary and the reintroduction of gas atoms into the matrix by the irradiation-induced re-solution of grain face bubbles. It also treats analytically the behavior of intragranular, intergranular, and grain edge bubbles under the assumption that both intragranular and intergranular bubbles are uniform in both radius and number density. Comparison of the present model with experimental data shows that the model's prediction produces reasonable agreement for fuel with centerline temperatures of 1000 to 1400 deg C, wide scatter band for fuel with centerline temperatures lower than 1000 deg C, and underprediction for fuel with centerline temperatures higher than 1400 deg C. 31 refs., 9 figs., 3 tabs. (author)