Fuel and fission gas response to simulated thermal transients: experimental results and correlation with fission gas release and swelling model

To evaluate the role of fission gas in hypothetical core disruptive accidents, experimental and analytical information describing the fission gas behavior in rapid temperature transients is urgently needed. A direct-electrical-heating apparatus was used to obtain information on the fission gas behavior and the response of mixed-oxide fuel elements to simulated thermal transient conditions. The experimental results indicate that fission gas response and swelling behavior are strongly dependent on the transient heating rate, and that fission gas can contribute significantly to the failure of a fuel stack during a temperature transient. The microstructural results from these tests were subsequently used to perform a limited verification of the fission gas release and swelling code, FRAS, which was developed to describe the fission gas release behavior in rapid temperature transients. A comparison of the measured intragranular bubble sizes (and in some cases bubble densities) with the calculated bubble sizes (and densities) revealed that the current version of the FRAS code is inadequate in some transient conditions. A nonequilibrium of bubble coalescence may be necessary to describe the fission gas behavior in such temperature transients