The Prediction of Transient Fission-Gas Release and Fuel Microcracking Under Severe Core-Accident Conditions

The interrelationship between fuel fracturing (microcracking), temperature scenario, and fission-gas-bubble response is investigated. The fission-gas-bubble behavior is described using the FASTGRASS computer code. A model, based on the work of DiMelfi and Deitrich describing ductile/brittle fuel behavior, has been incorporated into the FASTGRASS analysis. The predictions of fission-gas release, radial distribution of released gas, radial distribution of microcracking, and fuel temperatures are compared with the results of transient direct-electrical-heating experiments on irradiated light water reactor fuel. Finally, results of analyses for Three Mile Island Unit 2 type accident conditions are presented and implications for microcracking and fission-gas behavior during this accident are discussed.