Evaluation of Creep Damage and Diametral Strain of Fast Reactor MOX Fuel Pins Irradiated to High Burnups

In determining lifetime criteria of fast reactor fuel pins, creep damage due to fission gas pressure was evaluated on mixed-oxide fuel pins with austenitic stainless steel cladding irradiated to high burnups. The degree of creep damage of these fuel pins was expressed as cumulative damage fractions (CDFs), which were defined so that cladding breaching occurs when the CDF exceeds 1.0. The obtained CDFs for typical high-temperature fuel pins were on the order of 10−4–10−2 at the end of irradiation, indicating that these fuel pins had large safety margins against breaching due to creep damage. In order to investigate the factors that govern the lifetime of fuel pins, pin diametral increase and CDF were predicted under extended burnup conditions, and then were compared with their limit values. The predicted pin diametral increase reached its limit value earlier than did the CDF because of a significant increase in the cladding void swelling, suggesting that the lifetimes of fuel pins with austenitic stainless steel cladding are practically governed by the diametral increase rather than by the creep damage.