Review of the Canadian Approach to Fitness for Service Criteria for CANDU Fuel. Appendix 2

Anticipated operational occurrence (AOO) events have "the potential to challenge the safety of the reactor and might be reasonably expected to happen during the lifetime of a plant, with frequencies of occurrence equal to or greater than 10^-2 per reactor year" [9]. It needs to be ensured that the fuel remains fit for service when the reactor comes back to normal operation following an AOO. For this purpose, in Canadian nuclear industry, so-called 'fitness-for-service criteria' are used. The concept of fitness-for-service criteria for fuel is an integral approach to ensure that the fuel and fuel channel maintain their structural integrity under the environment that several fuel degradation mechanisms are stimulated by an AOO. Therefore, the fitness-for-service criteria are derived on the basis of assessments of associated fuel degradation mechanisms using a stylized conservative analysis approach. In the stylized analysis, the fuel thermomechanical behaviour is simulated taking into account coolant pressures, bundle powers and power ramps typical of AOOs, while keeping the fuel sheath temperature constant throughout the 60 seconds simulation time of the event. The contents of this Appendix are primarily taken from COG-12-2049. Fitness-for-service criteria are usually used by reactor operators as a simple and reliable measure to provide assurance that the performance of the fuel returned to normal operation following an AOO still remains within its acceptable range. Therefore, fitness-for-service criteria are specific to fuel design and also to transients considered.