An Assessment of the Core Degradation Frequency in a Typical Large LMFBR Design for Internal Accident Initiators—A Comparison with PWR Predictions

A comparative assessment of the core degradation frequency due to internal accident initiators between a typical large liquid-metal fast breeder reactor (LMFBR) design and pressurized water reactors (PWRs) has been performed. For the PWR system, existing analyses have been utilized. For the reference LMFBR, an extensive analysis has been performed for two accident initiators, i.e., loss of off-site power and loss of main feedwater. Based on this analysis an estimate of ∼1 × 10^-6/reactor·yr has been obtained for the core degradation frequency of the reference LMFBR. This estimate is significantly smaller than the PWR core degradation frequency (∼6 ×10^-5/yr). A sensitivity analysis shows that the parameters having the largest impact on the unavailability of decay heat removal are (a) for the “loss of off-site power” initiator: human error and failure to restore off-site power, and (b) for the “loss of main feedwater” initiator: the leakage rates of the passive decay heat removal system and the adoption of the policy to repair the Na-NaK heat exchanger only during normal shutdowns. The results indicate that the LMFBR system has the potential of higher resistance than the PWR system to the accident initiators considered. The lower core degradation frequency estimated for the LMFBR system is due to the presence of two redundant and diverse reactor shutdown systems, with a self-actuated feature included in one of them, the incorporation of a passive decay heat removal system, and the significantly lower sensitivity of the reference LMFBR to primary system pipe breaks.