The Relationship between CDF and LERF in Risk-Informed Regulations

摘要

The risk management will be used in future regulations of nuclear safety and acceptable risk criteria are expected to play an important part in the framework. There are several metrics used in probabilistic risk assessment (PRA) to represent the risk. Core Damage Frequency (CDF) and Large Early Release Frequency (LERF) are two common metrics used in the risk-informed and performance-based regulations. With no publications are available for clearly addressing the relationship between CDF and LERF. We review in this paper the development of the definitions for CDF, Large Release Frequency (LRF) and LERF, together with the acceptable risk criteria. Our review of PRA data for five nuclear plants studied in NUREG-1150 indicates the equivalence between LRF and LERF because they are both defined by early fatalities (EF) before the evacuation. Based on this observation, we present linear relationship between CDF, LRF, LERF and EF for the five NUREG-1150 plants in double logarithmic scales. Thus the development of acceptable risk criteria may use the relationship between CDF and LERF. This study also proposes a new metric, Large Late Release Frequency (LLRF), representing the large release after the evacuation. The LLRF may be used to regulate the radioactive contamination for the off-site areas in serious accidents, e.g. the Fukushima and Chernobyl accidents. A hierarchical important analysis is also proposed to address issues that normal importance analysis may not properly represent for systems contributing to the LRF.