On the Concept of Inherent Core Damage Frequency: a Framework for Residual Risk Floors in Seismic Probabilistic...

In seismic probabilistic safety assessment (SPSA), ongoing efforts to reduce core damage frequency (CDF) encounter fundamental limitations due to modeling saturation, epistemic uncertainties, and the diminishing contribution of extremely low-probability events. This paper introduces and develops the concept of "inherent CDF"-a residual risk floor that persists when further reductions in CDF become practically or physically unattainable despite best-estimate modeling and conservative design. Drawing from recent literature, regulatory philosophy, and structural fragility modeling, the paper synthesizes the rationale for formalizing this floor within seismic PSA methodology. A conceptual framework is proposed to identify, justify, and declare the inherent CDF based on hazard-fragility convolution, uncertainty bounds, and plateauing risk behaviour at high ground motions. The implications for licensing, risk-informed decision-making, and safety optimization are examined, along with potential extensions to multi-hazard PSA domains. This study advocates for the integration of residual risk acknowledgment into modern safety frameworks, providing a technically grounded and transparent foundation for defining acceptable seismic risk in nuclear power plants.