Standards for Probabilistic Risk/Safety Assessments of Nuclear Power Plants and High-Level Nuclear Safety Goals—An Overview

Probabilistic Safety/Risk Assessment (PSA/PRA) has been widely accepted as a technology for investigating the risks posed by the design, construction and operation of hazardous facilities such as nuclear power plants. The first part of this paper provides an overview of the standards for PRA/PSA of nuclear power plants (NPP) that have recently been adopted or are currently under development by the American Nuclear Society (ANS) and the American Society of Mechanical Engineers (ASME). These standards establish a set of technical requirements that need to be met in carrying out a plant PRA/PSA which can be used to support risk-informed decisions concerning plant design and operation. Since the commercial NPP in the US are light-water reactors (LWRs), the standards that have and are being developed are based on the three levels of PRA, Level 1 (core damage), Level 2 (accident releases) and Level 3 (consequences), conventionally used in LWRs to analyze severe accidents. The focus of this paper is the standard proposed for Level 3 PRA, probabilistic assessment of consequences including radiation dose and induced health effects to offsite populations, land contamination, and economic impacts. When combined with the release frequencies calculated in Level 1 and Level 2, the output of a Level 3 PRA is used to estimate the plant risk from severe accidents, in particular the individual risks of early fatality and latent cancer fatality that are then compared to the quantitative health objectives (QHOs) of the Safety Goals established by the US Nuclear Regulatory Commission (USNRC). The second part of the paper is devoted to a discussion of the coverage of plant risk provided by the QHOs of the current USNRC safety goals. The QHOs establish risk limits for severe accidents in terms of their radiological consequences to affected individuals: in particular, the average individual health risks of early fatality and latent cancers from radiation exposure of members of the public living in the vicinity of a nuclear power plant. However, the experience of severe NPP accidents such as at Fukushima suggest that there are significant societal consequences besides the individual health risks of radiation exposure. These societal consequences include the potential relocation of large numbers of people for long periods of time, non-radiological health impacts of emergency measures like evacuation, significant property damage and community disruption, along with the substantial costs of recovery and decontamination. Based on related earlier work, it is argued that a safety goal for societal risk also needs to be considered and possibilities of a qualitative goal along with an associated quantitative objective are discussed.