3.0 PRA
PRA is the regulatory paradigm for the Nuclear Regulatory Commission. No nuclear facility in the United States can obtain a license to operate without a comprehensive PRA. Although initially formulated for nuclear power plants, PRA is widely used in just about every field.
From the NRC:
The NRC did not systematically quantify these probabilities until 1975, when the agency published the Reactor Safety Study (WASH-1400, NUREG/75-014). The NRC initially developed many of its regulations without considering numerical estimates of risk. Rather, those prescriptive, deterministic regulatory requirements were primarily based on experience, test results, and expert judgment. In developing those requirements, the NRC considered factors such as engineering margins and the principle of defense-in-depth.
Since 1975, the NRC and its licensees have advanced significantly in their knowledge of (and experience with) probabilistic risk assessment (PRA). PRA considers nuclear safety in a more comprehensive way by examining a broad spectrum of initiating events (circumstances that put a facility in an off-normal condition, such as a reactor trip or “scram ” at a nuclear power plant). As a result, PRA analysts ask the additional question of how likely it is that something will go wrong. Analysts then explore the frequency and consequences of various scenarios, giving a measure of risk.
Because of these advances, the Commission decided to implement “risk-informed,” and ultimately “performance-based,” approaches. Then, in 1993, Congress passed a law called the “Government Performance and Results Act” (GPRA). One objective of that law is to “improve Federal program effectiveness and public accountability by promoting a new focus on results, service quality, and customer satisfaction.”
In response to the GPRA, Federal agencies, including the NRC, developed strategies and plans for achieving that objective. In its Strategic Plan, the NRC committed to move toward risk-informed, performance-based regulation. As a result, when the NRC proposes a new regulation, the alternatives considered must include a performance-based alternative that enhances the focus on the effectiveness of the agency’s regulatory programs.
The philosophy of defense-in-depth is still used today. This means installing redundant and diverse systems or multiple barriers in order to reduce the frequency of an event or mitigate the consequences of an event. Performing a PRA is also a tool to inform system design as much as it is to quantify hazards.
A Level 1 PRA focuses on the development of an envelope of potential initiating events leading to core damage. A Level 2 PRA focuses on initiating events leading to releases of radioactivity from the plant. Finally, a Level 3 PRA determines the consequences of these initiating events in terms of environmental damage and public health. Overall, the total PRA identifies potential hazards, develops accident sequences, determines frequencies of these accidents, and identifies consequences of these accidents.
Fundamentally, the PRA starts with establishing an envelope of initiating events. These events are characterized into anticipated operational occurrences (AOO), which are events that carry high frequency but low consequence, such as events leading to routine maintenance activities. Design basis events (DBE) are less frequent but carry a higher consequence, and beyond design basis events (BDBE) which occur at very low frequencies, but have very high consequences. The NRC has regulations specific to each category. All of these events collectively form the licensing basis events, or the licensing basis envelope (LBE) that are used in development of the license application.
