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# History of the NRC's Risk-Informed Regulatory Programs

The NRC is moving toward a risk-informed, performance-based regulatory framework incrementally--in steps. This effort began long ago. Below is a summary of the major steps taken by the agency.

On this page:
        - Historical Summary of Implementaton Plans
        - WASH-1400: Measuring Probability for Reactor Safety
        - Early Deterministic Regulation
        - Probabilistic Risk Assessment
        - Integration of Risk Assessment and Performance Into Regulation

## Historical Summary of Implementaton Plans

Many of the present regulations are based on deterministic and prescriptive requirements that cannot be quickly replaced. Therefore, the current requirements are being maintained, while risk-informed and/or performance-based regulations are being developed and implemented.

The agency developed the Probabilistic Risk Assessment (PRA) Implementation Plan in 1994 to focus the agency's efforts on PRA-related activities. This plan was superseded in 2000 by the Risk-Informed Regulation Implementation Plan (RIRIP). Then, in April 2007, the NRC replaced the RIRIP with the Risk-Informed, Performance-Based Plan (RPP). Each of these plans has guided the NRC in developing risk-informed, performance-based regulations.

See the following for periodic status reports for each program, as well as additional information related to the agency's path toward performance-based regulation.
        - PRA Implementation Plan (1994 to 1999)
        - Risk-Informed Regulation Implementation Plan (RIRIP) (2000 to 2007)
        - Risk-Informed and Performance-Based Plan (RPP) (2007 to Present)

The NRC established its regulatory requirements for reactor, materials, and waste applications to ensure that "no undue risk to public health and safety" results from licensed uses of facilities and materials covered by the Atomic Energy Act. In other words, the NRC's requirements ensure that there is a low probability of accidents that could adversely affect the health and safety of the public.

## WASH-1400: Measuring Probability for Reactor Safety

For reactors, the NRC did not systematically quantify these probabilities until 1975, when the agency published the Reactor Safety Study (WASH-1400, NUREG/75-014). For non-reactor activities, the initiation of risk-informed regulation is not as clear-cut. In some areas, such as high-level waste disposal and transportation, the NRC and its licensees have used risk assessment since the 1970s. In other areas, the NRC's use of quantitative methods is still evolving.

## Early Deterministic Regulation

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*. As previously noted, the NRC's traditional deterministic approach involved asking only what can go wrong and what are the consequences. Nonetheless, the NRC assumed that undesirable events can occur and required plant designers to include safety systems capable of preventing and/or mitigating the consequences of accidents.

## Probabilistic Risk Assessment

Since 1975 however, 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.

## Integration of Risk Assessment and Performance Into Regulation

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 "*PRA Policy Statement* " (60 FR 42622, August 16, 1995) formalized the Commission's commitment to risk-informed regulation through the expanded use of PRA. The PRA Policy Statement states,
> 
*The use of PRA technology should be increased in all regulatory matters to the extent supported by the state of the art in PRA methods and data, and in a manner that complements the NRC's deterministic approach and supports the NRC's traditional defense-in-depth philosophy.PRA and associated analyses should be used in regulatory matters, where practical within the bounds of the state-of-the-art, to reduce unnecessary conservatism associated with current regulatory requirements, regulatory guides, license commitments, and staff practices. Where appropriate, PRA should be used to support the proposal for additional regulatory requirements in accordance with 10 CFR 50.109 (Backfit Rule). The existing rules and regulations shall be complied with unless these rules and regulations are revised.PRA evaluations in support of regulatory decisions should be as realistic as practicable and appropriate supporting data should be publicly available for review.The Commission's safety goals for nuclear power plants and subsidiary numerical objectives are to be used with appropriate consideration of uncertainties in making regulatory judgments on the need of proposing and backfitting new generic requirements on nuclear power plant licensees.*

The plans listed in the Historical Summary, above, describe the agency's efforts to implement the PRA Policy Statement since it was issued.

Page Last Reviewed/Updated Thursday, September 30, 2021

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Page Last Reviewed/Updated Thursday, September 30, 2021