Risk-Informed & Performance-Based Decision Making

Risk-Informed, Performance-Based (RIPB) principles enable an economical implementation of a graded approach so that resources and higher quality expectations are associated with the most important activities contributing to the desired outcome (e.g. safety, economics, plant availability).

It is a process where risk insights are considered together with other sources of insights (e.g. deterministic analysis, safety margin, engineering design features) that considers a broad set of potential challenges to safety and provides a logical means for prioritizing these challenges.

1. The likelihood of occurrence of an event.
2. The likelihood of an adverse response to that event whether by a structural failure, loss of coolant, component failure, etc.
3. The magnitude of the consequences resulting from the adverse failure (e.g. loss of life, economic damages, environmental damages, etc.) if that occurs.

Insights gained from conducting a risk assessment can then allow for better resource allocation by focusing on more risk-significant issues identified in the assessment.

The consensus in the engineering community is that a risk-informed analysis is the best source of information for priority setting and resource allocation.

An approach that relies upon the desired, measurable results or performance outcomes based on objective criteria rather than a prescriptive process, technique, or procedure.

Basically, performance-based approaches seek to explicitly identify performance objectives that collectively represent the desired outcome of a project and look at the result of a design or regulation to assess the performance. It formally allows for more flexibility in meeting performance criteria.

A performance-based approach must follow some basic steps:

1. Establish the means for defining objectives, identifying parameters to be observed, and setting performance criteria
2. Monitor performance to ensure allowable margins are not compromised
3. Verify that expected outcomes are realized. Expectations of performance may be set through simulations and/or prototype tests.

The engineering community believes that focusing on the performance objectives instead of a prescriptive “how-to” design approach will allow for more innovation in methods without sacrificing adequacy of safety requirements.

The U.S. Nuclear Regulatory Commission (NRC) has been working to add RIPB regulations since 1994 with the development of the Probabilistic Risk Assessment Implementation Plan. That’s because the majority of the regulations are based on deterministic and prescriptive requirements i.e. what can go wrong and how to fix the problem. The majority of the current regulations were developed without considering numerical estimates of risk. Without incorporating risk assessments into the regulations, the NRC assumed that undesirable events can occur and required plant designers to include safety systems and defense-in-depth principles capable of preventing and/or mitigating the consequences of accidents.

In 1999, the NRC adopted the following definition: An approach in which risk insights, engineering analysis and judgment including the principle of defense-in-depth and the incorporation of safety margins, and performance history are used, to (1) focus attention on the most important activities, (2) establish objective criteria for evaluating performance, (3) develop measurable or calculable parameters for monitoring system and licensee performance, (4) provide flexibility to determine how to meet the established performance criteria in a way that will encourage and reward improved outcomes, and (5) focus on the results as the primary basis for safety decision-making. [SRM-SECY-98-0144].

The current belief is that using RIPB concepts will guide NRC requirements and regulatory attention to the issues that are most important to the health and safety of the public and the environment; and identify performance measures that ensure an adequate safety margin.