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February 14, 2000
The Honorable Richard A. Meserve
Chairman
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Chairman Meserve:
SUBJECT: |
IMPEDIMENTS TO THE INCREASED USE OF RISK-INFORMED REGULATION |
The ACRS has long advocated the transition to a risk-informed regulatory system. Over the last several years, we have discussed
potential impediments along the path toward risk-informed regulation.
This report responds to the Commission's request in the December 17, 1999 Staff Requirements Memorandum that the ACRS
provide examples of impediments to the increased use of risk-informed regulation, an evaluation of the significance of these
impediments, and, as appropriate, proposed solutions to identified problems. In our review, we had the benefit of the documents
referenced.
There can be a variety of views on what is meant by risk-informed regulation. One view, for example, could be that we start over
and redo the whole body of regulations making them risk informed without having a reactor type or design in mind. We believe
that this should be considered on a non-urgent long time frame.
Another view could be that there exists a body of regulations and a population of light water reactor plants whose designs have
resulted from meeting these regulations. As a result, risk-informed regulation would mean using risk insights gained from
performing probabilistic risk assessments (PRAs) on the existing plants to modify the regulations holistically or in selected areas to
make the regulations coherent, ensure that all requirements are necessary, and provide a focus on the more risk-significant issues.
In responding to the Commission's request, we take the latter view of what the agency intends in its efforts to risk inform the
regulations.
In this respect, we identify a number of conditions that we believe hinder the progress of risk informing the regulations and
implementing the changes for operating reactors. We have placed these "impediments" in two separate
categories,"cultural/institutional" and "technical."
The cultural/institutional impediments are characterized as being related to attitudes, impressions, institutional or organizational
barriers, processes, resource limits and similar such attributes. An important cultural/institutional impediment is the perception by
licensees that they will need to expend substantial resources to update their PRAs to an acceptable level, provide additional staffing
and resources to utilize and maintain the PRAs, and still have to comply with the current deterministic regulations. They fear that
risk considerations will be add-ons to the existing regulatory system that will impose additional burdens.
There are many more cultural/institutional impediments than there are technical ones. We have chosen not to focus on the
cultural/institutional impediments because it is our view that, as we risk inform the regulations in a technically defensible manner
and consistently apply these regulations, most of the cultural/institutional impediments will fade away naturally with time. On the
other hand, the technical impediments will not go away by themselves but will require significant effort and research for resolution.
We consider the more significant of the technical impediments to be:
1. |
PRA inadequacies and incompleteness in some areas. |
2. |
The need to revisit risk-acceptance criteria. |
3. |
Lack of guidance on how to implement defense in depth and on
how to impose sufficiency limits. |
4. |
Lack of guidance on the significance and appropriate use of
importance measures. |
5. |
Variation of PRA quality and scope and the need for Standards. |
While we consider it important that efforts be undertaken to overcome these impediments, we believe that the state-of-the-art of
PRA is sufficiently advanced that the agency can proceed with efforts to become more risk informed in its regulatory activities.
However, it will be necessary to craft the regulations in a conservative manner to accommodate these shortcomings and in such a
way that they can be easily evolved as improvements are made in the state-of-the-art of PRA. We also believe that the agency
ought not to underestimate the risk analysis capabilities that will be needed to sustain a risk-informed regulatory system.
Our views on each of the technical impediments are discussed below:
1. |
PRA Inadequacies and Incompleteness
Most of the current PRAs are inadequate for assessing risk
contributions from fires, seismic events, human performance,
organizational factors, and safety culture factors. They are
incapable of assessing the lifetime average risk contribution from
shutdown conditions. The reliability database is weak for
passive components and "nonsafety-related" systems and
components. |
2. |
Risk Acceptance Criteria
It is necessary to have risk acceptance criteria applicable to
individual licensees in a risk-informed regulatory system. The
initial efforts to risk inform the regulatory activities have utilized
two metrics for risk acceptance - mean values of core damage
frequency (CDF) and large, early release frequency (LERF).
The values for CDF and LERF used in Regulatory Guide 1.174
are consistent with the Commission's safety goals. These safety
goals were, however, originally intended to be goals (i.e., some
things to strive for) for the average risk status of the population
of plants as a whole. It is generally recognized that safety goals
are not risk acceptance values that would, for example, be
surrogates for adequate protection. |
In a risk-informed regulatory system, it is necessary to have risk
acceptance limits. If we are to have limits on CDF and LERF
that are "consistent" with "adequate protection," we believe
these would differ from those in Regulatory Guide 1.174. It is
important at this stage of risk-informing the regulations that
quantitative limits be incorporated into an expanded definition of
adequate protection. |
3. |
Defense-in-Depth
According to the Commission's White Paper (SECY-98-144):
Defense-in-Depth is an element of NRC's Safety Philosophy that
employs successive compensatory measures to prevent accidents
or mitigate damage if a malfunction, accident, or naturally
caused event occurs at a nuclear facility. |
|
If defense-in-depth is viewed as measures taken to compensate
for the PRA inadequacies and uncertainties, then there is a need
for guidance to help quantify how many compensatory measures
are necessary and how good these have to be. |
4. |
Importance Measures
We have noted that risk-informed decisions are often based on
categorizing structures, systems, and components according to
their importance in influencing changes to CDF and LERF. As
discussed in our report on importance measures, there is a need
for guidance on the appropriate interpretation and use of
importance measures. |
5. |
Need for Standards/PRA Quality and Scope
Most PRAs for existing reactors were developed in response to
Generic Letter 88-20 and Supplement 4 requesting the individual
plant examination (IPE) and individual plant examination for
external events (IPEEEs). It has been noted that there is much
variation in the scope and quality of these PRAs. |
NRC has requested the American Society of Mechanical
Engineers and the American Nuclear Society to develop
Standards to ensure that the technical quality of PRAs is
sufficient to support the regulatory review and approval of
licensee risk-informed applications. We believe that
development of appropriate PRA Standards is important to risk-informing the regulations. However, we believe it is important
that standards not stifle the continuing improvement of PRA
methods. |
We believe that the quality of any PRA is reflected in the
quantified uncertainty distribution. It is important that the
Standards include guidance on the appropriate determination of
uncertainties (epistemic and aleatory) and the NRC staff needs
guidance on how to consistently use these in the decisionmaking process.
As stated above, even though impediments exist, the agency has
the capabilities necessary to make significant progress in
developing and implementing risk-informed regulations. |
|
Sincerely,
/s/
Dana A. Powers
Chairman |
References:
1. |
Staff Requirements Memorandum dated December 17, 1999,
from Annette L. Vietti-Cook, Secretary of the NRC, to John T.
Larkins, ACRS, regarding Commission meeting with ACRS
November 4, 1999. |
2. |
U.S. Nuclear Regulatory Commission, Regulatory Guide 1.174,
"An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing
Basis," July 1998. |
3. |
U.S. Nuclear Regulatory Commission, Policy Statement, "Safety
Goals for the Operations of Nuclear Power Plants," 10 CFR Part
50, August 21, 1986. |
4. |
SECY-98-144, Memorandum dated June 22, 1998, from L.
Joseph Callan, Executive Director for Operations, NRC, Subject:
White Paper on Risk-Informed and Performance-Based
Regulation. |
5. |
Letter dated March 25, 1999, from Dana A. Powers, Chairman,
ACRS, to William D. Travers, Executive Director for
Operations, NRC, Subject: Proposed ASME Standard for
Probabilistic Risk Assessment for Nuclear Power Plant
Applications (Phase 1). |
6. |
Report dated February 11, 2000, from Dana A. Powers,
Chairman, ACRS, to Richard A. Meserve, Chairman, NRC,
Subject: Importance Measures Derived from Probabilistic Risk
Assessment. |
7. |
U.S. Nuclear Regulatory Commission, Generic Letter 88-20,
dated November 23, 1988, Subject: Individual Plant Examination
for Severe Accident Vulnerabilities. |
8. |
U.S. Nuclear Regulatory Commission, Generic Letter 88-20,
Supplement 4, dated June 28, 1991, Subject: Individual Plant
Examination for External Events. |
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