This sounds like exotic science and engineering...underneath it all is unquestionable self interested assumptions and judgements. There is no facts and evidence here. It's pseudoscience built on a house of cards.
The common useage of the nuclear word "safety" is grossly corrupted by the explanation of the below.
(Neat NRC explanations and definitions)
Risk-Informed and
Performance-Based Regulation
The
NRC has established its regulatory requirements, in both reactor and materials
applications, to ensure that "no undue risk to public health and
safety" results from licensed uses of Atomic Energy Act (AEA) materials
and facilities.
The
objective of these requirements has always been to assure that the
probabilities of accidents with the potential for adversely affecting public
health and safety are low. For reactors, these probabilities were not
quantified in a systematic way until 1975 when the Reactor Safety Study
(WASH-1400) was published. For non-reactor activities, the situation is more
complex. In some areas, high-level waste disposal and transportation, risk
assessment has been in use since the 1970s; in others, such quantification is
still evolving. Consequently, most of NRC's regulations were developed without the
benefit of quantitative estimates of risk. The perceived benefits of the
deterministic and prescriptive regulatory requirements were based mostly on
experience, testing programs and expert judgment, considering factors such as engineering
margins and the principle of defense-in-depth.
There
have been significant advances in and experience with risk assessment methodology
since 1975. Thus, the Commission is advocating certain changes to the
development and implementation of its regulations through the use of risk-informed,
and ultimately performance-based, approaches. The Probabilistic Risk Assessment
(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, in part, "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."
The
transition to a risk-informed regulatory framework is expected to be
incremental. Many of the present regulations are based on deterministic and
prescriptive requirements that cannot be quickly replaced. Therefore, the
current requirements will have to be maintained while risk-informed and/or
performance-based regulations are being developed and implemented.
To
understand and apply the commitment expressed in the PRA Policy Statement, it
is important that the NRC, the regulated community, and the public at large
have a common understanding of the terms and concepts involved; an awareness of
how these concepts (in both reactor and materials arenas) are to be applied to
NRC rulemaking, licensing, inspection, assessment, enforcement, and other
decision-making; and an appreciation of the transitional period in which the
agency and industry currently operate
1.
Risk and Risk Assessment: This paper defines risk in terms that can be applied to the
entire range of activities involving NRC licensed use of AEA materials. The
risk definition takes the view that when one asks, "What is the
risk?" one is really asking three questions: "What can go
wrong?" "How likely is it?" and "What are the consequences?"
These three questions can be referred to as the "risk triplet." The traditional
definition of risk, that is, probability times consequences, is fully embraced
by the "triplet" definition of risk.
The
first question, "What can go wrong?" is usually answered in the form
of a "scenario" (a combination of events and/or conditions that could
occur) or a set of scenarios. The
second question, "How likely is it?" can be answered in terms of the
available evidence and the processing of that evidence to quantify the
probability and the uncertainties involved. In some situations, data may exist
on the frequency of a particular type of occurrence or failure mode (e.g., accidental
overexposures). In other situations, there may be little or no data (e.g., core
damage in a reactor) and a predictive approach for analyzing probability and
uncertainty will be required. The
third question, "What are the consequences?" can be answered for each
scenario by assessing the probable
range of outcomes (e.g., dose
to the public) given the uncertainties. The outcomes or consequences are the
"end states" of the analyses. The choice of
consequence measures can be whatever seems appropriate for reasonable decision-making in a particular regulated activity and could
involve combinations of end states. A risk assessment is a systematic method for addressing the risk
triplet as it relates to the performance of a particular system (which may
include a human component) to understand likely outcomes, sensitivities, areas
of importance, system interactions and areas of uncertainty. From this
assessment the important scenarios can be identified.
2. Deterministic and Probabilistic
Analyses: All safety regulation
ultimately is concerned with risk and addresses the three questions discussed
in item 1 above. In practice, NRC addresses these three questions through the
body of regulations, guidance, and license conditions that it uses to regulate
the many activities under its jurisdiction. The current body of regulations,
guidance and license conditions is based largely on deterministic analyses and is
implemented by prescriptive requirements. As described in the PRA Policy Statement,
the deterministic approach to regulation establishes requirements for
engineering margin and for quality assurance in design, manufacture, and
construction. In addition, it assumes that adverse conditions can exist and
establishes a specific set of design basis events (i.e., what can go wrong?).
The deterministic approach involves implied, but unquantified, elements of
probability in the selection of the specific accidents to be analyzed as design
basis events. It then requires that the design include safety systems capable
of preventing and/or mitigating the consequences (i.e., what are the
consequences?) of those design basis events in order to protect public health
and safety. Thus, a deterministic analysis explicitly addresses only two
questions of the risk triplet. In addition, traditional regulatory analyses do
not integrate results in a comprehensive manner to assess the overall safety impact
of postulated initiating events.
PRA and other risk assessment methods (also described in the PRA
Policy Statement) consider risk (i.e., all three questions) in a more coherent,
explicit, and quantitative manner. Risk assessment methodology examines systems
and their interactions in an integrated, comprehensive manner. Probabilistic
analysis explicitly addressesa broad spectrum of initiating events and their event frequency.
It then analyzes the consequences of those event scenarios and weights the
consequences by the frequency, thus giving a measure of risk. Since risk assessment methods were first used to gain a better
understanding of the risk associated with some of the activities and facilities
that the NRC regulates, substantial event data and increased sophistication and
experience in the use of certain risk assessment methods (e.g., Probabilistic Risk
Assessment (PRA), IntegratedSafety Assessment (ISA), and Performance Assessment (PA)) have
been acquired. Accordingly, there is now the opportunity to enhance the
traditional approach by more explicitly addressing risk and incorporating the
insights thus gained. While the traditional deterministic approach to regulation has
been successful in ensuring no undue risk to public health and safety in the
use of nuclear materials, opportunities for improvement exist. Given the broad
spectrum of equipment and activities covered, the regulations can be
strengthened and resources can be allocated to ensure that they are focused on
the most risk-significant equipment and activities, and to ensure a consistent
and coherent framework for regulatory decision-making. The different
"risk-informed" and/or "performance-based" approaches to
regulation described below, if properly applied singly or in combination, would
provide such a framework. 3. "Risk
Insights": The term "risk
insights," as used here, refers to the results and findings that come from
risk assessments. The end results of such assessments may relate directly to public
health effects as in the Commission's Safety Goals for the Operation of Nuclear
Power Plants. For specific applications the results and
findings may take other forms. For example, for reactors these
include such things as identification of dominant accident sequences, estimates
of core damage frequency (CDF)(1)
and large early release
frequency (LERF)(2), and importance measures of structures, systems,
and components. On the other hand, in other areas of NRC
regulation, findings and results include risk curves(3) for disposal facilities for radioactive wastes, frequency of and
costs associated with accidental smelting of sealed sources at steel mills,
frequency of occupational exposures, predicted dose from decommissioned sites
and many others. Risk insights have already been incorporated successfully into
numerous regulatory activities, and have proven to be a valuable complement to
traditional deterministic approaches. Given the current maturity of some risk
assessment methodologies and the current body of event data, risk
insights can be incorporated more explicitly into the regulatory process in a
manner that will improve both the efficiency and effectiveness of current regulatory
requirements. 4. "Risk-Based
Approach": Regulatory decision-making is
required in both the development of regulations and guidance and the
determination of compliance with those regulations and guidance. A
"risk-based" approach to regulatory decision-making is one in which
such decision-making is solely based on the numerical results of a risk
assessment. This places heavier reliance on risk assessment results than is currently
practicable for reactors due to uncertainties in PRA such as completeness. Note
that the Commission does not endorse an approach that is
"risk-based"; however, this does not invalidate the use of
probabilistic calculations to demonstrate compliance with certain criteria,
such as dose limits.
5. "Risk-Informed
Approach": A "risk-informed" approach
to regulatory decision making represents a philosophy whereby risk insights are
considered together with other factors to establish requirements that better focus
licensee and regulatory attention on design and operational issues commensurate
with their importance to public health and safety. A "risk-informed"
approach enhances the deterministic approach by: (a) allowing explicit
consideration of a broader set of potential challenges to safety, (b) providing
a logical means for prioritizing these challenges based on risk significance,
operating experience, and/or engineering judgment, (c) facilitating
consideration of a broader set of resources to defend against these challenges,
(d) explicitly identifying and quantifying sources of uncertainty in the
analysis (although such analyses do not necessarily reflect all important
sources of uncertainty), and (e) leading to better decision-making by providing
a means to test the sensitivity of the results to key assumptions. Where
appropriate, a risk-informed regulatory approach can also be used to reduce
unnecessary conservatism in purely deterministic approaches, or can be used to
identify areas with insufficient conservatism in deterministic analyses and
provide the bases for additional requirements or regulatory actions.
"Risk-informed" approaches lie between the "risk-based" and
purely deterministic approaches. The details of the regulatory issue under
consideration will determine where the risk-informed decision falls within the
spectrum.
6. "Risk-Informed
Approach and Defense-in-Depth": The concept of defense-in depth(4) has always been and will
continue to be a fundamental tenet of regulatory practice in the nuclear field,
particularly regarding nuclear facilities. Risk insights can make the elements
of defense-in-depth more clear by quantifying them to the extent practicable.
Although the uncertainties associated with the importance of some elements of
defense may be substantial, the fact that these elements and uncertainties have
been quantified can aid in determining how much defense makes regulatory sense.
Decisions on the adequacy of or the necessity for elements of defense should reflect
risk insights gained through identification of the individual performance of
each defense system in relation to overall performance.
7. "Performance-Based
Approach": A regulation can be either
prescriptive or performance-based. A prescriptive requirement specifies
particular features, actions, or programmatic elements to be included in the
design or process, as the means for achieving a desired objective. A
performance-based requirement relies upon measurable (or calculable) outcomes
(i.e., performance results) to be met, but provides more flexibility to the licensee
as to the means of meeting those outcomes. A performance-based regulatory
approach is one that establishes performance and results as the primary basis
for regulatory decision-making, and incorporates the following attributes: (1)
measurable (or calculable) parameters (i.e., direct measurement of the physical
parameter of interest or of related parameters that can be used to calculate
the parameter of interest) exist to monitor system, including facility and
licensee, performance, (2) objective criteria to assess performance are established
based on risk insights, deterministic analyses and/or performance history, (3)
licensees have flexibility to determine how to meet the established performance
criteria in ways that will encourage and reward improved outcomes; and (4) a
framework exists in which the failure to meet a performance criterion, while undesirable,
will not in and of itself constitute or result in an immediate safety concern.
The measurable (or calculable) parameters may be included in the regulation
itself or in formal license conditions, including reference to regulatory
guidance adopted by the licensee. This regulatory approach is not new to the
NRC. For instance, the Commission previously has approved performance-based approaches
in 10 CFR Parts 20, 50 (Option B, Appendix J and the Maintenance Rule,10
CFR50.65), 60, and 61. In particular, the Commission weighed the relative
merits of prescriptive and performance-based regulatory approaches in issuing
10 CFR Part 60.
A performance-based approach can be implemented without the use of
risk insights. Such an approach would require that objective performance
criteria be based on deterministic safety analysis and performance history. This
approach would still provide flexibility to the licensee in determining how to
meet the performance criteria.Establishing objective performance criteria for performance
monitoring may not be feasible for some applications and, in such cases, a
performance-based approach would not be feasible.
As applied to inspection, a performance-based approach tends to
emphasize results (e.g., can the pump perform its intended function?) over
process and method (e.g., was the maintenance technician trained?). Note that a
performance-based approach to inspection does not supplant or displace the need
for compliance with NRC requirements, nor does it displace the need for enforcement
action, as appropriate, when non-compliance occurs. (5)As applied to licensee assessment, a performance-based approach
focuses on a licensee's actual performance results (i.e., desired outcomes),
rather than on products (i.e., outputs). In the broadest sense, the desired
outcome of a performance-based approach to regulatory oversight will be to
focus more attention and NRC resources on those licensees whose performance is
declining or less than satisfactory. 8. "Risk-Informed,
Performance-Based Approach": A risk-informed, performance-based approach to regulatory
decision-making combines the "risk-informed" and
"performance-based" elements discussed in Items 5 and 7, above, and
applies these concepts to NRC rulemaking, licensing, inspection, assessment,
enforcement, and other decision-making. Stated succinctly, a risk-informed,
performance-based regulation is 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
regulatory decision-making.
The
definitions and concepts in this paper have proven suitable for application to
nuclear power plants and certain nonreactor activities (e.g., PA of geologic
repositories). While different in detail, these activities are similar in terms
of system complexity and the application of probabilistic methods to the
determination of safety. In simpler situations, the concepts and definitions
should prove equally suitable provided that NRC adopts a flexible framework for
the implementation of risk-informed, and ultimately performance-based,
regulation across the full spectrum of the materials, processes, and facilities
regulated by the NRC.
1.
CDF is the frequency of the combinations of initiating events, hardware
failures, and human errors leading to core uncovery with reflooding of the core
not imminent.
2.
LERF is the frequency of those accidents leading to significant, unmitigated
releases from containment in a timeframe prior to effective evacuation of the
close-in population such that there is a potential for early health effects.
3.
Risk curves (also known as Complementary Cumulative Distribution Functions
(CCDFs) or Farmer curves) are estimates of the probability that a given
consequence will be exceeded.
4.
Defense-in-depth is an element of the 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. The
defense-in-depth philosophy ensures that safety will not be wholly dependent on
any single element of the design, construction, maintenance, or operation of a
nuclear facility. The net effect of incorporating defense-in-depth into design,
construction, maintenance, and operation is that the facility or system in
question tends to be more tolerant of failures and external challenges.
5.
Not every aspect of licensed activities can or should be inspected using this
approach. For example, if a licensee is unsuccessful in meeting the criteria
defined by a performance-based regulation, the inspector should then focus on
the licensee's process and method, to understand the root cause of the
breakdown in performance, and to understand how future poor performance may be
avoided.
