【Research Note】

Risk Management and Decision-Making for Nuclear Safety:
A Critique of Probabilistic Risk Assessment (PRA)※

原子力発電安全性におけるリスク・マネジメントと意思決定 :
確率論的リスク評価批判
SAKURAI Toru
桜 井徹Contents
1. Introduction: Fukushima Nuclear Accident and Probabilistic Risk Assessments
2. Definition of Risk and Uncertainty(1) General definitions of risk by English dictionaries(2) A distinction between risk and uncertainty by Frank Knight
3. Concept of PRA: Rasmussen Report and the Problems(1) Content and characteristics of Rasmussen Report(2) The problems of the Rasmussen Report
4. Institutionalization of PRA in the U.S. Nuclear Regulatory Scheme as Complemental
Instrument to the Deterministic(1) Three Mile Island Accident and PRA(2) PRA adopted as a formal regulatory assessment measure by the 1995 statement(3) PRA in the 1998 Integrated Risk-Informed Decision Making
5. Increasing Recognition of Uncertainties in PRA by U.S. NRC Statements(1) 2009 guidance on the treatment of uncertainties(2) Revised guidance after Fukushima(3) Questions about PRA in the 2016 Report
6. Conclusion

1. Introduction: Fukushima Nuclear Accident and Probabilistic Risk Assessments
There are two kinds of safety assessments or approaches to contribute to thedecision-making for nuclear safety management: deterministic and probabilistic.
According to the explanation of the International Nuclear Safety Group of theInternational Atomic Energy Agency (IAEA), the main elements of the deterministic aresafety criteria, defense-in-depth and safety margins, while the ones of the probabilistic arecomprehensive integrated analysis of potential accident scenarios and it can handle an※ This research note is made on the presentation in 16th International Symposium on Public Sector

Management held at Deutsche Universität für Verwaltungswissenschaften Speyer on July 5th, 2019.

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unrestricted number of potential components failures and human errors. In other words,
the former is a qualitative assessment of acceptable risk of undesired consequences, thelatter is a quantitative assessment of risk from a broad spectrum on internal and external
hazards (INSAG-IAEA 2011, p. 16).
Originally, the deterministic prevailed, but after the 1990s, the importance of theprobabilistic has been identified, and now both the deterministic and the probabilistic are
required to be integrated.
Certainly, the probabilistic, called PRA (probabilistic risk assessment) was morecost-efficient than the deterministic. A joint study of two institutes in the USA and Japan
noted the cost-saving effect as follows: “the PRAs identified that many of the expensive
backfits each plant was originally facing would have negligible impact on risk andidentified low-cost modifications that had more significant risk impact. The PRA
essentially saved the licensees hundreds of millions of dollars and improved risk (B. John
Garrick Institute for the Risk Sciences and Nuclear Research Center 2017, p. 6).
At the same time, however, the PRA has been criticized as an indirect cause of the
Fukushima Nuclear Accident on 11 March 2011. The accident, that is, explosions andmeltdown of four reactors at the Fukushima Daiichi Nuclear Power Plant operated by the
Tokyo Electric Power Company (TEPCO), had occurred by the stoppage of thecooling-water supply because the electric power for the water supply was cut off by thetsunami. The direct cause of the accident was that no improvements for protective
measures had been made to prevent the loss of power under such conditions. The decision
not to make improvements was affected by the PRA.
According to Ohmae, an MIT-trained nuclear engineer who is also widely regarded
as Japan’s top management guru, the decision-making was based on two assumptions of
the low probability of such an accident. The one assumption was that ‘the probability oflong-term stoppage of external electric supply was so unlikely that we do not have toassume it might take place’. The other was that ‘the probability of a 15-meter tsunami
hitting Japan is so low that you do not have to assume such a disaster because it is likely totake place once in 10,000,000 years’ (Ohmae 2012).
The government investigation report also criticized the applicability of PRA. “In that
instance, when it deals with designing general machinery or buildings, the conventional
approach of estimating the risk as “Risk = Probability of occurrence x Consequence ofdamage” will be applicable. However, in the government, as well as in private entities, a
new approach to safety measures and emergency preparedness should be established for adisaster which potentially could bring about serious damage in a broad area like a gigantic
tsunami or the severe accident at the Fukushima Nuclear Power Station, regardless of its
probability of occurrence” (Investigation Committee 2012, p. 484).
In spite of those critiques, PRA is still made use of for the justification for reopeningof reactors that have been shut down. For example, the probability of volcanic eruption
could be too low (Japan Times 2019.).
Considering that situation about PRA, it is important to explore the nature and『商学研究』第39号― 130 ―

problems of PRA and to clarify whether the PRA could contribute to the perfect safety of
nuclear plant operations. The exploring and clarifying will be made by investigating thedevelopment of U.S. NRC (Nuclear Regulatory Commission) documents.
This note is constituted in the following. Before describing the development of
documents, the different definitions of risk and the peculiarity of the definition used under
PRA will be noted. To the next point, the first report, named the Rasmussen Report,
which introduced PRA in the field of nuclear safety assessment, will be examined. By
publishing the 1995 statement of the NRC, PRA, along with the deterministic, became an
important element of nuclear safety assessment, but, partly from a little before Fukushima,and decisively after Fukushima, criticism and questions about PRA have been increasing.
That is the third part of this paper.
2. Definition of Risk and Uncertainty(1) General definitions of risk by English dictionaries
Webster’s Third New International Dictionary lists four meanings of risk: 1. The
possibility of loss, injury, disadvantage, or disruption. 2. Someone or something thatcreates or suggests a hazard or adverse chance. 3. The chance of loss or the perils to thesubject matter of insurance covered by a contract and 4. The product of the amount that
may be lost and the probability of losing it.
From these, we can learn that risk is concerned with possibility, chance or probability.
The first three definitions are also general meanings, to which other English dictionaries
such as the Oxford English Dictionary (OED) also refer.
The definition we should specifically note is the fourth one: the product of theamount that may be lost and the probability of losing it. Risk is calculated as probability ofoccurrence multiplied by the consequence of damage. This is the special meaning of risk,
which is examined in detail later.
Why is it noticeable and special? The answer for noticeability is because thatdefinition was used in the first report which introduced the probabilistic method relating to
nuclear safety: U.S. NRC (Nuclear Regulatory Commission) (1975). Reactor Safety
Study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants,
WASH-1400 (NURG 75/014). The answer for specialty is because the application of risk
made quantitative comparisons possible between different accidents and hazards andshows a lower risk of nuclear accidents.
(2) A distinction between risk and uncertainty by Frank Knight
Considering the distinction between risk and uncertainty made by American
economist, Frank Knight, however, we should not be content with the definitions of risk.
Knight pointed out in the book published in 1921, Risk, Uncertainty and Profit, that theterm risk is loosely used, but that risk should be defined in relation to uncertainty. In― 131 ―

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Chapter One, it is explained that proper risk is measurable uncertainty, while
unmeasurable uncertainty is true uncertainty. In another chapter, a distinction between risk
and uncertainty is not only one between measurable and unmeasurable, but also a
distinction between objective and subjective probability. Moreover, he stated thefollowing:
The practical difference between the two categories, risk and uncertainty, is that in
the former the distribution of the outcome in a group of instances is known (either throughcalculation a priori, from statistics of past experience), while in the case of uncertainty
this is not true, the reason being in general that it is impossible to form a group of
instances, because the situation dealt with is in a high degree unique (Knight 1921, p.
233).
From this, it is understood that Knight saw the distinction between risk anduncertainty in the distinction between measurable and unmeasurable as well as between
known and unknown.
Although Knight’s distinction was not adopted by famous German sociologist, Niklas
Luhmann (Luhmann 1991, p. 1), many researchers have supported and developed thedistinction (in the research field of project management, see Saunders 2016). In his risk
perception research, American psychologist, Paul Slovic, located 81 hazards on two
factors of risk. The first factor dread risk or not, that is, uncontrollable or controllable. The
second is unknown/unobservable or known/observable. Nuclear accident belongs to thecategory of dread and unknown risk. The second factor could be understood almost thesame as Knight’s distinction.
Does the risk in PRA have the same meaning as proper risk in the definition of
Knight? If so, how would it deal with unmeasurable and/or unknown uncertainty? If it is
not the case, another question arises. Could all risk or uncertainty, including unmeasurable
and unknown, be dealt with in PRA?
3. Concept of PRA: Rasmussen Report and the Problems(1) Content and characteristics of Rasmussen Report
The Rasmussen Report (U.S. NRC 1975) defined risk as consequence/unit time =Frequency (events/unit time) x Magnitude (consequences/event) like the above mentioned
fourth definition by the Third Webster dictionary. Based on the definition, the report
calculated the societal risk and average individual risk of death from auto accidents that
occurred in 1971 as following:
In that year, there were about 150,000 auto accidents in the United States and one in
300 accidents resulted in a fatality. From these figures, societal risk is calculated as 15x106(accidents per year) x one death/300 accident = 50,000 death per year, and if the U.S.
consists of 200,000,000 people, the average individual risk is expressed as probability ofdeath, that is, 50,000 death per year/200,000,000 persons = 2.5x10-4. Similarly, the societal
and individual risk of injuries and property damage in auto accidents are calculated.
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On the ground of such a probability method, the report assessed the risks of nuclear
accidents (mainly leading to the melting of the reactor core) and those of other man-madeaccidents as well as natural disasters. According to Table 1, the approximately individual
risk of early fatality of nuclear accident (100 reactors) was calculated as 2x10-10, while
those of motor vehicle accident, air travel accident, railway accident and hurricane
damages were calculated as 3x10-4, 9x10-6, 4x10-6, 43x10-7 respectively (cf. Table 1).
Table 1. The 1969 fatality number and approximate risk in accident-type

Accicident

Total numberType

for 1969

Approximate
individual Risk
Early fatality

Motor vehicle

55791

3 x 10-4

Air travel1778

9 x 10-6

Railway1148

4 x 10-690

4 x 10-7

Hurricanes
NuclearAccident

2 x 10-10-(100 reactors)

Source: U.S. NRC 1975, p. 112, selected.

From these calculations, the report concluded that “reactor risks are predicted to besmaller than many other man-made and natural risks” (U.S. NRC 1975, p. 132). The
conclusion was not surprising considering the following proposition of the report on the
public attitude toward accident probability: relation of acceptability for the risk andbenefit by use of automobile or nuclear.
A death risk level per person per year of 10-3 “is generally unacceptable (to the
public), and when it occurs, immediate action is taken to reduce it. / At an accidental risk
level of 10-4, people are less inclined to concerted action but are willing to spend money toreduce the hazard ” (U.S. NRC 1975, p. 11). “Risks of accidental death at a risk level of
10-5 per person per year are still recognized in an active sense. . . . and people accept” acertain amount of inconvenience to avoid risks, at this level. . . . Accidents with a probabilityof death of 10-6 or less per person per year are “apparently not of great concern to theaverage person” (U.S. NRC 1975, p. 11).
Adding the benefit to attitude towards accident probability, the range of acceptability
could become larger (Figure 1).

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Figure 1. A benefit-risk pattern

Source: U.S. NRC 1975, p. 17(2) The problems of the Rasmussen Report
However, the conclusion on the probabilistic method should be criticized, especiallyon the following two points.
The first is concerned with the definition of risk as the product of frequency(events/unit time) and magnitude (consequences/event), which does not, as the report also
admits, differentiate with respect to the magnitude of the consequences of accidents. The
product of one accident per year and one death per accident is equal to the product of one
accident divided by 10,000 years and 10,000 deaths per accident.
One should distinguish between big magnitude at low probability with small one athigh probability. The report took this problem of risk aversion, as: “Society generally
views the single large consequence event less favorably than the total of small events
having the same average risk” (U.S. NRC 1975, p. 12), or “In general, single large
accidents are viewed less tolerantly than multiple smaller accidents, even though theaverage annual consequences of the two are equal” (U.S. NRC 1975, p. 12).
As mentioned above, Slovic (1987) pointed out the importance of the distinction
between the dread accidents and the not dread. The Rasmussen Report assessed risk
magnitude only as death or injury. Slovic described the damages of nuclear accident whichshould not be confined to death, injury and to the amount of property damage, as thefollowing:『商学研究』第39号― 134 ―

“Early theories equated the magnitude of impact to the number ofpeople killed or injured, or to the amount of property damaged. However, theaccident at the Three Mile Island (TMI) nuclear reactor in 1979 provides a
dramatic demonstration that factors besides injury, death, and property
damage impose serious costs. Despite the fact that not a single person died,and few if any latent cancer fatalities are expected, no other accident in our
history has produced such costly societal impacts. The accident at TMI
devastated the utility that owned and operated the plant. It also imposed
enormous costs on the nuclear industry and on society, through stricter
regulation (resulting in increased construction and operation costs) ” (Slovic
1987, p. 283).
We, living after not only the nuclear accident at TMI in 1979, but also the ones at
Chernobyl in 1986 and at Fukushima in 2011, have recognized the magnitude of thenuclear accidents.
Though all damages the Fukushima Nuclear Accident have brought are not yet
entirely clear, its magnitude is shown from some figures. Many inhabitants had to be
evacuated from a 30km radius around the plant. The evacuees numbered even now 42,929
as of May 2019 (Fukushima Prefecture Head Office for Disaster Control 2019).
Compensations, which up to now have been paid, amounted to about 9 trillion yen. A fundneeded for decommissioning and decontamination is estimated for each at 8 trillion.
Consequently, the least estimated amount is expected to be about 22 trillion yen for
so-called disposal and restoration (Committee for the Reform of Tokyo Electric Power
Company and Fukushima Daiichi Nuclear Power Plant 2016) .
The second point in which we might criticize the Rasmussen Report is that it thought
all risks are measurable, in other words, that unmeasurable risk, unknown risk by Slovic,
or uncertainties by the terminology of Frank Knight, were omitted. Slovic used the term
“unknown risk” in the meaning of such risk that is not observable, unknown to those
exposed, with delayed effect, new and unknown to science.
4. Institutionalization of PRA in the U.S. Nuclear Regulatory Scheme as
Complemental Instrument to the Deterministic(1) Three Mile Island Accident and PRA
In spite of the above-mentioned criticism, Probabilistic Risk Assessment was
supported and implemented by licensees (ex. Zion and Indian Point Plant in 1981 and1982 respectively) and partly in certain areas by regulatory staff because of cost efficiency.
In particular, after criticism of the Three Miles Island Nuclear Accident in 1979, theusefulness of PRA came to be identified because PRA had found a small LOCA (Loss of
Cooler Accident) as a principal risk before the TMI accident (The B. John Garrick
Institute for the Risk Sciences and Nuclear Research Center 2017, p. 9).
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(2) PRA adopted as a formal regulatory assessment measure by the 1995
statement
PRA was finally adopted as a formal regulatory assessment measure by the 1995
NRC statement (U.S. NRC 1995). However, it did not mean that PRA replaced the NRC’s
traditional deterministic assessment measure as defense-in-depth. In fact, PRA wasdeemed to be complemental to the deterministic. That is represented by the following
passage of the statement:“The use of PRA technology should be increased to the extent
supported by the state-of-the-art in PRA methods and data and in a mannerthat complements NRC’s deterministic approach . . .and supports the NRC’s
traditional defense-in-depth philosophy . . .PRA and associated analyses (e.g.,sensitivity studies, uncertainty analyses and importance measures) should be
used in regulatory matters . . .where practical within the bounds of state-of-the-art, to reduce unnecessary conservatism associated with current regulatory
requirements, regulatory guides, license commitments, and staff practices.”(requoted from U.S. NRC 2009, p. 1, original: U.S. NRC 1995, p. 8).
(3) PRA in the 1998 Integrated Risk-Informed Decision Making
In 1998, when regulatory guide 1.174 (U.S. NRC 1998) was issued, the NRC
regulatory scheme came to be called Integrated Risk-Informed Decision-Making (IRIDM).
Not to say, it is decision-making based on the integration of the deterministic and theprobabilistic assessment measures. From the word “integration”, one might think that thedeterministic and the probabilistic (PRA) would have the same weight in decision-making.
In reality, as of the 1995 statement (U.S. NRC 1995, p. 5), PRA was placed ascomplementary. The first and second points of 5 Principles of Risk - Informed Integrated
Decision-making were stated as following: The first principle is that the proposed change
meets the current regulations unless it is explicitly related to a requested exemption or rulechange. The second is that the proposed change is consistent with the defense-in-depth
philosophy (U.S. NRC 1998, p. 4).
At the same time, however, it is noteworthy that the U.S. NRC has identified theweak point of PRA: uncertainties.
5. Increasing Recognition of Uncertainties in PRA by U.S. NRC Statements(1) 2009 guidance on the treatment of uncertainties
A 1995 NRC statement had evaluated positively the treatment of uncertainty by PRA
as follows:“The treatment of uncertainties is an important issue for regulatory decisions.
Uncertainties exist . . . from knowledge limitation . . . . A probabilistic approach has exposedsome of these limitations and provided a framework to assess their significance and assist『商学研究』第39号― 136 ―

in developing a strategy to accommodate them in the regulatory process” (U.S. NRC 1995,
p. 6, also cited by U.S.NRC 2013, p. 1).
However, at the same time the statement added the passage: “The NRC staff must
continue to develop new and improved PRA methods and regulatory decision-making
tools” (U.S. NRC 1995, p. 8). It is without a doubt that one of the important issues fordeveloping new and improved PRA was the treatment of uncertainties in the integrated
decision-making.
Concerning that issue, the 2009 U.S. NRC main report titled “Guidance on theTreatment of Uncertainties Associated with PRAs in Risk-Informed Decision Making”
stated two noteworthy points.
The first point is that the report classified more accurately the uncertainties in PRA
and tried to make clear the limit for resolving uncertainties in the integrated
decision-making.
According to the U.S. NRC Report: PRA is part of a risk informed analysis as
illustrated in point three, Perform a Risk-Informed Analysis of IRIDM (Figure 2).
Figure 2. Elements of integrated risk-informed decision-making process

Source: U.S. NRC (2009), p.10.

The report classified uncertainties in two stages. The first distinction is drawn between
aleatory uncertainty and epistemic uncertainty. The former is “associated with random
nature of events” and such kind of uncertainties could be fully implemented by PRA. The
latter epistemic uncertainties are categorized into three types of uncertainties: Completenessuncertainty, parameter uncertainty and model uncertainty (U.S. NRC 2009, p.13).
The completeness uncertainty, which is divided further in two categories: the known,
but not included in the PRA model and the unknown. In order to deal with the known typeof completeness uncertainty, conservative- and bounding-types of analyses could be used.
For the unknown type of completeness uncertainty, other methods such as safety margins― 137 ―

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could be made use of (U.S. NRC, 2009, pp.15-16).
Parameter uncertainty, which relates to the uncertainty in the determination of the
input parameter values to quantify the frequencies and probabilities of the events in the
PRA logic model, could be dealt with risk calculations on the state-of-knowledge
correlation (U.S. NRC 2009, pp.13-14, 31).
Model uncertainty relates to the uncertainty associated with some aspect of a PRA
model that can be represented by any one of several different modeling approaches, noneof which is clearly more correct than another. It could be addressed by making
assumptions (U.S. NRC 2009, p. 14).
The main report, published in 2007, pointed out, as above mentioned, the parallel
existence of three types of uncertainties and the necessary and alternative measures forcorrecting the uncertainties.
(2) Revised guidance after Fukushima
The second noteworthy point of the report is that it was two times revised (2013 asdraft report for comment [U.S. NRC 2013] and 2017 as final report [U.S. NRC 2017]) andthat finally the use of PRA might be more carefully checked in each stage of uncertainties.
In particular, with complete uncertainties, even if the application of PRA scope andlevel of detail would be appropriate for the application in the first step of the risk-informed
review process, it is possible that PRA as a risk-informed application is rejected in thesecond stage, where the PRA model would be unacceptable, or the significance of theunacceptability would be determined (cf. Figure 3).
Figure 3. Overview of the risk-informed review of completeness uncertainty

Source: U.S. NRC (2017), p.127.

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From the second point, one must recognize that reliabilities on the use of PRA indecision-making was less and less stable.
Behind that instability was the impact of Fukushima Daiichi reactor accident.
(3) Questions about PRA in the 2016 Report
PRA itself, in more accurate expression, the use of PRA in regulatory
decision-making became the subject to be criticized. That can be understood from the factthat U.S. NRC had to publish the 2016 report, Probabilistic Risk Assessment andRegulatory Decision-making: Some Frequently Asked Questions. The report described thefollowing:
In 2011, following the Fukushima Dai-ichi reactor accidents, somecritics raised concerns regarding the realism of the PRAs that provide thetechnical information needed in the NRC’s risk-informed programs. Twoprincipal complaints raised were (1) PRA-based estimates of the likelihoodof major accidents were significantly smaller than simple statistical estimates
based on international events (notably the accidents at Three Mile Island,
Chernobyl, and Fukushima), and (2) PRAs did not predict the accident
scenario experienced at Fukushima (U.S.NRC 2016, p. ix).
The report tried to tackle those questions by answering that the first question isoversimplified and the second is a reflection of a lack of understanding of the nature of
PRA. In spite of further explanations, the report made the following conclusion:
PRA models, as with any model, are imperfect representations of reality.
Furthermore, unlike many models, PRA models also have to address thepossibility of unlikely situations for which empirical data are (fortunately)sparse or even nonexistent. It is, therefore, important from a model validity
viewpoint, as well as a broader safety viewpoint, to look for lessons from
accidents and other significant operational events if and when such eventsoccur (U.S.NRC 2016, p. 49).
6. Conclusion

From the hitherto analysis, it has become clear that the PRA was,at the first stage, adopted as an efficient and low-cost assessment
method, then prevailed, however, in particular after Fukushima, itcannot remove uncertainties.
In other words, PRA is inappropriate as the means ofdecision-making concerning nuclear power plant safety. PRA could― 139 ―

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not realize the risk zero that should be needed in nuclear power
plants because of accident severity. Of course, an alternative to PRA
could be adopted, the deterministic assessment and measure, which
could, in turn, lead to high cost. Therefore, the realistic alternative torisk-zero is the exit from nuclear power plants.
A list of references
1. Committee for the Reform of Tokyo Electric Power Company and Fukushima Daiichi
Nuclear Power Plant (2016), Reform, A Proposal for the Reform of Tokyo ElectricPower Company (Japanese)(https://www.meti.go.jp/committee/kenkyukai/energy_environment/touden_1f/pdf/16
1220_teigen.pdf).
2. Fukushima Prefecture Head Office for Disaster Control (2019), Bulletin on Damage
Situation of the 2011 Tohoku-Pacific Ocean Earthquake, No. 1759 as of June 5,2019 (Japanese).
3. International Nuclear Safety Group (INSAG-IAEA,2011) A Framework for an
Integrated Risk Informed Decision Making Process.
4. Investigation Committee on the Accident at Fukushima Nuclear Power Stations ofTokyo Electric Power Company (Investigation Committee 2012), Final Report VI(https://www.cas.go.jp / jp/ seisaku/icanps/eng/07VIfinal.pdf) .
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6. Knight, Frank H. (1921[1967]) Risk, Uncertainty and Profit, Sentry Press (reprint).
7. Luhmann, Niklas (2003) Soziologie des Risikos, Walter de Gruyter.
8. Ohmae, Kenich (2012) “World is ignoring most important lesson from Fukushima
nuclear accident”, The Christian Science Monitor(https://www.csmonitor.com/Commentary/Global-Viewpoint/2012/0405/World-is-ign
oring-most-important-lesson-from-Fukushima-nuclear-disaster).
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Probability and Uncertainty”, pp.1-30 (http://www.ier.hit-u.ac.jp/extra/18.Sakai.pdf).
10. Saunders, Fiona (2016) “Differentiating between risk and uncertainty in the project
management literature”, MANCHESTER 1824
(http://fionasaunders.co.uk/wp-content/uploads/2016/07/Differentiating-between-Riskand-Uncertainty-in-the-Project-Management-Literature.pdf).
11. Slovic, Paul (1987) “Perception of risk”, Science, Vol.236, pp.280-285.
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Risk-informed Decision Making: A survey of United State Experience , UCLA
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Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants,[WASH-1400 (NURG 75/014)](http://journeesdetudes.org/atomescrochus/recherche/rapport-rasmussen.pdf).
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Assessment Methods in Nuclear Regulatory Activities: Final Policy Statement(https://www.nrc.gov/ reading-rm/doc-collections/commission/policy/60fr42622.pdf).
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Report: NUREG-1855, Vol.1(https://www.nrc.gov/docs/ML0909/ML090970525.pdf).
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for Comment, NUREG-1855, Revision. 1 (URL missing).
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and Regulatory Decisionmaking: Some Frequently Asked Questions, NUREG-2201(https://www.nrc.gov/docs/ML1624/ML16245A032.pdf).
19. U.S. NRC (Nuclear Regulatory Commission) (2017) Guidance on the Treatment ofUncertainties Associated with PRAs in Risk-Informed Decision Making: Final Report,
NUREG-1855, Revision. 1 (https://www.nrc.gov/docs/ML1706/ML17062A466.pdf).
All of the URLs mentioned above were finally accessed on January 31, 2023.

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