NRC Defines Risk Terms in a New Glossary

John Nakoski
Office of Nuclear Regulatory Research
Division of Risk Analysis

As with many subjects, when you’re talking about Probabilistic Risk Assessment (PRA) terms, it can get confusing when the same terms mean different things. To reduce this confusion, the NRC’s Office of Nuclear Regulatory Research decided to produce a glossary of risk-related terms.

glossaryIn November 2013, we published NUREG-2122, “Glossary of Risk-Related Terms in Support of Risk-Informed Decisionmaking.” This glossary provides a single source for definitions of terms used in risk-informed activities. This document clarifies the terms and provides the proper context for using them. For example, the glossary defines the word “assumption” as “a decision or judgment that is made in the development of a model or analysis.” It then describes how “assumption” is used within the context of a PRA.

We developed the list of terms by first reviewing documents used to support risk-informed activities. Both experienced and new staff contributed to this initial list to ensure terms were not excluded too early. We then screened the list to develop the final glossary by considering each term’s significance to risk-informed decisionmaking.

After we finalized the list, we identified definitions that already existed for these terms. Where a definition already existed, we left it as is. Also, when a definition was consistent among the various sources, it was used as the basis for the final definition in the glossary.

To help in understanding the definitions we used, we included cross-references to other terms in the glossary. Also, for the more complex terms, we included plain language background information to clarify the definition used by experts. This supporting information eliminated technical jargon and — we hope — makes the terms more accessible to non-technical readers.

For example, the glossary goes into quite a bit of detail about “Bayesian analysis,” an uncommon term for the lay audience. The glossary explains that the term applies to a type of analysis used when occurrences of an event are so rare or nonexistent that data from actual event occurrences (frequentist approach) are not reliable.

In addition to defining the terms, the glossary describes how they are used in risk-informed decisionmaking and offers some insights on their history. Where multiple definitions exist, we included them in the descriptions along with an explanation of their differences.

This is our attempt to make what we do a bit less opaque to the non-scientist. We hope it reduces ambiguity and promotes common understanding for better communication. If you have thoughts about future adjustments, feel free to let us know in the comments.

Author: Moderator

Public Affairs Officer for the U.S. Nuclear Regulatory Commission

9 thoughts on “NRC Defines Risk Terms in a New Glossary”

  1. On page 83 of NUREG 2122, “Risk Profiles (Plants)” is defined as: The major results generated by a PRA that characterize plant risk.” Additional clarifying information is also provided that states:

    A plant risk profile presents a concise synopsis of the major PRA results. This synopsis may consist of numerous characterizations of risk, including:

    • Core damage frequency and large early release frequency for internally and externally initiated events during various modes of operation.
    • Percentage contributions to core damage frequency and large early release frequency by initiating event and accident sequence type.
    • Ranking of the contribution of individual basic events and cutsets to core damage frequency and large early release frequency, based on various importance measures.
    • Comparison of PRA results to PRAs for other plants.
    • Qualitative risk insights on plant design features.

    The information that comprises the plant risk profile includes site-specific design and operating information that [is] maintained by the licensee.

    Consistent with this definition, the “Risk Profile” of a plant is maintained by and can be “found” with the licensee. The NRC does not maintain the “Risk Profiles” for nuclear power plants as licensees are not required by regulation to have PRAs nor to provide the NRC with risk information. However, the NRC does use risk information in its assessment of the performance of licensees of operating nuclear power plants. Information on the background and results of the NRC’s oversight of operating reactors can be found at: In addition, licensees can voluntarily submit risk information as part of a license amendment request. This information is used by the NRC consistent with the guidance provided in Regulatory Guide 1.174, “An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions
    on Plant Specific Changes to the Licensing Basis,” (

    John Nakoski

  2. Help please, for the moderator or anyone else concerning NUREG 2122’s mention of nuclear facility “risk profiles.” Where may the nuclear plant “Risk Profiles” be found? Is there a singular risk profile number assigned to each nuclear facility?

  3. I’d like to add that the NRC still fails to take into consideration the proven fact that Nature can destroy any land based nuclear reactor, any place anytime 24/7 just like it did at Fukushima and no engineering design work or NRC specifications can prevent that from happening.

    Fukushima proved that even though the probability of a meltdown was a 1 in 10,000 year or even a 1 in a 100,000 year event, not 1 but 3 meltdowns could occur almost on the same day! Probability dictates that an event is just as likely to happen today as it is sometime in the future, yet the NRC tends to always imply that if something does happen it will be far into the distant future, which is illogical at best and simply dangerous in the worst case. Using scientific to somehow imply that something is safe when it is not is the worst thing those with scientific knowledge can do, because it make scientists look like “snake oil salesmen”.

    The USA cannot afford a nuclear accident for any reason and therefore it is up to the NRC to review each nuclear power plant operator and determine what changes need to be made, in order to insure Zero Tolerance when it come to compliance with all safety regulations , because even then Nature can strike at any time…

  4. This is a good start and I encourage the NRC to apply this new glossary to their ongoing investigations, especially like those about the replacement steam generator debacle at San Onofre and St. Lucie .

    Since the lay public has a right to be informed as to what is actually being done to identify the root causes of all accident and incidents, so that the rest of the US Nuclear Fleet can be operated in a safer manner!

    Sure there will be always be Experts that disagree with everything that other Experts on the opposite side of any debate say, but when the NRC repeated refuses to accept that more than one steam generator tube can fail at one time, ex-NRC Experts, knowledgable Engineers and others must question why the NRC refuses to make this change. since San Onofre proved that more than one steam generator tube could fail at one time and because not one but 8 tubes failed in-situ testing a cascade of failing tubes could under the correct conditions lead to draining the reactor core coolant!

    I also agree that the global nuclear industry has the money it needs to fund all the peer reviewed studies it wants, that end up saying whatever they want them to, or they simply will not publish them…

    We also know that exactly three years ago today, all the Japanese nuclear Experts (along with many other nuclear Experts across the World) that said that “modern” nuclear power plants were safe and had so many safety features that they would not meltdown because they were so well designed, were proven terribly wrong by Fukushima’s triple meltdowns and that it will take decades if not about 100 years to deal with its on-going pollution of the Pacific Ocean, that is, if nothing BIG goes BAD before then.

    Also in all fairness, mankind will have to employ NEW types of equipment that have never ever been built, in order to deal with the new problems Fukushima has created. Also, until fully decommissioned, the Japanese will continue to contaminate massive amounts of sea water with radioactivity daily, that will all end up in the Pacific Ocean unless the UN sanctions the Japanese with penalties which should be used to finance Solar (of all flavors) R&D and it’s installation in developing Countries, if they will agree to not use nuclear. This will enable mankind to begin the transition to Solar while at the same time reduce the need for our Earth’s limited resources.

    Yes I also agree that the Coal Industry has many health problems associated with it, which the Nuclear industry is all too eager to point out; but the SAME THING COULD BE SAID ABOUT THE NUCLEAR INDUSTRY; since it also mines radioactive Uranium ore which is then processed into highly radioactive fuel rods of several different types. Once this radioactive fuel is used in a reactor, it then produces huge amounts of additional radioactive waste that will have as yet unknown effects on mankind over the enormous timespan that it will take to render all of it harmless! Because this radioactive timespan dwarfs anything currently affecting mankind, it is completely unscientific to say today, what the harmful effect of our using nuclear power plants in the twentieth and twenty-first century will be generations from now!

    For example, should highly radioactive “dirty” material from Fukushima be used in a terrorist weapon at some point in the future, its affect on man must be placed directly upon the nuclear industry that created it, because without building the nuclear power plants it would have never existed to cause harm to man’s health. This is yet another potential “future” health problem that cannot be discounted since there is so much radioactive waste material unaccounted for at Fukushima and many other locations globally!

    It is no longer fair for the nuclear industries spokespersons, the IAEA and/or regulators like the NRC to try to limit Energy discussions to only the positive points that favor using nuclear while at the same time shrugging off all other negative points as not being relevant!

    Some of the above was also posted at :

  5. Quote from page 2-11 NUREG 2122 – “…no special risk knowledge is presumed by the audience, a basic understanding of nuclear safety is assumed.” Is the public to assume that the NRC’s goal in protecting the public is an ASSUMED mission or a practiced mission? Are folks to ASSUME that the NRC understands nuclear safety?

    The NRC utilizes a model of a parachutist on page 4-9 to discuss an accident sequence. Your path sequence is flawed as there are no allowances for an unintended event occurring prior to an attempt to deploy the “chute,” a very long list in the case of a parachute accident, in the sequence. On Page 4-40, the Event Tree example, goes into more detail in the case of the parachutist accident scenario, it is still lacking the detail required. You display an over simplification of a defined model for accident sequences in risk analysis, why? Am I to ASSUME your entire sequence of accident evaluations/investigation definitions of all events are flawed if the “parachutist scenario” is the defining methodology the NRC utilizes in an accident or event investigation? Note: You do define the term “External Hazard;” however, it is not listed in the “tree” as a required item to investigate. Applicable in this case, e.g. tampering, missile (foreign object) strike, medical condition of the jumper, amongst other factors.

    It is appreciated that the NRC considers beyond-design-basis accidents (BDBA) in the definitions of risk analysis. Unfortunately you issue forth what seems to be a conflicting statement in your BDBA description on page 4-15, quote: “A nuclear facility must be designed and built to withstand a design-basis accident (DBA) without threatening public health and safety. However, the nuclear facility is not necessarily designed to withstand BDBAs.” What is it, either it is designed and built to withstand the accident or it is not? If the design does not contain the necessary elements to withstand the BDBA how will the nuclear facility be built to withstand the BDBA? The NRC’s description of the design vs. building a nuclear facility appears to be an example of flawed logic; before such a complex building may be built, it must be designed. Logic flaws in risk analysis process do not build confidence in the organization performing the risk analysis. Maybe this is a communications failure, which should also be part of risk assessment, it is not listed.

    More questions, where may the nuclear plant “Risk Profiles” be found? Is there a singular risk profile number assigned to each nuclear facility?

    Your sharing the NUREG document as well as facilitating this discussion is appreciated.

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