Probabilistic risk assessment

Last updated

Probabilistic risk assessment (PRA) is a systematic and comprehensive methodology to evaluate risks associated with a complex engineered technological entity (such as an airliner or a nuclear power plant) or the effects of stressors on the environment (probabilistic environmental risk assessment, or PERA). [1]

Contents

Risk in a PRA is defined as a feasible detrimental outcome of an activity or action. In a PRA, risk is characterized by two quantities:

  1. the magnitude (severity) of the possible adverse consequence(s), and
  2. the likelihood (probability) of occurrence of each consequence.

Consequences are expressed numerically (e.g., the number of people potentially hurt or killed) and their likelihoods of occurrence are expressed as probabilities or frequencies (i.e., the number of occurrences or the probability of occurrence per unit time). The total risk is the expected loss: the sum of the products of the consequences multiplied by their probabilities.

The spectrum of risks across classes of events are also of concern, and are usually controlled in licensing processes – it would be of concern if rare but high consequence events were found to dominate the overall risk, particularly as these risk assessments are very sensitive to assumptions (how rare is a high consequence event?).

Probabilistic risk assessment usually answers three basic questions:

  1. What can go wrong with the studied technological entity or stressor, or what are the initiators or initiating events (undesirable starting events) that lead to adverse consequence(s)?
  2. What and how severe are the potential detriments, or the adverse consequences that the technological entity (or the ecological system in the case of a PERA) may be eventually subjected to as a result of the occurrence of the initiator?
  3. How likely to occur are these undesirable consequences, or what are their probabilities or frequencies?

Two common methods of answering this last question are event tree analysis and fault tree analysis – for explanations of these, see safety engineering.

In addition to the above methods, PRA studies require special but often very important analysis tools like human reliability analysis (HRA) and common-cause-failure analysis (CCF). HRA deals with methods for modeling human error while CCF deals with methods for evaluating the effect of inter-system and intra-system dependencies which tend to cause simultaneous failures and thus significant increase in overall risk.

PSA for nuclear power plants

One point of possible objection interests the uncertainties associated with a PSA. The PSA (Probabilistic Safety Assessment) has often no associated uncertainty, though in metrology any measure shall be related to a secondary measurement uncertainty, and in the same way any mean frequency number for a random variable shall be examined with the dispersion inside the set of data.

For example, without specifying an uncertainty level, the Japanese regulatory body, the Nuclear Safety Commission issued restrictive safety goal in terms of qualitative health objectives in 2003, such that individual fatality risks should not exceed 10−6/year. Then it was translated in a safety goal for nuclear power plants: [2]

The second point is a possible lack of design in order to prevent and mitigate the catastrophic events, which has the lowest probability of the event and biggest magnitude of the impact, [2] and the lowest degree of uncertainty about their magnitude. A cost-effective of the factor of safety, contribute to undervaluate or completely ignore this type of remote safety risk-factors. Designers choose if the system has to be dimensioned and positioned at the mean or for the minimum level of probability-risk (with related costs of safety measures), for being resilient and robust in relation to the fixed value.

Such external events may be natural hazard, including earth quake and tsunami, fire, and terrorist attacks, and are treated as a probabilistic argument. [2] Changing historical context shall condition the probability of those events, e.g. a nuclear program or economic sanctions.

See also

Related Research Articles

<span class="mw-page-title-main">Safety engineering</span> Engineering discipline which assures that engineered systems provide acceptable levels of safety

Safety engineering is an engineering discipline which assures that engineered systems provide acceptable levels of safety. It is strongly related to industrial engineering/systems engineering, and the subset system safety engineering. Safety engineering assures that a life-critical system behaves as needed, even when components fail.

<span class="mw-page-title-main">Fault tree analysis</span> Failure analysis system used in safety engineering and reliability engineering

Fault tree analysis (FTA) is a type of failure analysis in which an undesired state of a system is examined. This analysis method is mainly used in safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk and to determine event rates of a safety accident or a particular system level (functional) failure. FTA is used in the aerospace, nuclear power, chemical and process, pharmaceutical, petrochemical and other high-hazard industries; but is also used in fields as diverse as risk factor identification relating to social service system failure. FTA is also used in software engineering for debugging purposes and is closely related to cause-elimination technique used to detect bugs.

SAPHIRE is a probabilistic risk and reliability assessment software tool. SAPHIRE stands for Systems Analysis Programs for Hands-on Integrated Reliability Evaluations. The system was developed for the U.S. Nuclear Regulatory Commission (NRC) by the Idaho National Laboratory.

Risk assessment determines possible mishaps, their likelihood and consequences, and the tolerances for such events. The results of this process may be expressed in a quantitative or qualitative fashion. Risk assessment is an inherent part of a broader risk management strategy to help reduce any potential risk-related consequences.

Human reliability is related to the field of human factors and ergonomics, and refers to the reliability of humans in fields including manufacturing, medicine and nuclear power. Human performance can be affected by many factors such as age, state of mind, physical health, attitude, emotions, propensity for certain common mistakes, errors and cognitive biases, etc.

WASH-1400, 'The Reactor Safety Study', was a report produced in 1975 for the Nuclear Regulatory Commission by a committee of specialists under Professor Norman Rasmussen. It "generated a storm of criticism in the years following its release". In the years immediately after its release, WASH-1400 was followed by a number of reports that either peer reviewed its methodology or offered their own judgments about probabilities and consequences of various events at commercial reactors. In at least a few instances, some offered critiques of the study's assumptions, methodology, calculations, peer review procedures, and objectivity. A succession of reports, including NUREG-1150, the State-of-the-Art Reactor Consequence Analyses and others, have carried-on the tradition of PRA and its application to commercial power plants.

A hazard analysis is used as the first step in a process used to assess risk. The result of a hazard analysis is the identification of different types of hazards. A hazard is a potential condition and exists or not. It may, in single existence or in combination with other hazards and conditions, become an actual Functional Failure or Accident (Mishap). The way this exactly happens in one particular sequence is called a scenario. This scenario has a probability of occurrence. Often a system has many potential failure scenarios. It also is assigned a classification, based on the worst case severity of the end condition. Risk is the combination of probability and severity. Preliminary risk levels can be provided in the hazard analysis. The validation, more precise prediction (verification) and acceptance of risk is determined in the risk assessment (analysis). The main goal of both is to provide the best selection of means of controlling or eliminating the risk. The term is used in several engineering specialties, including avionics, food safety, occupational safety and health, process safety, reliability engineering.

NUREG-1150 "Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants", published December 1990 by the Nuclear Regulatory Commission (NRC) is a follow-up to the WASH-1400 and CRAC-II safety studies that employs the methodology of plant-specific Probabilistic Risk Assessment (PRA). The research team, led by Denwood Ross, Joseph Murphy, and Mark Cunningham, concluded that the current generation of nuclear power plants exceeded NRC safety goals.

IEC 61508 is an international standard published by the International Electrotechnical Commission (IEC) consisting of methods on how to apply, design, deploy and maintain automatic protection systems called safety-related systems. It is titled Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems.

Core damage frequency (CDF) is a term used in probabilistic risk assessment (PRA) that indicates the likelihood of an accident that would cause severe damage to a nuclear fuel in a nuclear reactor core. Core damage accidents are considered extremely serious because severe damage to the fuel in the core prevents adequate heat removal or even safe shutdown, which can lead to a nuclear meltdown. Some sources on CDF consider core damage and core meltdown to be the same thing, and different methods of measurement are used between industries and nations, so the primary value of the CDF number is in managing the risk of core accidents within a system and not necessarily to provide large-scale statistics.

The technique for human error-rate prediction (THERP) is a technique used in the field of human reliability assessment (HRA), for the purposes of evaluating the probability of a human error occurring throughout the completion of a specific task. From such analyses measures can then be taken to reduce the likelihood of errors occurring within a system and therefore lead to an improvement in the overall levels of safety. There exist three primary reasons for conducting an HRA: error identification, error quantification and error reduction. As there exist a number of techniques used for such purposes, they can be split into one of two classifications: first-generation techniques and second-generation techniques. First-generation techniques work on the basis of the simple dichotomy of ‘fits/doesn’t fit’ in matching an error situation in context with related error identification and quantification. Second generation techniques are more theory-based in their assessment and quantification of errors. ‘HRA techniques have been utilised for various applications in a range of disciplines and industries including healthcare, engineering, nuclear, transportation and business.

A Technique for Human Event Analysis (ATHEANA) is a technique used in the field of human reliability assessment (HRA). The purpose of ATHEANA is to evaluate the probability of human error while performing a specific task. From such analyses, preventative measures can then be taken to reduce human errors within a system and therefore lead to improvements in the overall level of safety.

<span class="mw-page-title-main">Risk</span> Probability of loss of something of value

In simple terms, risk is the possibility of something bad happening. Risk involves uncertainty about the effects/implications of an activity with respect to something that humans value, often focusing on negative, undesirable consequences. Many different definitions have been proposed. The international standard definition of risk for common understanding in different applications is "effect of uncertainty on objectives".

Risk management tools allow the uncertainty to be addressed by identifying and generating metrics, parameterizing, prioritizing, and developing responses, and tracking risk. These activities may be difficult to track without tools and techniques, documentation and information systems.

Probability bounds analysis (PBA) is a collection of methods of uncertainty propagation for making qualitative and quantitative calculations in the face of uncertainties of various kinds. It is used to project partial information about random variables and other quantities through mathematical expressions. For instance, it computes sure bounds on the distribution of a sum, product, or more complex function, given only sure bounds on the distributions of the inputs. Such bounds are called probability boxes, and constrain cumulative probability distributions.

P-boxes and probability bounds analysis have been used in many applications spanning many disciplines in engineering and environmental science, including:

Probability distribution fitting or simply distribution fitting is the fitting of a probability distribution to a series of data concerning the repeated measurement of a variable phenomenon. The aim of distribution fitting is to predict the probability or to forecast the frequency of occurrence of the magnitude of the phenomenon in a certain interval.

Event tree analysis (ETA) is a forward, top-down, logical modeling technique for both success and failure that explores responses through a single initiating event and lays a path for assessing probabilities of the outcomes and overall system analysis. This analysis technique is used to analyze the effects of functioning or failed systems given that an event has occurred.

Katrina Groth is an American mechanical engineer and professor. Groth is an Associate Professor in Mechanical Engineering at the University of Maryland, College Park, where she is the associate director for research for the Center for Risk and Reliability and the director of the Systems Risk and Reliability Analysis lab (SyRRA). Groth previously served as the Principal Research & Development Engineer at Sandia National Laboratories.

<span class="mw-page-title-main">High explosive violent reaction</span> Type of explosion

A high explosive violent reaction (HEVR) includes reactions ranging from a fast deflagration of the high explosive (HE), up to and including a detonation of the high explosive. The explosive wave may be subsonic or supersonic.

References

  1. ↑ Goussen, Benoit; Price, Oliver R.; Rendal, Cecilie; Ashauer, Roman (2016). "Integrated presentation of ecological risk from multiple stressors". Scientific Reports. 6: 36004. Bibcode:2016NatSR...636004G. doi:10.1038/srep36004. PMC   5080554 . PMID   27782171.
  2. 1 2 3 Song, Jin Ho; Kim, Tae Woon (2014). "Severe Accident Issues Raised by the Fukushima Accident and Improvements Suggested". Nuclear Engineering and Technology. 46 (2): 207–216. doi: 10.5516/NET.03.2013.079 .
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.