Risk and Safety Analysis of Nuclear Systems

John C. Lee, Norman J. McCormick

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781118043455
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

The book has been developed in conjunction with NERS 462, a course offered every year to seniors and graduate students in the University of Michigan NERS program.

The first half of the book covers the principles of risk analysis, the techniques used to develop and update a reliability data base, the reliability of multi-component systems, Markov methods used to analyze the unavailability of systems with repairs, fault trees and event trees used in probabilistic risk assessments (PRAs), and failure modes of systems. All of this material is general enough that it could be used in non-nuclear applications, although there is an emphasis placed on the analysis of nuclear systems.

The second half of the book covers the safety analysis of nuclear energy systems, an analysis of major accidents and incidents that occurred in commercial nuclear plants, applications of PRA techniques to the safety analysis of nuclear power plants (focusing on a major PRA study for five nuclear power plants), practical PRA examples, and emerging techniques in the structure of dynamic event trees and fault trees that can provide a more realistic representation of complex sequences of events. The book concludes with a discussion on passive safety features of advanced nuclear energy systems under development and approaches taken for risk-informed regulations for nuclear plants.

CHAPTER 1

RISK AND SAFETY OF ENGINEERED SYSTEMS

1.1 RISK AND ITS PERCEPTION AND ACCEPTANCE

Risk and safety concerns for the engineering of nuclear power plants are somewhat analogous to the opposing yin and yang energies that represent the ancient Chinese understanding of how things work. The outer circle represents “everything”, while the “yin” (black) and “yang” (white) shapes within the circle represent the interaction of two energies that cause everything to happen. As such, risk (yin) is the performance downside of a nuclear system and safety (yang) is what happens when the system performs its intended function. In the Chinese interpretation of yin-yang, there is a continuous movement between the two energies, just as there is when a nuclear system operates. Just as the Chinese have observed, risk and safety are intertwined, even though the engineering principles for each have a different emphasis.

Risk is the combination of the predicted frequency of an undesired initiating event and the predicted damage such an event might cause if the ensuing follow-up events were to occur. In essence, it combines the concepts of “How often?” with “How bad?”

In this book we are concerned with probabilistic risk assessment (PRA) and the methods used to analyze the safety of nuclear systems. For this reason we are investigating risks that might occur to society as a whole, rather than risks that might be incurred by an individual in society. A PRA typically models events that only very rarely occur. Hence it differs from an investigation in which there is an operating history from which to predict risks. Although most of the licensing and regulations governing the current generation of operating nuclear power plants are based on deterministic assessment of the consequences of postulated accidents and operating conditions, there is an increasing emphasis placed on implementing PRA techniques in licensing decisions. With this perspective, the terminology probabilistic safety analysis often is used to represent the safe assessment that combines the elements of both probabilistic and deterministic methods. Thus, the dichotomy between risk and safety has become somewhat fuzzy in recent years.

When thinking about a complex technology it is not difficult to conjecture a series of questions: What if undesired event A happened? Or if undesired event B happened? Or if undesired event C happened? … To scientifically answer such questions requires clearly defining what the consequences of events A, B, C, … are, but an often overlooked aspect is the frequency of occurrence of such events. Risk analysis techniques are needed to assess both the frequency and the consequence of an undesired event while safety analysis techniques are for preventing the occurrence of such events.

Perception of the risk associated with any human activity, including that associated with the utilization of man-made systems, is quite subjective. This can be illustrated by the way the news media typically report on airplane crashes involving the injury or death of even a few passengers and crew, while the annual casualties of 40,000 to 50,000 individuals due to automobile accidents in the United States do not receive special coverage. The distinction between perhaps a few hundred casualties resulting from airplane accidents and a much larger number of deaths from automobile accidents in the United States every year can be characterized in two ways: (a) voluntary versus involuntary risks and (b) distributed versus acute or catastrophic risks. We consider the risk associated with traveling in private automobiles a voluntary one that is under our personal control, in contrast to the involuntary risk involved with commercial airline flights in which we do not have control. Similarly, an automobile-related accident typically does not result in a large number of casualties so the risk is distributed, while a catastrophic airline crash can result in a large number of casualties.

Acceptability of risk is often inversely proportional to the consequences. In the risk space shown in Fig. 1.1, the abscissa represents the consequences or dreadfulness and the ordinate the observability or familiarity of the hazard. Events in the upper right quadrant, entailing significant consequences and significant unfamiliarity or limited observability, generally require strict regulations. In the case of postulated accidents in nuclear systems, the consequences could be significant although the probability of the accidents is predicted to be very small. Thus, the traditional method of risk evaluation is often subject to public skepticism, despite the extensive efforts made in implementing scientific principles in the design, construction, and operation of nuclear systems.

Figure. 1.1 Risk space illustrating acceptability of risks as a function of consequences and observability.

Source: Reprinted with permission from [Mor93]. Copyright © 1993 Scientific American, a division of Nature America, Inc.

Risks are incurred in everyday life by everyone, of course. So what distinguishes such risks from those from the operation of a nuclear power plant, for example? An important distinction in whether an individual accepts a risk is whether he or she has control over the risk to be incurred. Other factors are important as well and have been summarized in Table 1.1.

Table. 1.1 Factors Affecting Acceptance of Risks

Effect	Opposite Effect
Assumed voluntarily	Incurred involuntarily
Consequences occur immediately	Consequences delayed
Consequences reversible	Consequences irreversible
Consequences short term	Consequences long term
No alternatives available	Many alternatives available
Small uncertainty	Large uncertainty
Common hazard	Unknown or “dreaded” hazard
Exposure is necessary	Exposure is optional
Incurred occupationally	Incurred nonoccupationally
Incurred by other people	Not incurred by other people
Source: Modified and expanded from [Low76].

The use of nuclear power for the generation of electricity has the disadvantage of many factors working against its acceptance. By its very nature, a probabilistic analysis of any system can never yield a result for a “risk known with certainty.” The potential for a delay of the effects and the irreversible consequences following a catastrophic event at a radioactive waste disposal site are contributing effects to the siting of such sites, for example. Public concerns over the potential for delayed climate changes arising from the buildup of CO2 also can be understood in the context of the Table 1.1 factors.

One might think that the response of the public to modern medical imaging methods might provide a clue for the eventual acceptance of nuclear power. Widespread acceptance of x-rays shows that a radiation technology can be tolerated once its use becomes familiar, its benefits clear, and its practitioners trusted. In spite of the two most widely publicized nuclear power accidents, at Three Mile Island Unit 2 and Chernobyl, the nuclear power safety record is outstanding in light of the benefits obtained from the electricity generated without CO2 emissions. But yet several decades have passed, with countries like France generating upward of 80% of its electricity by nuclear power, and the acceptance of nuclear power in the United States has remained lower than most engineers with a nuclear background could have imagined at those earlier times.

It can be argued that unfavorable media publicity has played a role in the lack of acceptance of nuclear power by a large fraction of the U.S. population. An outstanding example of this is what transpired after the Three Mile Island nuclear reactor accident in March 1979 in which some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. Indeed, for 18 years the Pennsylvania Department of Health maintained a registry of more than 30,000 people who lived within 5 miles of Three Mile Island at the time of the accident, but that was discontinued in mid 1997 without any evidence of unusual health trends in the area. Yet an explosion at the Union Carbide India pesticide plant in Bhopal in December 1984 released toxic gas in the form of methyl isocyanate and its reaction products over the city. The estimated mortality of this accident is believed to have been between 2500 and 5000 people, with up to 200,000 injured [Meh90]. But such an accident was largely ignored by the media in comparison to the publicity surrounding the Three Mile Island accident. One reason for this disparity was that the consequences of the Bhopal accident were known within days while the effects of the Three Mile Island accident took years to assess.

Industrial facilities such as nuclear reactors and chemical plants have been studied, by the techniques presented in this book, for their risks to the public at large. But such investigations are entirely different than what people do in making their own individual decisions about risks in their everyday lives. Because ordinary citizens do not have direct control over how their electricity is generated or various products are manufactured, the operation of such industrial facilities must lead to the probability of undesired consequences much lower than the risks from everyday occurrences.

For common risks leading to unnatural human deaths incurred involuntarily by an individual, for example, the probability of occurrence loosely can be bounded between 10−6/yr and 10−2/yr. The lower bound is set by the risk of death from natural events, such as lightning, flood, earthquakes, insect and snake bites, etc. (about one death per year per million people) and the upper bound arises by the death rate from disease (about one death per year per 100 people). The lower bound is not, however, appropriate for a large-scale commercial facility like a nuclear power or chemical plant.

One can argue that the risks from the operation of plant A need not necessarily be as small as those from operation of plant B if one perceives the benefits of the products produced by plant A to be greater than those from plant B. An early comparative risk assessment of technologies for the generation of electricity was performed by Inhaber [Inh82]. He investigated the production of electricity in MWe-yr from 11 different sources: coal, oil, nuclear, natural gas, hydroelectric, wind, methanol, solar space heating, solar thermal, solar photovoltaic, and ocean thermal sources (but did not consider ocean tidal, for example). One innovative feature of the study was to put the technologies for each power source on equal footing by also as...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Preface
• Permissions and Copyrights
• List of Tables
• List of Figures
• Chapter 1: Risk and Safety of Engineered Systems
• Chapter 2: Probabilities of Events
• Chapter 3: Reliability Data
• Chapter 4: Reliability of Multiple-Component Systems
• Chapter 5: Availability And Reliability of Systems With Repair
• Chapter 6: Probabilistic Risk Assessment
• Chapter 7: Computer Programs for Probabilistic Risk Assessment
• Chapter 8: Nuclear Power Plant Safety Analysis
• Chapter 9: Major Nuclear Power Plant Accidents and Incidents
• Chapter 10: PRA Studies of Nuclear Power Plants
• Chapter 11: Passive Safety and Advanced Nuclear Energy Systems
• Chapter 12: Risk-Informed Regulations and Reliability-Centered Maintenance
• Chapter 13: Dynamic Event Tree Analysis
• Appendix A: Reactor Radiological Sources
• Appendix B: Some Special Mathematical Functions
• Appendix C: Some Failure Rate Data
• Appendix D: Linear Kalman Filter Algorithm
• Index
• EULA