Quantitative Safety Goals Research Articles

Quantitative safety goals for fusion power plants: Rationales and suggestions

“How safe is safe enough?” is a key and fundamental question for any risky engineering and safety goals has been extensively studied in many fields, such as chemical industry, nuclear industry, etc. Fusion reactors are advanced nuclear reactors which are expected to be more attractive in terms of safety and environmental impact, and remarkable progress has been made toward the realization of fusion energy in this regard. However, fusion safety is not guaranteed automatically: accidents could happen with any possible future fusion reactor, potentially resulting in releases of radioactive materials. Meanwhile, there are also safety issues about radioactive wastes and occupational exposure, due to high neutron radiation produced in the reactor. To ensure the safety of fusion reactors from the origin, defining safety goals is very necessary to guide reactor design. In this paper, the evolution of safety goals for nuclear reactors is reviewed and a hierarchical framework for fusion quantitative safety goals is established in the four aspects of occupational radiation exposure, routine releases of radioactive materials, accidents and radwastes, based on the characteristics of fusion reactors from a designer's perspective as well as the experiences from the pressurized water reactor, Generation-IV, and International Thermonuclear Experimental Reactor. These safety goals will provide reference for designers of fusion reactors These safety goals will be helpful to guide the designer of fusion reactors to seek and develop appropriate means to enhance safety.

Consistency issues in quantitative safety goals of nuclear power plants in Korea

Abstract As the safety level of nuclear power plants (NPPs) relates to the safety of individuals, society, and the environment, it is important to establish NPP safety goals. In Korea, two quantitative health objectives and one large release frequency (LRF) criterion were formally set as quantitative safety goals for NPPs by the Nuclear Safety and Security Commission in 2016. The risks of prompt and cancer fatalities from NPPs should be less than 0.1% of the overall risk, and the frequency of nuclear accidents releasing more than 100 TBq of Cs-137 should not exceed 1E-06 per reactor year. This paper reviews the hierarchical structure of safety goals in Korea, its relationship with those of other countries, and the relationships among safety goals and subsidiary criteria like core damage frequency and large early release frequency. By analyzing the effect of the release of 100 TBq of Cs-137 via consequence analysis codes in eight different accident scenarios, it was shown that meeting the LRF criterion results in negligible prompt fatalities in the surrounding area. Hence, the LRF criterion dominates the safety goals for Korean NPPs. Safety goals must be consistent with national policy, international standards, and the goals of other counties.

A DYNAMIC RISK ASSESSMENT MODELING BASED ON FUZZY ANP FOR SAFETY MANAGEMENT SYSTEMS

Risk assessment in large organizations with extensive operational domains has been a challenging issue. Employing an efficient method along with realistic pair comparisons, applying subjective inferences of organization experts, and purging the intrinsic ambiguity of inferences, are not reflected in current airlines' safety management. Traditional two-dimensional risk assessment for risk management of safety hazards, however, is no longer sufficient to comply with this complexity. A new model for risk management and a novel formula for risk index calculation, based on a fuzzy approach, are presented in this study. In this new model, unlike in the traditional approach, the latent aftermath of safety reports, especially those which affect the continuity of the business, is also taken into account. In this model, along with the definition of a new structure for risk management, risk analysis should be restructured. To that end, a two-dimensional classic risk formula was replaced with three-dimensional (nonlinear) exponential ones, considering “the impact on the business” as a source of risk and hazard. For measuring the safety risk using the Fuzzy hierarchical evaluation method, considering experts' opinions, three criteria in four different operational fields were developed. This method employs a Fuzzy ANP to help quantify judgments, make qualitative judgments in the traditional method, and weigh the priority of elements contributing to risk. Also, it provides a tool for top-level as well as expert level management to monitor safety more precisely, monitor the safety level within their departments or organizations, set quantitative safety goals and provide feedback for improvement as well as find the most critical areas with the least cost. In this study, an airline has been selected as a case study for the risk assessment of reports based on the new model.

Life-safety risks and optimisation of protective measures against terrorist threats to infrastructure

A decision support analysis considers fatality risks and cost-effectiveness of protective measures, expressed in terms of expected cost spent on risk reduction per life saved for terrorist threats to infrastructure. The analysis is applicable to any item of infrastructure, but in this paper, it is applied to commercial buildings in the US. Risks may be compared with risk acceptance criteria in the form of quantitative safety goals. The risk acceptability and cost-effectiveness of protective measures includes cost of the protective measures, attack probability, reduction in risk due to protective measures, probability of fatality conditional on successful terrorist attack and number of exposed individuals. The risk-based approach developed herein provides a means for initial risk screening based on the broad levels of risk and its acceptability.

Experience with quantitative safety targets on international level

Abstract On international level, probabilistic safety assessment represents the state of the art in the licensing process for new industrial facilities, but increasingly also for evaluating the safety level of older industrial plants, e. g. as part of periodic safety reviews in case of nuclear installations. Quantitative safety goals or criteria have not yet reached the same level of acceptance. However, the role of such goals/criteria depends on the type of industry. Most of the countries consider such criteria as safety targets rather than as sharply defined boundary values. The Netherlands and the United Kingdom are exceptions because they require demonstration of compliance with legally binding safety goals in the licensing procedure for all types of industrial activities.

Risk acceptability and cost-effectiveness of protective measures against terrorist threats to built infrastructure considering multiple threat scenarios

Decisions are often needed about the need and/or extent of protective measures against explosive blast loads on built infrastructure. A decision support analysis considers fatality risks and cost-effectiveness of protective measures expressed in terms of expected cost spent on risk reduction per life saved for terrorist threats to infrastructure. The analysis is applicable to any item of infrastructure, but in this paper is applied to casualties arising from building facade glazing damage. Risks may be compared with risk acceptance criteria in the form of quantitative safety goals. The risk acceptability and cost-effectiveness of protective measures includes cost of the protective measures, attack probability, reduction in risk due to protective measures, probability of fatality conditional on successful terrorist attack and number of exposed individuals.

Basic Considerations on Defining Safety Goals

A study on considerations and strategies for developing safety goals applicable to all nuclear facilities is presented. A scheme of basic safety goals and subsidiary safety goals is assumed, and quantitative basic safety goals for radiation protection of individuals in the vicinity of nuclear facilities are discussed. The risk limit rules are based on comparative risk factors for various health effects, including annual dose, health detriments, acute fatality, latent cancers, and severe hereditary effects. The comparative factor of 1% is used for nonfatal health detriments and 0.1% for fatal effects. A risk limit profile is generated from the risk factors, biological effects, and statistics of injury and mortality rates from general causes. The approaches have potential for developing integrated and comprehensive safety goals.

A Unified Approach for Calculating Core Melt Frequency Caused by Internal and External Initiating Events

A unified approach for calculating the core melt frequency of a specific reactor caused by both internal and external accident initiators is demonstrated. Two classes of internal initiators are examined: transients, of which turbine trip is the chosen example, and loss‐of‐coolant events of various sizes. The concepts of hazard and fragility analysis first proposed for seismic risk analysis are linked to the frequencies of internal initiating events, and to the plant response as a function of the event intensity. Uncertainties are propagated using discrete probability distribution (DPD) arithmetic. Advantages of this approach include mathematical and conceptual consistency, and an improved uncertainty analysis, which are important considerations if risk studies are to be utilized in decision‐making based on quantitative safety goals.

Quantitative safety goals for risk management of industrial facilities

This paper discusses the issues, constraints and conflicts involved in risk management for industrial facilities and proposes a global and explicit approach to numerical safety goals. The use of such quantitative targets is part of a more general strategy of risk management. The factors involved are technical, organizational, ethical, social, legal and economic. In order to set priorities among safety measures, the first step is a probabilistic risk analysis, which should not be limited to technical parameters but should also include organizational factors. A review of precedents and “floating numbers” commonly used as safety targets is presented. A general structure of safety goals is then proposed with illustrative numbers. The case of existing facilities and the treatment of uncertainties are discussed briefly.

Probabilistic Reliability Analysis, Quantitative Safety Goals, and Nuclear Licensing in the United Kingdom

Although unpublicized, the use of quantitative safety goals and probabilistic reliability analysis for licensing nuclear reactors has become a reality in the United Kingdom. This conclusion results from an examination of the process leading to the licensing of the Sizewell B PWR in England. The licensing process for this reactor has substantial implications for nuclear safety standards in Britain, and is examined in the context of the growing trend towards quantitative safety goals in the United States.

Safety goals for nuclear power plant regulation

A survey and comparison has been made of 10 widely disseminated safety goal proposals originating in Canada, the U.K. and the U.S. When the proposals are placed in a directly comparable numerical context, they are remarkably similar in the inferred levels of safety. The development of quantitative safety goals by the U.S. Nuclear Regulatory Commission is also reviewed. While progress has been made in structuring technically and politically defensible goals, because of the way the regulatory philosophy has evolved in the U.S. the implementation of such goals could well turn out to be more difficult than defining the goals themselves. The safety goals are useful in providing guidance to the designers, operators and regulators of nuclear power plants with regard to public safety concerns as well as plant reliability and engineering considerations.

On the usefulness of quantitative safety goals for state regulation of energy systems

The potential usefulness of quantitative safety goals for energy systems in the State of California is evaluated. Five energy-related risk issues, previously dealt with by state regulatory agencies, are examined for the role played by risk in decision-making and the possible role that quantitative safety goals might have played. Several other energy-related risk situations are identified and briefly discussed. It is concluded that quantitative safety goals do not appear to be needed for risk management of energy systems in California. However, there does appear to be a role for a central office of risk management in the State governmental structure. Such an office would provide a reference and evaluation service for other agencies. The potential usefulness of establishing threshold levels of risk for different kinds of action is also noted.

Quantitative safety goals through more adequate risk management and risk assessment

The paper offers an overview of the current regulatory background within the United States, emphasising nuclear regulation, related to the establishment of quantitative safety goals as a way to respond to the key issue of ‘How safe is safe enough?’ The approach described in the paper, based on use of probabilistic risk assessments techniques is centred around definition of quantitative safety regions. Regions are termed: design basis, safety margin, the design capability, and safety research. The design basis region is bounded by the frequency of 10 −4 per reactor year and consequences of no identifiable public injury. The frequency of 10 −4 per reactor year, is associated with the total projected lifetime of a commercial US nuclear power programme. Events which have a 50% chance of happening are included in the design basis region. In the safety margin region, which extends below the design basis region, protection is provided against events whose probability of not happening during the expected course of the US nuclear power programme is within the region of 50 to 90%. Setting the lower mean frequency to this region of 10 −5 per reactor year is equivalent to offering 90% assurance that an accident of given severity will not happen. A rare event with a mean frequency below 10 −5 can be predicted to occur. However, accidents predicted to have a probability of less than 10 −6 are 99% certain not to happen at all, and are thus not anticipated to affect public health and safety. Safety goals associated with individual risk to a maximum exposed member of the public, general societal risk, and public property risk are proposed in the paper. The derivation and rationale behind the above stated goals are briefly discussed.

Insights from On-Going PRA Studies

Earlier papers (Ref. 1, 2) dealt with the subject of use of probabilistic risk assessment (PRA) techniques in safety and licensing. The last year's paper discussed an approach to establishing quantitative safety goals in the regulatory process (Ref. 3). Other authors, notably Saul Levine (e.g. Refs. 4,5), dealt with the same subjects from the perspectives of the RSS (Reactor Safety Study). PRA, as a safety assessment tool, experienced a tortuous acceptance. Prior to the Three Mile Island accident, despite existence of several studies, PRA was on the way to extinction. In the aftermath of TMI it was noted that WASH-1400 contains a similar accident sequence. This understanding began the rebirth of WASH-1400 which was completed by virtue of Kemeny Commission and Rogovin Inquiry recommendations. As a result, use of PRA techniques is rapidly becoming widespread. The objective of this paper is to provide some insights from on-going plant specific PRA Studies. Generic studies have been prolifically discussed elsewhere and are listed here primarily for the sake of completeness. The paper discusses briefly Limerick (Ref. 6), Big Rock Point (Ref. 7) and Sequoyah (Ref.8) studies from the standpoint of an outside party.

Industrial risks

A brief look is taken at the status of progress, or the lack of it, towards a quantitative approach to the estimation and assessment of risk for several technologies in the United States. The increase of interest in the consideration of comparative risks in decision-making is also discussed. Finally, a recently proposed trial approach to quantitative safety goals for light-water nuclear power reactors is summarized. The approach is divided into two major tasks: the predominantly social and political task of setting safety criteria, and the technical task of estimating the risks and deciding whether the safety criteria have been met. The safety criteria include the following: limits on hazard states within the reactor; limits on risk to the individual; limits on societal risk; a cost-effectiveness criterion as low as reasonably achievable; a small element of risk aversion.

Quantitative Safety Goals

An extensive heritage of quantitative safety knowledge now makes it feasible to consider quantitative safety goals for nuclear power plants. Comparative risk is the foundation for some of the deliberations, but most of the discussion is based on knowledge of probabilistic risk assessment (PRA) studies. The licensing notion of design basis accidents is retained for those families of events having mean frequencies above 10-4/reactor-year with PRA being used to screen the events. The consequence limit discussed, for individual risk, is that of no identifiable public injury for design basis accidents. This approach yields a dose limit of 5 rem at 10-4/reactor-year. A limit line is derived for the individual risk having a slope of -1.3. Societal risks are measured by latent cancers which could result from accidents. For safety policies which allow balancing of risks, more latent cancers can be tolerated because of the large and variable spontaneous cancer rate from non-nuclear causes. This may become a suitable and feasible policy for the present generation of light water reactors. If, on the other hand, safety policies were to emphasize nuclear risk in an attempt to lower it, a much smaller number of latent cancers might become an appropriate goal. A future generation of nuclear power plants might make such an approach more feasible. Risks from damage to public property are considered on a first cut basis.

Use of PRA in evaluating safety of nuclear power

This paper offers an overview of uses and limitations of probabilistic risk assessment (PRA) techniques in evaluating safety of nuclear power in the US as seen and experienced by the authors. It discusses subjects like PRA contributions to the defense-in-depth philosophy adopted in the US many years ago for licensing nuclear power plants, needs to expand its present role, present pitfalls, the Three Mile Island (TMI) accident sequence, retrospective and prospective safety improvements, quantitative safety goals and the nuclear fuel cycle.

Conceptual Questions in Air Traffic Control Design

ATC system organization is discussed with regard to safety, capacity and delay. The derivation of explicit relations between these is believed possible. This could form a framework for system improvement studies. Adoption of quantitative safety goals would be helpful. Improvements in runway capacity and terminal operations and determination of minimum spacing between parallel runways are discussed. The overall system is viewed as a lightly coupled but heavily loaded system. Tighter coupling to shift delays to the ground and to decrease airborne delays due to occasional en route saturation is reluctantly viewed as necessary. Expansion of terminal capacity is urgently needed to reduce total delay and to reduce need for overall system coupling. Various analyses are needed to understand the organic functioning of the system and to properly frame conceptual questions which need resolution.