Risk-informed Analysis Research Articles

Integrated Decision Support for Disaster Risk Management: Aiding Preparedness and Response Decisions in Wildfire Management

A central challenge in disaster risk management (DRM) is that there are key dependencies and uncertainty between the decisions made at the mitigation, preparedness, response, and recovery stages. Decision support systems for disaster management require information systems that allow timely and reliable integration of data sources from different domains, including information on hazards and vulnerabilities for risk analysis, as well as organizational and logistical information for decision analysis. We propose an analytics-centered framework that integrates predictive and prescriptive models responding to unique characteristics of DRM. The framework relies on probabilistic risk assessment and uses optimization-based simulation of the response phase as a means to inform decisions at the preparedness stage. This paper presents a case study regarding the analysis of preparedness and response decisions for wildfire control in Uruguay. Numerical results illustrate insights from the risk-informed analyses. For instance, slight reductions in the preparedness budget can lead to disproportionate losses during the response stage, whereas slight increases have little effect unless explicitly directed to control high-consequence scenarios. Motivated by a real-world problem, this case study emphasizes the challenges for integrated information systems that enable the potential of analytical decision support frameworks for DRM.

On the use of time-dependent success criteria within risk-informed analyses. Application to LONF-ATWS sequences in PWR reactors

The classical Probabilistic Safety Analysis (PSA) does not include any time dependence explicitly. However, the success criteria (SC) could evolve during the cycle for some initiating events. In that sense, there is a type of sequence in which this time-dependency is quite important, the family of Anticipated Transient without Scram (ATWS) sequences in Pressurized Water Reactors. Therefore, a new risk-informed approach is proposed in this paper, which makes it possible to obtain the time-dependent SC evolution of the safety functions affected by the Moderator Temperature Coefficient (MTC) value. Then, the evolution of the ATWS conditional core damage probability (CCDP) could be obtained using a PSA model. To quantify the CCDP, the average values of the time-dependent failure probabilities must be computed. Finally, the comparison between the CCDP obtained through the application of the classical PSA approach and the new one makes it possible to quantify the impact of time-dependence on the SC of the headers that this new risk-informed ATWS approach can provide.

Considerations for Implementing Presidential Memorandum-20 Guidelines for Nuclear Safety Launch Authorization for Future Civil Space Missions

National Security Presidential Memorandum-20 (NSPM-20) (Launch of Spacecraft Containing Space Nuclear Systems) provides updated guidelines for launch authorization for three categories of proposed launches of spacecraft with space nuclear systems: Federal government civil space including the National Aeronautics and Space Administration (NASA), Federal government defense and intelligence, and commercial. These space nuclear systems provide power, heat, and/or propulsion to the spacecraft. NSPM-20 requires a rigorous, risk-informed safety analysis and launch authorization process. The launch approval process of previous NASA missions each took several years and cost multimillion dollars. NSPM-20 provides guidelines to potentially streamline the process and improve cost and schedule. This technical note examines three NSPM-20 guidelines on a future example interplanetary mission (EIM) as to their potential implementation feasibility for three accident categories: Earth gravity assist reentry, solid propellant fires, and flight termination system design. It is found that the safety technology for these accidents were constantly improved over the last several missions, but in some cases may not be adequate for direct use in the EIM’s launch authorization process.

Preliminary Reliability Allocation Method for Fusion Facility Based on Probabilistic Safety Assessment

The construction of ITER indicates that the development of fusion energy has entered the engineering stage. Reliability assurance, which is essential in the design and safety supervision of a new reactor, is an urgent problem to be solved in the process of fusion reactor engineering. But, up to now, the reliability work of a fusion device has not considered the security of the fusion reactor. This paper proposes a method to establish a reliability index for fusion safety–related components. The reliability index is a useful indicator to evaluate the reliability of the system, and it is also the regulatory basis for regulatory authorities. First, this paper gives the recommended values of probability safety goals for a fusion reactor. Second, the reliability requirements for the fusion safety system that meet the safety goals are calculated. In this part, a Probability Safety Assessment (PSA) is adopted to establish the risk models and calculate the undesired consequences of a fusion reactor. Based on the PSA analysis results, a risk-informed approach is used to categorize the plant structures, systems, and components of the fusion reactor as four categories according to their safety significance. Last, the reliability index of the safety-related components is given based on the results of the risk-informed safety categorization and PSA analysis results. The validation of the reliability index system is still being studied, and this work is expected to support the reliability evaluation and safety supervision of a fusion reactor in the future.

Applying an Information Based Decision Analysis to Spectrum Management Regulatory Decisions

This paper contains the development and application of an interdisciplinary framework for making informed spectrum management regulatory decisions. This paper addresses the question, can a framework that includes technical, risk, and benefits/cost assessments serve as a tool for decision makers when making spectrum management regulatory decisions? In 2010, the U.S. President issued a Presidential Memorandum, directing the Department of Commerce, working with the Federal Communications Commission (FCC), to make spectrum available over the next ten years for expanded wireless broadband use. Consequently, the Spectrum Pipeline, Mobile Now and AIRWAVES Acts have echoed a desire for expansion in commercial spectrum availability. With the expanded spectrum available for allocation, regulators require a repeatable, defensible, and effective approach to inform decisions about new spectrum allocations. This approach must consider; service rules, sharing methods, auction details, incumbent user relocation, and incumbent system adaptation and evolution. Regulators’ ability to make dispassionate spectrum management regulatory decisions is a key to expanding services like mobile broadband in a fair, well-reasoned manner. Using an informed decision-making process could add necessary rigor and provide a definitive decision framework for repurposing spectrum to meet market demands. This paper develops a decision-making framework for spectrum sharing based on existing well-known risk management strategies, including work done by the Nuclear Regulatory Committee, the National Institute of Standards and Technology, the Federal Communications Commission and benefit-cost assessments. Contrary to decision-making techniques that only consider the worst-case scenarios involving the worst performing devices, information-based decision-making frameworks allow decision makers to analyze the continuum of effects across varying conditions and devices, as well as the economic implications of those effects. Risk management frameworks ask (i) what can go wrong? (ii) how likely is it? and (iii) what are the consequences? This paper proposes and applies a decision analysis framework to a spectrum allocation, or service rule change request proposed by regulators. This decision analysis framework was developed by: • investigating state-of-the-art risk management strategies to identify an approach that could be applicable to spectrum management regulatory changes. • evaluating harm-claim interference measurements, considering both out of band and co-channel coexistence scenarios. • performing a technical interdisciplinary decision analysis considering aggregate consumer welfare, including both technical offerings, availability, and impact on consumers, as well as an analysis of the value of all services affected. The decision analysis framework proposed is based on existing risk-informed interference assessments and benefits-cost analyses. The framework has also been tested and proven using a test-case. The framework helps provide appropriate information regulators need to identify and analyze key performance indicators describing harmful interference and performance objectives of the systems in question, as well as economic data describing aggregate welfare. Furthermore, this framework proposes benefit-cost analysis for quantifying economic impact of changing spectrum allocation and service rules.

Risk-Informed Analysis of Accidents at Reactor Facilities with VVER

An accident with ‘rupture of steam line’ as the initial event was studied for a reactor facility with VVER-1000. A computational analysis was performed of different flow scenarios of an accident with singular and multiple failures of the safety systems. The realization probability and the consequences were evaluated for each accident sequence. The consequences of the accidents are ranked and the most instructive combinations of failures of systems and blockings are determined from the standpoint of risk. The presented method is based on multivariate calculations and an integrated combination of deterministic and probabilistic approaches to safety validation.

Risk-informed analysis of the large break loss of coolant accident and PCT margin evaluation with the RISMC methodology

For general design basis accidents, such as SBLOCA and LBLOCA, the traditional deterministic safety analysis methodologies are always applied to analyze events based on a so called surrogate or licensing sequence, without considering how low this sequence occurrence probability is. In the to-be-issued 10 CFR 50.46a, the LBLOCA will be categorized as accidents beyond design basis and the PCT margin shall be evaluated in a risk-informed manner. According to the risk-informed safety margin characterization (RISMC) methodology, a process has been suggested to evaluate the risk-informed PCT margin. Following the RISMC methodology, a load spectrum of PCT for LBLOCA has been generated for the Taiwan’s Maanshan Nuclear Power plant and 14 probabilistic significant sequences have been identified. It was observed in the load spectrum that the conditional PCT generally ascends with the descending sequence occurrence probability. With the load spectrum covering both aleatory and epistemic uncertainties, the risk-informed PCT margin can be evaluated by either expecting value estimation method or sequence probability coverage method. It was found that by comparing with the traditional deterministic methodology, the PCT margin evaluated by the RISMC methodology can be greater by 44–62K. Besides, to have a cumulated occurrence probability over 99% in the load spectrum, the occurrence probability of the sequence referred is about 5.07*10−3, whereas for the traditional surrogate or licensing sequence generally applied in the deterministic methodology, the occurrence probability is only about 5.46*10−5.

Risk-Informed Interference Assessment: A Quantitative Basis for Spectrum Allocation Decisions

The trade-off between the benefits of a new radio service allocation and its risk to incumbents has to date been based on “worst case” analyses that focus on the single scenario with the most severe negative consequence, regardless of its likelihood. This is no longer a tenable approach since it leads to over-conservative allocations that limit the social benefits of new services while giving incumbents more protection than they need.This paper describes an alternative to worst case reasoning: the use of quantitative risk assessment (QRA) to analyze the harm that may be caused by changes in radio service rules. QRA considers both the likelihood and the consequences for multiple hazard scenarios, not just the most severe consequence, regardless of likelihood, as in worst case. While quantitative risk assessment has been used for decades in many regulated industries it has not yet been applied to spectrum management. This paper identifies four key elements of a systematic, quantitative analysis of radio interference hazards, and illustrates them with examples: (1) make an inventory of all significant harmful interference hazard modes; (2) define a consequence metric to characterize the severity of hazards; (3) assess the likelihood and consequence of each hazard mode; and (4) aggregate them into a basis for decision making. The four elements are illustrated by examples from spectrum policy questions past and present; the focal case study is the determination of geographic exclusion zones to protect weather satellite earth stations from co-channel cellular mobile transmitters.In order to reap the benefits of risk-informed interference analysis, the paper rec-ommends that regulators (1) develop know-how through a variety of educational initiatives; (2) use quantitative risk assessment in their analysis of harmful interference, and publish the results; and (3) pilot this approach in proceedings with limited scope, like selected site-specific license waiver proceedings. There is also a role for legislators and the executive branch: (1) they can require risk-informed assessments as part of their spectrum oversight activities; (2) when presented with politically charged claims of harmful interference, they can avoid the temptation of nightmare scenarios and instead make assessments that consider both the likelihood and consequences of interference harms; and (3) they can support regulators that use risk-informed interference assessments rather than basing decisions on worst case analysis.

An integrated methodology to predict the likelihood of spurious actuation of AC circuits due to fires in commercial nuclear power plants

Research efforts by the nuclear power industry and, most recently, its regulatory agency, the U.S. Nuclear Regulatory Commission (USNRC), have provided information useful for risk-informed analysis of the effects of spurious actuation (“hot shorting”) of alternating current (AC) electrical circuits at nuclear power plants. The results from these efforts have been integrated into a methodology to predict the likelihood of spurious actuation of AC circuits due to fires in commercial nuclear power plants.

Development of CFD fire models for deterministic analyses of the cable issues in the nuclear power plant

Additional fire barriers of electrical cables are required for the nuclear power plants (NPPs) in Taiwan due to the separation requirements of Appendix R to 10 CFR Part 50. The risk-informed fire analysis (RIFA) may provide a viable method to resolve these fire barrier issues. However, it is necessary to perform the fire scenario analyses so that RIFA can quantitatively determine the risk related to the fire barrier wrap. The CFD fire models are then proposed in this paper to help the RIFA in resolving these issues. Three typical fire scenarios are selected to assess the present CFD models. Compared with the experimental data and other model’s simulations, the present calculated results show reasonable agreements, rendering that present CFD fire models can provide the quantitative information for RIFA analyses to release the cable wrap requirements for NPPs.

A feasibility assessment of the use of nanofluids to enhance the in-vessel retention capability in light-water reactors

Nanofluids, colloidal dispersions of nanoparticles, exhibit a substantially higher critical heat flux (CHF) compared to water. As such, they could be used to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented by certain light-water reactors. It is envisioned that, at normal operating conditions, the nanofluid would be stored in dedicated storage tanks, which, upon actuation, would discharge into the reactor cavity through injection lines. The design of the injection system was explored with risk-informed analyses and computational fluid dynamics. It was determined that the system has a reasonably low failure probability, and that, once injected, the nanofluid would be delivered effectively to the reactor vessel surface within seconds. It was also shown analytically that the increase in decay power removal through the vessel using a nanofluid is about 40%, which could be exploited to provide a higher IVR safety margin or, for a given margin, to enable IVR at higher core power. Finally, the colloidal stability of a candidate alumina-based nanofluid in an IVR environment was experimentally investigated, and it was found that this nanofluid would be stable against dilution, exposure to gamma radiation, and mixing with boric acid and lithium hydroxide, but not tri-sodium phosphate.

Methods and practices used in incident analysis in the Finnish nuclear power industry

According to the Finnish Nuclear Energy Act it is licensee’s responsibility to ensure safe use of nuclear energy. Radiation and Nuclear Safety Authority (STUK) is the regulatory body responsible for the state supervision of the safe use of nuclear power in Finland. One essential prerequisite for the safe and reliable operation of nuclear power plants is that lessons are learned from the operational experience. It is utility’s prime responsibility to assess the operational events and implement appropriate corrective actions. STUK controls licensees’ operational experience feedback arrangements and imlementation as part of its inspection activities. In addition to this in Finland, the regulatory body performs its own assessment of the operational experience. Review and investigation of operational events is a part of the regulatory oversight of operational safety. Review of operational events is done by STUK basically at three different levels. First step is to perform a general review of all operational events, transients and reactor scram reports, which the licensees submit for information to STUK. The second level activities are related to the clarification of events at site and entering of events’ specific data into the event register database of STUK. This is done for events which meet the set criteria for the operator to submit a special report to STUK for approval. Safety significance of operational events is determined using probabilistic safety assessment (PSA) techniques. Risk significance of events and the number of safety significant events are followed by STUK indicators. The final step in operational event assessment performed by STUK is to assign STUK’s own investigation team for events deemed to have special importance, especially when the licensee’s organisation has not operated as planned. STUK launches its own detail investigation once a year on average. An analysis and evaluation of event investigation methods applied at STUK, and at the two Finnish nuclear power plant operators Teollisuuden Voima Oy (TVO) and Fortum Power and Heat Oy (Fortum) was carried out by the Technical Research Centre (VTT) on request of STUK at the end of 1990s. The study aimed at providing a broad overview and suggestions for improvement of the whole organisational framework to support event investigation practices at the regulatory body and at the utilities. The main objective of the research was to evaluate the adequacy and reliability of event investigation analysis methods and practices in the Finnish nuclear power industry and based on the results to further develop them. The results and suggestions of the research are reviewed in the paper and the corrective actions implemented in event investigation and operating experience procedures both at STUK and at utilities are discussed as well. STUK has developed its own procedure for the risk-informed analysis of nuclear power plant events. The PSA based event analysis method is used to assess the safety significance and importance measures associated with the unavailability of components and systems subject to Technical Specifications. The insights from recently performed PSA based analyses are also briefly discussed in the paper.