A Simulation-Based Dynamic Approach for External Flooding Analysis in Nuclear Power Plants

A practical approach for evaluating the strength of knowledge supporting risk assessment models

PRA (Probabilistic Risk Assessment) has been used in nuclear and other industries for a number of years. Generally, the PRA model is based on a completed or as-built design and is updated as the plant or system is modified. This provides the benefit of ensuring that the new modification or design makes the system more reliable or at least that the new “risk” is understood and acceptable. Using PRA as a design tool has been discussed for years, but the disadvantages have often outweighed the advantages of such an endeavor. Given the high risk environment and the emphasis on a “safe” and “reliable” replacement for the Space Shuttle, NASA chose to incorporate the use of PRA methodology into the design requirements for the Constellation Program in order to allow management to make better risk informed decisions. A variety of “lessons” and planning considerations can be learned from NASA's experience. The most obvious advantage of using PRA methodology as a design tool is that PRA helps identify potential risks early in the design process, allowing designers to mitigate or eliminate unacceptable risks. Areas for improvement that can occur when using PRA models early in the design process, include: understanding how to address the problems of limited design information and data, planning for the delay between the current PRA model and the “latest” design changes, how to use and document the series of assumptions needed to supplement the lack of detail in the design, how to use the model for “trade” studies, and how to interpret the numbers and “cut-sets” to make appropriate “risk” based decisions. Understanding and planning for these circumstances would improve the effectiveness, cost and schedule impacts of using this methodology to mitigate or eliminate a variety of safety risks in the design stage instead of re-work after the system has been built.

There are many ways of reducing the O&M costs; this work presents an innovative framework of reducing O&M costs by utilizing the onsite Flex equipment at NPPs. Flex strategies were postulated by the U.S. Nuclear Regulatory Commission (NRC) in the wake of Fukushima Dai-ichi accident to address beyond-design-basis accidents and improve plant flexibility. Onsite Flex includes equipment such as portable pumps, generators, batteries, compressors, and other supporting equipment or tools, all stored in a dedicated and secure building designed to withstand external hazards. In the past years, several NPPs have invested in procuring and maintaining the onsite Flex asset that stands unutilized most of the time. Recently there have been active efforts to develop strategies through which NPPs can take credit for the Flex equipment. This work focuses on identifying areas where Flex equipment can be utilized during normal plant operation and develop a framework that would aide in reduction of O&M costs without impacting plant safety. Earlier work in the current project, published in INL/EXT 18 51531, had presented the risk- and cost-analysis framework for utilizing FLEX portable equipment during technical specification required shutdown due to component failure. The licensee event report (LER) database of the NRC shows that the commercial NPPs in the US reported 86 technical specification required shutdowns since year 2010. When a component failure or unavailability leads to a technical specification required shutdown, the NPPs suffers both direct costs in terms of loss of revenue arising from the loss of generation and indirect costs in form of reporting and inspection required by the NRC. The earlier work developed the following framework and implemented it on a demonstration probabilistic risk assessment (PRA) model to utilize the portable Flex equipment when a component failure could potentially lead to a technical specification required shutdown: 1. Identify the components, the failure or unavailability of which would result in a 10 CFR 50.73(a)(2)(i)A-reportability requirement, postulated by the NRC for technical specification required shutdown to be reported in NRC’s LER database. 2. Identify the Flex equipment that can be utilized as a standby to the failed component. 3. Develop a probabilistic risk assessment (PRA) model that incorporates the Flex equipment within the current plant PRA model. 4. Perform PRA calculations to determine change in core damage frequency and change in risk-informed allowable outage time. 5. Perform cost-benefit analysis to determine the economic feasibility of implementing the Flex equipment The current work, presented in this report, implement the above framework in a plant PRA model obtained from a US commercial nuclear power plant by prestaging onsite FLEX equipment as a redundancy to the existing auxiliary feedwater system, and pre-coordinating with offsite FLEX centers. The new configuration increased the allowable outage time by about a fifty percent overall for the diesel generator being out of service. The benefits to NPPs of utilizing the Flex equipment during normal operations, and performing the PRA developed in this work, include: 1. A risk-informed plant shutdown alternative to the current technical specification required shutdown. 2. Extension of allowable outage time to initiate technical specification required shutdown. 3. Reducing economic impact of component failure, avoiding plant shutdown, and maximizing generation. 4. Save the direct and indirect costs associated with technical specification required shutdown.

An integral Pressurized Water Reactor (iPWR) type SMR was studied featuring Passive Safety Systems (PSSs). Different from active systems, PSSs are easily influenced by system parameters referred to as phenomenological factors such as heat loss, flow friction, oxidation, non-condensable gases and void fraction due to the low driving force of natural circulation. The system parameters also contribute to the uncertainty and dependency of PSS leading to the system unreliability. Thus, efforts are made to improve the reliability of PSS. A classical Probabilistic Risk Assessment (PRA) model describing active systems does not consider time evolution nor event ordering for PSS that dynamic PRA can accommodate. Here we developed and realized coupling between LabVIEW and CAFTA. Isolation Condenser System (ICS) is taken as the benchmark system due to the simple design in single phase without phase change phenomena in order to mainly remove decay heat and secondarily depressurize the reactor pressure vessel (RPV). A classical PRA model of ICS using CAFTA is coupled with real-time simulation of primary loop and ICS in LabVIEW, leading to a dynamic simulation result. The difference in failure probability using dynamic versus classical PRA revealed that for one there are more component demands with different event ordering, such that improved PSS reliability in the iPWR-type SMR designs is possible.

Pertaining to nuclear power plant (NPP) safety, the flooding has been realized as one of the important phenomenon after Fukushima Daiichi accident, as inundation of Fukushima site caused an extended SBO up to 14 days for few units. It is important to analyze all possible internal and external flood hazards which may impact the safety of nuclear facility. This article covers Probabilistic Safety Assessment (PSA) of an internal flooding case only. There are numerous water storages in nuclear power plants, which may cause internal flooding. This paper gives a detailed approach to perform internal Flood PSA highlighting all tasks such as criteria for selection of flooding sources, type of floods according to break sizes, calculation of flooding frequency based on failure probabilities of large volume tanks & high pressure pipelines. The impact of flooding on plant safety has been assessed through flood mitigation by operator action or plant systems, which have been modeled in event tree of the Risk spectrum software. A case study of internal flooding due to component cooling water pipes and failure of high pressure piping of different systems has been performed to analyze impact of flooding on plant safety. The features to control and overcome this phenomenon in plant are also suggested in this paper.

Experience with probabilistic risk assessment in the nuclear power industry

Integrated Level 1–Level 2 decommissioning probabilistic risk assessment for boiling water reactors

The current approach to evaluating the risk of nuclear power plant (NPP) operation relies on static probabilities of component failure, which are based on industry experience with the existing fleet of nominally similar light water reactors (LWRs). As the nuclear industry looks to advanced reactor designs that feature non-light water coolants (e.g., liquid metal, high temperature gas, molten salt), this operating history is not available. Many advanced reactor designs use advanced components, such as electromagnetic pumps, that have not been used in the US commercial nuclear fleet. Given the lack of rich operating experience, we cannot accurately estimate the evolving probability of failure for basic components to populate the fault trees and event trees that typically comprise probabilistic risk assessment (PRA) models. Online equipment prognostics and health management (PHM) technologies can bridge this gap to estimate the failure probabilities for components under operation. The enhanced risk monitor (ERM) incorporates equipment condition assessment into the existing PRA and risk monitor framework to provide accurate and timely estimates of operational risk.

PRA as a management tool: organizational factors and risk-based priorities for the maintenance of the tiles of the space shuttle orbiter

Probabilistic risk assessment for milk in dark chocolate, cookies and other baked goods with PAL sold in Canada

This report concentrates on dynamic probabilistic risk analysis of optical elements for complex characterizationof damages using physical model of solid state lasers and predictable level of ionizing radiation and space weather.The following main subjects will be covered by our report: (a) a solid-state laser model; (b) mathematical modelsfor dynamic probabilistic risk assessment; and (c) software for modeling and prediction of ionizing radiation. Aprobabilistic risk assessment method for solid-state lasers is presented with consideration of some deterministicand stochastic factors. Probabilistic risk assessment is a comprehensive, structured, and logical analysis methodaimed at identifying and assessing risks in solid-state lasers for the purpose of cost-eectively improving theirsafety and performance. This method is based on the Conditional Value-at-Risk measure (CVaR) and theexpected loss exceeding Value-at-Risk (VaR). We propose a new dynamical-information approach for radiationdamage risk assessment of laser elements by cosmic radiation. Our approach includes the following steps: (a)lasermodeling, modeling of ionizing radiation in”uences on laser elements, (b) probabilistic risk assessment methods,and (c) risk minimization. For computer simulation of damage processes at microscopic and macroscopic levelsthe following methods are used: (a) statistical; (b) dynamical; (c) optimization; (d) acceleration modeling, and(e) mathematical modeling of laser functioning. Mathematical models of space ionizing radiation in”uence onlaser elements were developed for risk assessment in laser safety analysis. This is a so-called black box orinput-output model, which seeks only to reproduce the behaviour of the systems output in response to changesin its inputs. The model inputs are radiation in”uences on laser systems and output parameters are dynamicalcharacteristics of the solid laser.Keywords: risk analysis, laser elements, damage, global optimization, prediction

Procedure for the analysis of errors of commission during non-power modes of nuclear power plant operation

Complex engineered systems are increasingly being tasked with operating beyond their design lifecycle. In some industries such as the civilian nuclear power industry, lifecycle extensions are becoming common. Industries with high consequence of failure such as aerospace, critical infrastructure, defense, and others often perform probabilistic risk assessments to determine if a system is within acceptable risk thresholds. However, it is uncommon to account for physical aging, technological obsolescence, and knowledge obsolescence and retention in a probabilistic risk assessment. This paper introduces a system aging risk evaluation method that uses the living probabilistic risk assessment concept and includes the aforementioned aging, obsolescence, and retention issues that commonly cause issues with aging systems. The method allows for practitioners to determine if these issues may increase system risk beyond acceptable thresholds. While developing failure data for a probabilistic risk assessment model that accurately captures how physical aging, technological obsolescence, and knowledge obsolescence and retention is challenging, the resulting analysis this method provides is likely to justify the required effort. This method may be useful beyond analyzing existing systems reaching the end of their lifecycles and could be used as a design driver for new systems that are expected to operate past their intended end of life date.

The software Fire Risk Investigation in 3D (FRI3D) has been developed over the last 2 years to integrate 3D spatial modeling with existing fire probabilistic risk assessment (PRA) models and fire simulation codes. The goal of this research and development is to automate many of the fire analysis manual tasks to reduce industry efforts in the initial fire modeling and operational costs for the model maintenance and evaluations required during normal plant operations. The tasks for Fiscal Year (FY) 2021 include first testing the FRI3D modeling capabilities by importing an industry fire model into FRI3D and making a 3D model of a complex/high-risk significant area. (For this work, the switchgear room was chosen.) Then, the second task of FY 2021 is to develop a dynamic fire PRA process that can help optimize traditional fire PRA models. The switchgear room model will be used for the dynamic fire PRA work. This report describes the work and insights learned when using FRI3D software to model both a Nuclear Regulatory Report (NUREG) example models and a full industry switchgear room.

Across all of its program areas, the United States Environmental Protection Agency conducts risk assessments to evaluate the potential adverse effects of environmental hazards on ecological and human health. Traditionally, these assessments rely on deterministic methods that use point estimates for key parameters and incorporate uncertainty factors and precautionary assumptions to account for uncertainties in data and variability in environmental conditions, exposure pathways, and population characteristics. However, these approaches are unnecessarily conservative for the general population and yet fail to transparently account for the vulnerabilities of susceptible populations. Probabilistic risk assessment (PRA) offers a more refined approach that uses distributional data to better characterize uncertainty and response. By leveraging empirical data and probabilistic modeling, PRA allows for a more transparent, precise quantification of risk that ensures targeted protection for susceptible populations. This article examines how PRA enhances each phase of risk assessment-hazard identification, dose-response assessment, exposure assessment, and risk characterization-to produce a more scientifically robust assessment of risk for susceptible populations, including both human and ecological receptors. We review the literature and discuss practical applications of PRA in regulatory contexts to illustrate its advantages over deterministic approaches. Finally, we discuss key implementation challenges and propose future research directions to advance risk assessment methodologies and improve policy decision-making.