Seismic Isolation Working Meeting Gap Analysis Report
J Coleman PhD
Stephen McDuffie2014
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Abstract
The ultimate goal in nuclear facility and nuclear power plant operations is operating safely during normal operations and maintaining core cooling capabilities during off-normal events including external hazards. Understanding the impact of external hazards, such as flooding and earthquakes, have on nuclear facilities and NPPs is critical to deciding how to manage these hazards to expectable levels of risk. From a seismic risk perspective the goal is to manage seismic risk. Seismic risk is determined by convolving the seismic hazard with seismic fragilities (capacity of systems, structures, and components (SSCs)). There are large uncertainties associated with evolving nature of the seismic hazard curves. Additionally there are requirements within DOE and potential requirements within NRC to reconsider updated seismic hazard curves every 10 years. Therefore opportunity exists for engineered solutions to manage this seismic uncertainty. One engineered solution is seismic isolation. Current seismic isolation (SI) designs (used in commercial industry) reduce horizontal earthquake loads and protect critical infrastructure from the potentially destructive effects of large earthquakes. The benefit of SI application in the nuclear industry is being recognized and SI systems have been proposed, in the American Society of Civil Engineers (ASCE) 4 standard, to be released in 2014, for Light Water Reactors (LWR) facilities using commercially available technology. However, there is a lack of industry application to the nuclear industry and uncertainty with implementing the procedures outlined in ASCE-4. Opportunity exists to determine barriers associated with implementation of current ASCE-4 standard language. Based on discussions with nuclear vendors there is opportunity to apply SI solutions for systems and components in nuclear facilities and NPPs. There may be a need to seismically isolate systems and components such as diesel generators and reactor pressure vessels. However, SI procedures and designs proposed for SI of an entire facility may not be appropriate because the mass of many systems and/or components is relatively small, and their geometry is very different. For these systems and/or components there may be a need to provide three-dimensional seismic isolation in addition to lateral SI. To start this activity a working meeting was convened on August 19 th 2014 with representatives from DOE, National Laboratories, Industry, EPRI, and NRC to discuss three SI topics: 1) general background on current SI progress in the U.S., 2) limitations associated with implementing procedures outlined in ASCE-4 for SI solutions of entire nuclear power plant, and 3) to determine potential SI solutions for systems and components and gaps associated with developing standardized technologies, methods, and numerical tools for these solutions. Additionally, the working meeting highlights what systems and/or components could benefit from SI. Issues related to implementation of SI were discussed at the meeting and these issues help in identification of areas and needs to perform research and development (R&D).
Related papers
ASCE 4 provisions for seismic isolation of safety-related nuclear structures
Seismic isolation is a viable method of protecting nuclear safety-related structures from the damaging effects of earthquake shaking. The soon-to-be completed 2013 edition of ASCE Standard 4 will include detailed provisions and commentary (Section 7.7) to enable the horizontal seismic isolation of nuclear facilities such as nuclear power reactors and waste storage facilities. Although the provisions and commentary focus on building-type structures, they can be applied, in principle, to other structures, systems, and components, including small modular reactors and safety-related systems such as diesel generators. The performance expectations associated with the provisions, and their integration with ASCE 43-05, are presented together with the design basis for the isolated superstructure and safety-related secondary systems, the isolators, the foundation and the umbilical lines that cross the isolation interface.
Seismic isolation of nuclear power plants
Seismic isolation is a viable strategy for protecting safety-related nuclear structures from the effects of moderate to severe earthquake shaking. Although seismic isolation has been deployed in nuclear structures in France and South Africa, it has not seen widespread use because of limited new build nuclear construction in the past 30 years and a lack of guidelines, codes and standards for the analysis, design and construction of isolation systems specific to nuclear structures. The funding by the United States Nuclear Regulatory Commission of a research project to the Lawrence Berkeley National Laboratory and MCEER/University at Buffalo facilitated the writing of a soon-to-be-published NUREG on seismic isolation. Funding of MCEER by the National Science Foundation led to research products that provide the technical basis for a new section in ASCE Standard 4 on the seismic isolation of safety-related nuclear facilities. The performance expectations identified in the NUREG and ASCE 4 for seismic isolation systems, and superstructures and substructures are described in the paper. Robust numerical models capable of capturing isolator behaviors under extreme loadings, which have been verified and validated following ASME protocols, and implemented in the open source code OpenSees, are introduced.
Regulatory gaps and challenges for licensing advanced reactors using seismic isolation
INL/EXT-15 36945, 2016
Over the last decade, particularly since implementation of the certified design regulatory approaches outlined in 10 CFR 52, “Licenses, Certifications, and Approvals for Nuclear Power Plants,” interest has been increasing in the use of seismic isolation (SI) technology to support seismic safety in nuclear facilities. In 2009, the United States (U.S.) Nuclear Regulatory Commission (NRC) initiated research activities to develop new guidance targeted at isolated facilities because SI is being considered for nuclear power plants in the U.S. One product of that research, which was developed around a risk-informed regulatory approach, was a draft NRC nuclear regulatory commission (NUREG) report that investigated and discussed considerations for use of SI in otherwise traditionally-founded large light water reactors (LWRs). A coordinated effort led to new provisions for SI of LWRs in the forthcoming standard ASCE/SEI 4-16, “Seismic Analysis of Safety-Related Nuclear Structures.” The risk-informed design philosophy that underpinned development of the technical basis for both of these documents led to a set of proposed performance objectives and acceptance criteria that was developed to serve as the foundation for future NRC guidance on the use of SI and related technology. Although the guidance provided in the draft SI NUREG report and ASCE/SEI 4-16 provides a sound basis for further development of nuclear power plant designs incorporating SI, these initial documents were focused on surface-founded or near-surface-founded LWRs and were, necessarily, limited in scope. For example, there is limited information in either the draft NUREG report or ASCE/SEI 4-16 related to nonlinear analysis of soil-structure systems for deeply-embedded reactors, the isolation of components, and the use of vertical isolation systems. Also missing from the draft SI NUREG report are special considerations for licensing of isolated facilities using the certified design approach in 10 CFR 52 and a detailed discussion of seismic probabilistic risk assessments (SPRAs) for isolated facilities. To identify and address limitations in the initial guidance, Idaho National Laboratory (INL) has initiated several projects focused on further developing the technical and licensing underpinnings for facilities using SI technology. These efforts include a 2014 workshop focused on SI, development of new structural analysis tools and methodologies, and development of this report to identify and describe regulatory gaps and challenges related to licensing of advanced reactors using SI. Nearly all of the gaps and challenges identified in the report fall outside the scope of current efforts (including those at INL). This report provides information for developing a roadmap for future activities related to SI of advanced reactors. Although design optimization and commercial aspects related to the use of SI have been identified in Coleman and Sabharwall (2014) and elsewhere as possible issues or areas of opportunity, only topics that may impact efficient and successful licensing are addressed in this report. Because efforts to date related to regulatory guidance development (e.g., the draft SI NUREG report) have principally considered designs similar to those currently being licensed, the existing literature (some of which is discussed in this report) is reflective of traditional LWR designs. All of the regulatory gaps and challenges that apply to LWRs also apply to advanced reactors; and often the LWR case provides a simplified example as compared to the range of cases found in advanced reactors. Advanced reactor designs often lead to additional gaps and challenges not faced in LWR design. Although this report discusses advanced reactors broadly, the exact set of challenges and potential solutions for any particular reactor design is technology-specific. SI offers potential significant economic benefits for advanced reactors because the isolation system can be used to reduce the site-dependent seismic demands below pre-qualified levels in the certified design. Site-independent SSCs such as reactor vessels and steam generators would be analyzed, designed and qualified just once. Their protective isolation systems would be site-specific. vi Advanced reactors will often be designed and constructed very differently from LWRs, regardless of whether or not they employ SI and damping devices. The key technical advances in civil and structural engineering needed to deploy advanced reactors are: (1) development of performance-based seismic design and assessment procedures for non-LWR reactors; (2) development and deployment of analysis methodologies suitable for computing the response of deeply-embedded power reactors, including nonlinear time domain, soil-structure-interaction analysis; and (3) development, prototyping, and deployment of two-dimensional and three-dimensional isolation systems suitable for components ranging in size and complexity from diesel generators to reactor vessels. Advances in Items 1 and 2 are needed regardless of whether seismic protective measures (i.e., such as those noted in Item 3) are deployed. Not covered in this report are other important technical advances in civil and structural engineering needed for economical deployment of advanced reactors such as development and deployment of modular construction strategies used to minimize “one-off” field work, schedule delays, and construction cost. The identified gaps and challenges are addressed throughout this report and summarized in Section 7. High-impact/high-value topics that should be addressed in the short term, and that must be satisfactorily completed before substantial progress on other tasks can be made, include the following topics listed below. The number in parentheses refers to the subsection number in this report where the topic is discussed. These topics are also highlighted in Table 4. • Verification and validation of models and tools for nonlinear soil structure interaction of embedded facilities (2.1.3) • Guidance for ground motion selection and modification for design and SPRA (2.1.5) • Procedures for SPRA of nonlinear soil-isolator-structure systems (2.3.1, 2.3.2) • Component isolation systems (2.3.3) • Certified design process for deeply-embedded advanced reactors (3.1, 3.2)
Seismic Isolation of Safety-Related Nuclear Structures
The forthcoming update of ASCE 4 includes expanded provisions for analysis, design of seismically isolated nuclear structures and testing of seismic isolation bearings. The paper describes the provisions and their technical basis. The analysis and design provisions are consistent with the ASCE 43-05 performance objectives for nuclear structures, namely, 1) 1% probability of unacceptable performance for 100% Design Basis Earthquake (DBE) shaking, and 2) 10% probability of unacceptable performance for 150% DBE shaking.
Seismic Isolation of Major Advanced Reactor Systems for Economic Improvement and Safety Assurance
2020
The nuclear industry is currently at a severe economic disadvantage, mainly due to the extremely large capital costs involved constructing in new nuclear power plants. Recent nuclear constructions have seen large cost overruns and schedule delays, making the nuclear sector less attractive to investors. Advanced reactor concepts currently under development in the United States are striving to improve the economics of nuclear power primarily by using passive and 'walk-away' safe technologies. The recently published report, 'Future of Nuclear Energy in a Carbon-Constrained World' (Buongiorno et al., 2018) by the Massachusetts Institute of Technology Energy Institute (MIT EI) has shown that a significant contributor to the capital cost of NPPs is the 'civil works' i.e., the construction of the balance of plant (BoP: all SSCs except those involved in power generation, e.g., reactor vessel) such as the buildings, containment dome, foundation, etc., which amount to almost half of the overnight capital cost. It also suggests that using advanced construction technologies such as seismic isolation and high-performance concrete to significantly reduce the capital costs of advanced reactors. It is therefore imperative that innovations in advanced reactors are not only in the reactor technology, but also in the design, procurement, and construction of the balance of plant and in the civil works. This report demonstrates the application of two cost-cutting solutions on generic advanced reactor designs. These solutions include, (1) seismic base isolation, and (2) risk-and cost-based seismic design optimization including seismic isolation of individual components. Capital cost (USD millions) Seismic risk Initial, unoptimized, and not seismically isolated 99.20 5.16 x 10-5 Optimized, without including component seismic isolation 83.22 4.92 x 10-5 Optimized, including component seismic isolation 78.61 4.86 x 10-5
Seismic protection technology for nuclear power plants: a systematic review
Journal of Nuclear Science and Technology, 2014
Seismic protection systems (SPS) have been developed and used successfully in conventional structures, but their applications in nuclear power plants (NPPs) are scarce. However, valuable research has been conducted worldwide to include SPS in nuclear engineering design. This study aims to provide a state-ofthe-art review of SPS in nuclear engineering and to answer four significant research questions: (1) why are SPS not adopted in the nuclear industry and what issues have prevented their deployment? (2) what types of SPS are being considered in nuclear engineering research? (3) what are the strategies for location of SPS within NPPs? and (4) how may SPS provide improved structural performance and safety of NPPs under seismic actions? This review is conducted following the procedures of systematic reviews, where possible. The issues concerning the use of SPS in NPPs are identified: cost, safety, licensing and scarcity of applications. NPPs demand full structural integrity and reactor's safe shutdown during earthquake actions. Therefore, horizontal isolation may be insufficient in active seismic zones and isolation in the vertical direction may be required. Based on the results in this review, it is likely that next generation reactors in seismic zones will include state-of-the-art SPS to achieve full standardised design.
Development of Design Guidelines for Seismically Isolated Nuclear Reactors and Research and Development Work Performed by ENEA
Nuclear Technology, 1992
ENEA began work on seismic isolation in Italy in 1988 in cooperation with ISMES. Until now, work has been limited to horizontal systems and focused on high-damping steel-laminated elastomer bearings. Work consists of both the assessment of proposed design guidelines for isolated nuclear reactors (developed in collaboration with General Electric Company) and research and development (R&D) experimental and numerical studies, partly performed in support of the guideline development. Experiments include static and dynamic characterization of single bearings, analysis of a full-scale isolated structure and an actual building with in situ techniques, and shake table tests of scaled isolated structures. The main features of the guidelines document and R&D studies are described, and some initial measured data are presented.
Evaluation of ASCE 4-16 and AISC 43-18 (Draft) for use in the Risk-Informed Performance-Based Seismic Design of Nuclear Power Plant Structures, Systems, and Components
2020
This report describes the assessment of ASCE Standards 4-16 and 43-18 (Draft) for use in the Risk-Informed Performance-Based (RIPB) seismic design of structures, systems, and components (SSCs) at nuclear power plants. This work was performed for the U.S. Nuclear Regulatory Commission (NRC) Office of Regulatory Research (RES), to support potential endorsement of these industry standards for the design of nuclear power plants based on the RIPB approach. Currently, the NRC endorses a deterministic design approach for demonstrating the design adequacy of SSCs based on the Standard Review Plan (NUREG-0800) and NRC Regulatory Guides. In the RIPB approach, the design criteria are developed to achieve a target performance goal, which is defined by the annual frequency of occurrence of the design basis earthquake (i.e., Seismic Design Category) and the acceptable level of structural performance (i.e., Limit State) for the SSCs. ASCE 4-16 provides methods for performing a seismic analysis of structures to obtain the seismic response of these structures (e.g., building displacements, accelerations, in-structure response spectra) which are used in the design of the SSCs. ASCE 4-16 also provides methods for performing seismic analysis of SSCs to determine the seismic demands (e.g. member forces and displacements) needed to design individual SSCs. ASCE 43-18 (Draft) provides the criteria for the seismic design of SSCs using the seismic demands developed in ASCE 4-16. The use of ASCE Standard 43-18 (Draft), along with ASCE 4-16, provides the criteria for the seismic design of the SSCs. ASCE 43-18 (DRAFT), in turn, relies on other consensus codes and standards such as ACI 349 for reinforced concrete, AISC/N690 for steel structures, ASME Section III for pressure-retaining mechanical components and Containments, and IEEE-344 for Class 1E equipment. The goal of this technical review is to assess the adequacy of the provisions in these standards for use by the NRC in developing regulatory guidance for design of SSCs in nuclear power plants, based on the RIPB approach. The research reported herein describes the basis for acceptance of the new standards and identifies areas where additional staff guidance is needed for the seismic design of SSCs at nuclear power plants. This technical review has determined that ASCE 4-16 and ASCE 43-18 (Draft) provide an appropriate framework for the seismic design of SSCs at nuclear power plants using a Risk-Informed Performance-Based approach. However, some of the criteria in these standards warrant exceptions, qualifications, and/or clarifications. v
Characterizing the benefits of seismic isolation for nuclear structures: A framework for risk-based decision making
Seismic isolation has been implemented in many civil structures, including buildings, bridges, liquid natural gas tanks, and off shore oil platforms, both in the United States and other countries, to mitigate the damaging effects of earthquakes. Seismic isolation has also been implemented in nuclear structures in France and South Africa, but not yet in the United States, in either Department of Energy facilities or commercial nuclear power plants (NPPs). This is primarily due to a lack of guidelines, and codes and standards for the analysis, design and construction specific to seismically isolated nuclear structures. However, seismic isolation of nuclear structures has seen increased research interest in the recent years and the forthcoming version of the national consensus standard America Society of Civil Engineers (ASCE) Standard 4-16 (ASCE, 2016) "Seismic analysis of safety related nuclear structures", recently incorporated language and commentary (Chapter 12) for seismically isolating surface or near-surfacemounted nuclear facilities, including NPPs.
Isolation systems influence in the seismic loading propagation analysis applied to an innovative near term reactor
Nuclear Engineering and Design, 2010
Integrity of a Nuclear Power Plant (NPP) must be ensured during the plant life in any design condition and, particularly, in the event of a severe earthquake.
Seismic Protection of Small Modular Reactors
ASME 2011 Small Modular Reactors Symposium, 2011
The next edition of ASCE Standard 4 will include detailed provisions for the seismic isolation of structures, systems and components in safety-related nuclear structures. The provisions are based on those available in North America for buildings, bridges and other infrastructure but address issues particular to nuclear energy construction and take advantage of recent research funded by federal agencies, including the Nuclear Regulatory Commission and the National Science Foundation. The paper highlights these research products and their incorporation into ASCE Standard 4. Although the focus of the studies and ASCE Standard 4 is analysis of conventional light water reactors of 500+MWe, most of the conclusions are applicable to small modular reactors.
Seismic demands on secondary systems in base-isolated nuclear power plants
Earthquake Engineering & Structural Dynamics, 2007
Numerical models of a sample nuclear power plant (NPP) reactor building, both conventionally constructed and equipped with seismic protective systems, are analysed for both safe shutdown and beyond-designbasis earthquake shaking at two coastal sites in the United States. Seismic demands on secondary systems are established for the conventional and seismically isolated NPPs. The reductions in secondary-system acceleration and deformation demands afforded by the isolation systems are identified. Performance spaces are introduced as an alternate method for evaluating demands on secondary systems. The results show that isolation systems greatly reduce both the median and dispersion of seismic demands on secondary systems in NPPs. result in high seismic acceleration and deformation demands in the stiff NPP structural systems and extremely high demands on the safety-related secondary systems.
Analysis and design of seismic isolation systems for nuclear structures
ASCE 43-05 presents two performance objectives for the design of nuclear structures: 1) 1% probability of unacceptable performance for 100% Design Basis Earthquake (DBE) shaking, and 2) 10% probability of unacceptable performance for 150% DBE shaking. To develop procedures for the analysis and design of baseisolated nuclear power plants (NPPs) to meet the intent of ASCE 43-05, we performed a series of nonlinear response-history analyses to study the impact of the variability in both earthquake ground motion and mechanical properties of isolation systems on the seismic responses of base-isolated NPPs. Computations were performed for three representative sites (rock and soil sites in the Central and Eastern United States and a rock site in the Western United States), three types of isolators (lead rubber, Friction Pendulum TM and low-damping rubber bearings), and realistic mechanical properties for the isolators. Estimates were made of the ratio of the 99%-ile (90%-ile) response of isolation systems computed using a distribution of spectral demands and distributions of isolator mechanical properties to the median response of isolation systems computed using best-estimate properties and 100% (150%) spectrum-compatible DBE ground motions. Only the results for the soil site in the Central and Eastern United States and LR and FP bearings are presented.
Seismic performance assessment of base-isolated safety-related nuclear structures
Earthquake Engineering & Structural Dynamics, 2010
Seismic or base isolation is a proven technology for reducing the effects of earthquake shaking on buildings, bridges and infrastructure. The benefit of base isolation has been presented in terms of reduced accelerations and drifts on superstructure components but never quantified in terms of either a percentage reduction in seismic loss (or percentage increase in safety) or the probability of an unacceptable performance. Herein, we quantify the benefits of base isolation in terms of increased safety (or smaller loss) by comparing the safety of a sample conventional and base-isolated nuclear power plant (NPP) located in the Eastern U.S. Scenario-and time-based assessments are performed using a new methodology. Three base isolation systems are considered, namely, (1) Friction Pendulum TM bearings, (2) lead-rubber bearings and (3) low-damping rubber bearings together with linear viscous dampers. Unacceptable performance is defined by the failure of key secondary systems because these systems represent much of the investment in a new build power plant and ensure the safe operation of the plant. For the scenario-based assessments, the probability of unacceptable performance is computed for an earthquake with a magnitude of 5.3 at a distance 7.5 km from the plant. For the time-based assessments, the annual frequency of unacceptable performance is computed considering all potential earthquakes that may occur. For both assessments, the implementation of base isolation reduces the probability of unacceptable performance by approximately four orders of magnitude for the same NPP superstructure and secondary systems. The increase in NPP construction cost associated with the installation of seismic isolators can be offset by substantially reducing the required seismic strength of secondary components and systems and potentially eliminating the need to seismically qualify many secondary components and systems. Analysis of plant systems and accident sequences Characterization of seismic hazard Structural response simulation Damage assessment of NPP components Risk computation Figure 1. The procedure of seismic performance assessment of NPPs.
Advanced Seismic Base Isolation Methods for Modular Reactors
2010
Advanced technologies for structural design and construction have the potential for major impact not only on nuclear power plant construction time and cost, but also on the design process and on the safety, security and reliability of next generation of nuclear power plants. In future Generation IV (Gen IV) reactors, structural and seismic design should be much more closely integrated with the design of nuclear and industrial safety systems, physical security systems, and international safeguards systems. Overall reliability will be increased, through the use of replaceable and modular equipment, and through design to facilitate on-line monitoring, in-service inspection, maintenance, replacement, and decommissioning. Economics will also receive high design priority, through integrated engineering efforts to optimize building arrangements to minimize building heights and footprints. Finally, the licensing approach will be transformed by becoming increasingly performance based and technology neutral, using best-estimate simulation methods with uncertainty and margin quantification.
Acceptable Seismic-Induced Risks per U.S. Department of Energy Standards at Nuclear Facilities
Nuclear Technology, 2012
This paper investigates the potential impacts of the transition to the U.S. Department of Energy (DOE) Order 420.1B requirements and the criteria promulgated by the new DOE-STD-1189 on the current practice for seismic design of structures, systems, and components (SSCs). Addressed in the review is the modification of the prescribed methodology provided in ANSI/ANS-2.6-2004 by the new DOE standard. The new ANSI/ANS standards provide criteria and guidance in selecting the seismic design category (SDC) and the limit state (LS) for the SSCs that are important to safety. An unmitigated consequence analysis considering the uncertainties in estimating failure and the safety consequences of the failure may be performed to determine the SDC and the LS, which then are used to establish the level of peak ground acceleration and design response spectra. The new DOE-STD-1189 modifies the prescribed methodology provided in ANSI/ANS-2.6-2004 for calculation of unmitigated radiological dose consequence. Unmitigated consequence analysis is a procedure that has been used by the DOE for the purpose of incorporating safety in the design and operation of its nuclear facilities and is also used in 10 CFR 70, the U.S. Nuclear Regulatory Commission regulation applicable to fuel cycle facilities, and the associated Standard Review Plan (NUREG-1520). This paper identifies the iterative DOE double-pronged approach to seismic design, and a simplified example demonstrates the unmitigated seismic hazard consequence analysis.
Seismic Isolation of Lead-Cooled Reactors: The European Project Siler
Nuclear Engineering and Technology
Seismic Isolation of Reactor Assembly for a Fixed Base Accelerator Driven System Reactor Building
Journal of Civil Engineering and Architecture, 2016
In the SILER (Seismic-Initiated events risk mitigation in LEad-cooled Reactors) Project, it is interesting to apply seismic isolation technology for the reactor assembly of the fixed base reactor building for ADS (Acceleration Driven System) heavy liquid reactor MYRRHA (Multipurpose Hybrid Research Reactor for High-Tech Application) which contains the most critical safety related components, such as reactor vessel, safe shutdown and control rod mechanisms, primary heat exchangers, primary pumps, spallation target assembly and fuel assemblies, etc. The purpose of this paper is to investigate the possibility of an application of a partial seismic isolation to the safety critical components only, here, the reactor assembly. This paper presents the preliminary analysis results of the isolated reactor assembly and compares these with those of seismic isolated ADS reactor building. The analysis results show the reduction of the seismic acceleration response but the increase of the relative displacement for the reactor assembly. Some safety issues, especially, coolant's incapable covering the reactor core make difficult to apply for the partial seismic isolation of the ADS reactor assembly due to large relative displacement occurring the partial isolation system. Further study on the partial seismic isolation application of the critical safety components are also discussed.
Seismic demands on secondary systems in conventional and isolated nuclear power plants
Numerical models of a sample nuclear power plant (NPP) reactor building, both conventionally constructed and equipped with seismic protective systems, are analyzed for design-basis and beyond-design-basis earthquake shaking at a West Coast site in the United States. Seismic demands on secondary systems are established for the conventional and seismically isolated NPPs. The reductions in secondary-system acceleration and deformation demands afforded by the isolation systems are identified. The impact of using spectrum-matched ground motions for predicting demands on structural and secondary systems is addressed.
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