Seismic qualification of systems, structures, equipment and components

The purpose of this paper is to give an overview of the various qualification procedures available to the vendors of nuclear power plants and equipment for hopefully achieving NRC (Nuclear Regulatory Commission) plant licensing and overall guaranteed safe operation. These procedures usually involve computer-aided analyses for large systems and structures, but trend toward shaking table tests for small equipment and components.The dynamic analysis and testing required for seismic qualification can be covered in a practical manner by reference to several pertinent Regulatory Guides and Standards. They have been issued by the NRC on specific subjects, but often represent a consensus of more general standards prepared by ASME, IEEE, ASCE, ANSI and NEMA. These documents cover such diverse subjects as (a) reactor site criteria, (b) seismic design limits and loading combinations, (c) system damping values, and (d) recommended vibration test practices.The author has been directly concerned with IEEE Std 344 on seismic qualification practices and has therefore included the latest ideas and suggestions for revising this document. In general, there has been a continuing escalation in the g-level of seismic requirements. This present overview indicates a need for R&D work and re-examination of published documents to counterbalance unwarranted conservatism.     

Seismic response of temporary structures

An unanchored temporary structure has five modes of response to base excitation: rest, slide, rock, slide–rock, and free-flight. This paper is concerned with the conditions under which an unanchored object will respond in these modes and, in a rock mode, the maximum amount of tilting and the probability of toppling. The conditions for the five modes of response are derived based on equations of equilibrium and of motion. The rocking response and toppling probability are obtained by the Monte Carlo simulation technique by solving the equations governing rocking response numerically and repeatedly using simulated base motion. Details of the derivation and computation are not presented. Toppling probabilities are given graphically for a number of values specifying dimension of the object.     

Probabilistic safety assessment: quantitative process to balance design, manufacturing and operation for safety of plant structures and systems

Probabilistic safety assessment (PSA) has to date been successfully applied in various branches of technology. Rather than reviewing the details of development and applications, more general issues related to PSA and the specific use of them to balance design, manufacturing and operation for safety of plant structures and systems are addressed. Based on some industrial examples, insights are discussed and conclusions are given.     

Seismic analysis of thin shell structures

We consider in this paper a vertically erected, axisymmetric shell, resting on a horizontal foundation. The foundation is subjected to a time-dependent motion in both the vertical and horizontal directions. The motion may be produced by an event such as an earthquake or explosion. An estimate of the response of the shell to such excitation can be obtained from the solution for time-dependent boundary conditions. This solution is adapted here for an analysis with the response spectrum of earthquakes, which has been pioneered by Biot and Housner. The modes of free vibration are calculated by the multisegment, direct numerical integration method using classical shell theory. An actual design case of a containment vessel of a nuclear power plant built in the US is presented. An estimate of the dynamic response of the shell to an earthquake is obtained for all the relevant variables, such as stresses and displacements. As an example, an estimate for the axial stress of the response is given at various stations of the shell.     

Reactor design and safety

Before the resurgence of light water reactor (LWR) orders, the nuclear power industry should take this time to conduct overall design reviews and to discuss, through technical or professional societies, the design criteria for the next generation of LWRs. Such a review and open discussion of design criteria for future LWRs are needed to provide a critical, in depth self-evaluation by the nuclear industry itself during this hiatus. The results of this evaluation would be beneficial to the potential owners, reactor vendors, plant constructors, reactor operators, regulators and the public. Organizations, such as the Atomic Industrial Forum and the Institute for Energy Analysis could provide a convenient focal point for this review and evaluation process. A future advanced design should be conceived based on a balanced consideration of simplicity in design, safety and reliability in operation, and cost effectiveness. Therefore, it must systematically optimized by using the invaluable operating experience and accumulated research results so that the optimum of cost/effectiveness can be obtained.     

Seismic qualification of equipment — research needs

In general, industry has followed the Institute of Electric and Electronic Engineers' (IEEE) Recommended Practices for Seismic Qualification of Class 1E Equipment for Nuclear Power Stations (IEEE Standard 344-1975). However, the U.S. Nuclear Regulatory Commission Regulatory Guide 1.100 notes exceptions to a small part of the IEEE Standards.This paper describes research needed to reconcile the differencies between the IEEE Standard and the Regulatory Guide. In addition, the paper discusses the effects of shake table mass and stiffness on the dynamic response of equipment tested, and the effect attributable to the difference between methods of attaching to the shake table and the actual in-situ attachment method.     

ARIES-AT safety design and analysis

ARIES-AT is a 1000 MWe conceptual fusion power plant design with a very low projected cost of electricity. The design contains many innovative features to improve both the physics and engineering performance of the system. From the safety and environmental perspective, there is greater depth to the overall analysis than in past ARIES studies. For ARIES-AT, the overall spectrum of off-normal events to be examined has been broadened. They include conventional loss of coolant and loss of flow events, an ex-vessel loss of coolant, and in-vessel off-normal events that mobilize in-vessel inventories (e.g., tritium and tokamak dust) and bypass primary confinement such as a loss of vacuum and an in-vessel loss of coolant with bypass. This broader examination of accidents improves the robustness of the design from the safety perspective and gives additional confidence that the facility can meet the no-evacuation requirement under average weather conditions. We also provide a systematic assessment of the design to address key safety functions such as confinement, decay heat removal, and chemical energy control. In the area of waste management, both the volume of the component and its hazard are used to classify the waste. In comparison to previous ARIES designs, the overall waste volume is less because of the compact design.     

Seismic analysis and design of hyperbolic cooling towers

The safety of hyperbolic cooling towers is important to the continuous operation of a power plant. Depending upon the site, earthquake may govern the design of the tower. Methods of seismic analysis have been presented. It is concluded that the response spectrum method of analysis is of maximum practical use. A method to construct the design response spectra for various earthquake zones is presented. An earthquake motion consists of three components; however, it is shown that designing for one horizontal component only is adequate. The use of boundary conditions and the effects of inelastic action on analysis and design are discussed.     

Seismic mitigation of structures by using viscoelastic dampers

In this paper, the characteristics of energy-absorbing capacities of the viscoelastic damper and its applications to the seismic mitigation of structures during earthquakes are studied. An innovative analytical model for the viscoelastic damper accounting for the earthquake-like and wind-like loadings and temperature effects, in good agreement with experimental results, and finite element formulations for the viscoelastic damper are presented. The applications of proposed methodology to a structure incorporating viscoelastic dampers is examined while subjected to horizontal and vertical earthquake ground motions. Numerical results show that the responses of the structure equipped with added dampers to earthquake loadings are significantly reduced. The same concept can also be applied to many types of structures for vibration reductions.     

Dynamic testing of full-scale nuclear power plant structures and equipment

Three steps are required in the design of reliable nuclear power plants to be located in seismic areas. In addition to development of realistic analytical models, it is necessary to perform dynamic tests to verify the models and acquire the information needed to establish the dynamic parameters for modeling. The third and final step is to perform high level proof tests to validate analysis and test results. This report is an overview of dynamic testing methods.Testing can be performed in the laboratory or in the field. Laboratory tests are useful because a wide range of effects can be studied and test parameters are more easily controlled. Care must be exercised to insure that the laboratory situation faithfully reproduces the actual structure in such details as supports, appurtenances, appendages, and mounting methods. In general, laboratory methods permit high level excitation of structures weighing up to a few metric tons (a few facilities in the world have capabilities up to 100 t). Since actual structures of interest to nuclear power plant designers often weigh up to 10 000 t, field testing is also important. Test procedures have been developed, using portable structural vibrators, for testing structures as large as nuclear power plant containment buildings. High level tests can be performed using explosives buried in the soil to excite structures. Recent work performed by the author demonstrates that explosive tests which produce a predetermined, specified response spectrum can be conducted.     

Interaction of package design development and package design safety assessment

Abstract

In his plenary presentation at PATRAM 2010, Professor Bernhard Droste of BAM (Federal Institute for Materials Research and Testing), Berlin, Germany, reviewed recent developments in package design and safety assessment.     

Dynamic fragility concepts for equipment design and qualification

Fragility concepts are explored for use in the design and qualification of nuclear plant equipment and for relating the ultimate capability of equipment to that of the overall plant. In the most general sense, the fragility level of a device may depend on several different types of environmental stress or challenge factors (i.e., heat, nuclear radiation, vibration, etc.) that influence its operation. However, emphasis is concentrated on the dynamic and particularly the seismic fragility levels of equipment. A general definition of dynamic fragility and various methods for its measurement are described. The state of published data on nuclear equipment fragility is discussed, and limitations on its use are noted. From there, the concept of a standardized seismic fragility data base and its potential uses are considered. Various gaps in the methodology are identified, and recommendations for further research are suggested.     

Seismic considerations in siting and design of power plants

A methodology which provides guidelines for the preliminary evaluation of the safety of nuclear power stations subjected to strong vibratory ground motions from earthquakes is outlined. The methodology includes a procedure for estimating a spectral envelope of ground motion at the reactor site. On the basis of this ground motion the seismic response of structural systems and equipment of the power plant can be estimated. A comparison of the expected seismic response of these systems with their strength and functional capabilities yields an evaluation of the safety of the power plant systems studied.     

Seismic response of structures by the response spectrum method

The problems of the acceleration profile at the lower elevations of cantilever structures and the response of relatively rigid structures are explored. It is shown that the use of the conventional methods for the above problems provide very approximate results. An alternate combination of the modal responses is proposed that not only resolves the above problems but also provides better estimates of response for the complete range of structure frequencies. The procedure treats the relative and rigid body responses separately and then appropriately combines the two results. For the rigid range of frequencies (fundamental frequencies greater than about 2 Hz), the proposed procedure does not encounter any numerical difficulties because of the additive nature of the component responses; however, the application of the proposed procedure for very flexible structures causes accuracy problems since the rigid body effects tend to be subtractive from the flexural response of about equal magnitude. For this latter class of problems, the conventional approach of modal combination provides adequate results and avoids the above mentioned numerical difficulties.     

Seismic fragility of reinforced concrete structures in nuclear facilities

The failure and fragility analyses of reinforced concrete structures and elements in nuclear reactor facilities within the Seismic Safety Margins Research Program (SSMRP) at the Lawrence Livermore National Laboratory are evaluated. Receiving special attention are uncertainties in material modeling, behavior of low shear walls, and seismic risk assessment for nonlinear response. Problems with ductility-based spectral deamplification and prediction of the stiffness of reinforced concrete walls at low stress levels are examined. It is recommended that relatively low damping values be used in connection with ductility-based response reductions and that static nonlinear force-deflection curves be studied for better nonlinear dynamic response predictions.     

Experimental determination of damping in nuclear power plant structures and equipment

A summary of damping values for structures and equipment obtained in several full-scale dynamic tests performed on a research reactor, three experimental power reactor plants, and a commercial power reactor plant is given. The testing techniques used include steady state shakers, dynamite blasting, and snapback. In an appendix, guidelines are presented for required frequency resolution and record length for damping estimation from blast or snapback records. The influence of system non-linearity upon damping interpretations is also discussed.     

Seismic safety of nuclear power plants in eastern Europe

This paper summarizes the work performed by the International Atomic Energy Agency in the areas of safety review and applied research in support of programmes for the assessment and enhancement of seismic safety in Eastern Europe and in particular, WWER type nuclear power plants during the past seven years. Three major topics are discussed; engineering safety review services in relation to external events, technical guidelines for the assessment and upgrading of WWER type nuclear power plants, and the Coordinated Research Programme on "Benchmark study for the seismic analysis and testing of WWER type nuclear power plants". These topics are summarized in a way to provide an overview of the past and present safety situation in selected WWER type plants which are all located in Eastern European countries. The main conclusion of this paper is that even though there is now a thorough understanding of the seismic safety issues in these operating nuclear power plants, the implementation of seismic upgrades to structures, systems and components are lagging behind, particularly for those cases in which re-evaluation indicated the necessity to strengthen the safety related structures or install new safety systems.     

HTGR plant safety design bases

The high temperature gas-cooled reactor (HTGR) has inherent and design safety features that are sifnificant and unique, requiring a number of safety criteria and approaches that differ markedly from other reactor types. This paper briefly reviews the design of HTGR plants that have been built and are being offered in the United States. It then reviews the safety considerations involved in the design of the plants being offered. The unique features, their development, and their effects on safety criteria are described. The design bases of the prestressed concrete reactor vessel (PCRV) are given particular attention. Operating characteristics of the HTGR and plant response to transient conditions are discussed. The design-basis depressurization accident evolution and related HTGR safety requirements are discussed. Characteristics of the HTGR with respect to technical specifications are discussed, with particular emphasis on the PCRV and the core safety limit.     

Seismic design of the ITER main Tokamak components

In order to assess the structural performance of the ITER Main Components it is important to take into account not only the mutual dynamic interaction among them during a seismic event but also their interactions with the Tokamak Buildings (TB) complex. The seismic behavior of the TB is affected by the large dimensions of the building, the concrete basemat thickness that has to be sufficiently rigid to support the weight of the Tokamak, the presence of anti-seismic bearing (ASB) under the basemat, and the distribution of heavy equipment at higher levels. These factors require that the soil–structural interaction must be studied in detail, taking into account the specific effects such as the excavation influence and the building rocking motion due to seismic wave propagation. The study of the seismic behavior has been carried out using two different linear dynamic methodologies: power spectral density (PSD) and spectral analyses. The paper illustrates the main results of the seismic analyses and gives the seismic design input for the Tokamak components in terms of support loads, accelerations and displacements.