Issues in seismic isolation of nuclear power plants

The paper covers the issues involved in considering seismic isolation for nuclear plants. The application of isolation techniques to non-nuclear installations is discussed. Its potential application to nuclear components and plants is considered and the lack of actual, experimental verification of novel techniques is portrayed. Finally a cost comparison, based on certain preliminary assumptions of isolated and non-isolated nuclear plants is made.     

Vapor explosion studies for nuclear and non-nuclear industries

Energetic melt-water explosions are a well-established contributor to risk for nuclear reactors, and even more so for the metal casting industry. In-depth studies were undertaken in an industry-national laboratory collaborative effort to understand the root causes of explosion triggering and to evaluate methods for prevention. The steam explosion triggering studies (SETS) facility was devised and implemented for deriving key insights into explosion prevention. Data obtained indicated that onset of base surface-entrapment induced explosive boiling-caused trigger shocks is a result of complex combination of surface wettability, type of coating (organic versus inorganic), degree of coating wearoff, existence of bypass pathways for pressure relief, charring and non-condensable gas (NCG) release potential. Of these parameters NCGs were found to play a preeminent role on explosion prevention by stabilizing the melt-water steam interface and acting as a shock absorber. The role of NCGs was experimentally confirmed using SETS for their effect on stable film boiling using a downward facing heated body through which gases were injected. The presence of NCGs in the steam film layer caused a significant delay in the transitioning of film-to-nucleate boiling. The role of NCGs on explosion prevention was thereafter demonstrated more directly by introducing molten metal drops into water pools with and without NCG bubbling. Whereas spontaneous and energetic explosions took place without NCG injection, only benign quenching occurred in the presence of NCGs. Gravimetric analyses of organic coatings which are known to prevent explosion onset were also found to release significant NCGs during thermal attack by melt in the presence of water. These findings offer a novel, simple, cost-effective technique for deriving fundamental insights into melt-water explosions as well as for explosion prevention under most conditions of interest to metal casting, and possibly for nuclear reactor systems during severe accident conditions. Energetics of entrapment boiling induced shocks for explosive and non-explosive conditions were quantified using a modified zero-crossing technique. In honor of Professor R.T. Lahey Jr. the non-dimensional parameter “L_T” was proposed to delineate the explosion-onset boundary. Experimental evidence suggests that a system with L_T above a threshold value of ∼65 leads to the growth of perturbations and onset of propagating melt-water explosions. The data appear to offer valuable insights into explosion prevention in nuclear reactors during beyond-design basis accident conditions. An unresolved issue concerns the potential for trigger shocks from chemical ignition reactions between reactive metals in contact with oxide coatings such as rust.     

Evaluation methodology for seismic base isolation of nuclear equipments

An evaluation methodology for seismic base isolated nuclear equipments is proposed. The evaluation can be classified into two steps. In the first step, the seismic functional failure probability during the lifetime of equipment without base isolation devices is quantified in order to decide the applicability of the base isolated structure. The second step is comparative and calculates the ratio of the seismic failure frequency of the equipment without base isolation devices to that with them in order to evaluate the effectiveness of the base isolated structure. The sample evaluation considers the case of high voltage type emergency transformer with ceramic tubes.     

Criteria for seismic analysis of nuclear plant structures and substructures

This paper encompasses criteria used for seismic analysis of nuclear power plant structures such as supporting structures founded on ground, as well as substructures. Nuclear power plant equipment and systems can be treated as substructures. Modeling of structures and substructures is described. Since instructure response spectra play an important role in the design and analysis of nuclear power plant equipment, systems and components, methods for development of instructure response spectra as well as variations of input parameters considered in determining these spectra are described.When the principal contribution to the equipment response is due to flexibility of the supporting substructures, an analytical approach to the problem for obtaining reduced stiffness and associated mass matrices of supporting substructures with finite element representation for use in the dynamic analysis of equipment and supporting structures is presented. When supporting structures and equipment, that have inherently different damping properties, are included as intergral parts of the dynamic models, the approximate evaluation of the modal damping based on the weighted damping according to the modal energy stored in each component is outlined. Use of time history and response spectrum analyses is presented. The effects of relative displacements due to different motion of the support points of substructures in each significant mode of the supporting structures as well as procedures of combining modal responses are detailed.     

New Zealand seismic base isolation concepts and their application to nuclear engineering

This paper describes recent New Zealand experience with seismic base isolation, and in particular, the distinguishing feature of the New Zealand system - that of using mechanical energy dissipators to control response.There are presently 22 structures in New Zealand which use base isolation concepts for seismic protection. Nuclear power plants are not used in New Zealand, but the principles adopted for the above buildings and bridge structures are appropriate for nuclear structures and, as shown by use of hypothetical examples, significant benefits can be achieved for these structures also. The most important is a marked reduction for the in-structure acceleration spectra with consequential implications for the enhanced safety of secondary structures and equipment.The paper also reviews the experimental and analytical studies undertaken to validate the technique and addresses the issues of reliability and cost.     

Effects of foundation rotation on seismic inertia forces of nuclear power plant structures

Effects of foundation rotation on seismic inertia forces are studied in two different plane strain finite element investigations. A three-mass model is used to approximate the dynamic characteristics of a containment vessel of a nuclear power plant and its internal structure. In the first study a large finite element mesh is specified in order to eliminate unwanted reflections from the boundary. Motion input to the structure is limited to 2 sec duration. A quiet boundary technique is employed in the second investigation. As a result, earthquake motion inputs of any duration could be specified. Most results are based on portions of two recorded earthquake motions of 4 sec in length. Effects of foundation rotation and lateral soil-structure interaction are evaluted with program output. Results are presented in graphs and tables.     

DEMT experimental and analytical studies on seismic isolation

This paper reviews the experimental and theoretical studies performed at CEA/DEMT related to the overall behavior of isolated structures. The experimental work consists of the seismic shaking-table tests of a concrete cylinder isolated by neoprene sliding pads, and the vibrational tests on the reaction mass of the TAMARIS seismic facility. The analytical work consists of the development of procedures for dynamic calculation methods: for soil—structure interaction where pads are placed between an upper raft and pedestals, for time-history calculations where sliding plates are used, and for fluid—structure interaction where coupled fluid and structure motions and sloshing modes are important.Finally, this paper comments on the consequences of seismic isolation for the analysis of fast-breeder reactor (FBR) vessels. The modes can no longer be considered independent (SRSS Method leads to important errors), and the sloshing increases.     

Experimental investigation into the seismic behavior of nuclear power plant shear wall structures

Nuclear power plant structures are designed to resist large earthquakes. However, as new data are obtained on earthquake activity throughout the United States, plant design earthquake levels have increased. The U.S. Nuclear Regulatory Commission is sponsoring an analytical-experimental research program to obtain information on the structural response of nuclear power plant shear wall structures subjected to earthquake motions within and beyond their design basis. Using different size scale models constructed with microconcrete and prototypical concrete this research has demonstrated consistent results for measured values of stiffness at load levels within the design basis. Furthermore, the values are well below the theoretical stiffnesses calculated from an uncracked cross-section strength-of-materials approach. Current program emphasis is to assess the credibility of previous experimental work by beginning to resolve the ‘stiffness difference’ issue.     

Design considerations for concrete nuclear containment structures subjected to simultaneous pressure and seismic shear

Improvements in design code provisions for tangential shear in secondary concrete nuclear containment vessels are needed. This paper presents a brief summary of an experimental research program conducted at Cornell University on tangential shear. Six inch thick reinforced concrete panels were subjected to combined in-plane tension and shear as a behavioral model of a section of the wall of the containment under the combined loading of internal pressurization and seismic shear. Approximately 50 panels were tested. Parameters studied included: tension level and direction (biaxial or uniaxial), shear level and type (monotonic, cyclic, or a combined mode), sequence of applied loading, and reinforcing ratio and orientation.The results of the research indicate that current code provisions are overly conservative with regard to the amount of tangential shear to be carried by the orthogonally reinforced concrete. By increasing the allowable stress, the required amount of diagonal reinforcing would be reduced. This would result in savings in fabrication costs and construction time, and improved structural reliability through improved concrete placement. The research also indicates a need for a more exact consideration of containment displacements. Shear stiffnesses for the panels were extremely low, indicating that containment displacements may be larger than anticipated. The code provisions in this area are limited and unsubstantiated.     

Methodologies for rapid evaluation of seismic demand levels in nuclear power plant structures

A methodology for rapid assessment of both acceleration spectral peak and “zero period acceleration” (ZPA) values for virtually any major structure in a nuclear power plant is presented. The methodology is based on spectral peak and ZPA amplification factors, developed from regression analyses of an analytical database. The developed amplification factors are applied to the plant's design ground spectrum to obtain amplified response parameters. A practical application of the methodology is presented.This paper also presents a methodology for calculating acceleration response spectrum curves at any number of desired damping ratios directly from a single known damping ratio spectrum. The methodology presented is particularly useful and directly applicable to older vintage nuclear power plant facilities (i.e. such as those affected by USI A-46). The methodology is based on principles of random vibration theory. The methodology has been implemented in a computer program (SPECGEN). SPECGEN results are compared with results obtained from time history analyses.     

Evaluation of torsional effects in the seismic analysis of symmetric and unsymmetric nuclear power plant structures

This paper summarizes evaluation of torsional effects of symmetric and unsymmetric structures in the seismic analysis of nuclear power plants. The idealization of structures to predict the response due to earthquakes is described. The determination of stiffness or flexibility matrices of structures, using an approximate method and finite element technique, is outlined. The evaluation of foundation-structure interaction for structures founded on soft soils, hard rocks, and piles is presented. The analytical methods of analysis such as time-history and spectrum approach are discussed.     

核电厂大型组合结构的有限元抗震分析方法研究

在现代核电站抗震设计中,有限元法是各类相关设备抗震分析与评价的重要数值仿真工具。对于形状复杂、部件众多的大型组合结构,采用整体三维建模的有限元模型通常需要很大的存储和计算规模,超出现有的计算条件。因此需要首先研究组合结构各个部件的动力学特性,从而建立合理的三维简化力学模型,并以该模型为基础进行有限元数值仿真。本文以某地车-吊车组合结构为例,给出此类大型组合结构的抗震分析方法,并将等效静力法与反应谱法相结合,对该结构进行分析,最后根据相关法规对各子结构进行评价,以确保总体组合结构在极限安全地震条件下能够保持结构完整性。     

Variability in dynamic characteristics and seismic response due to the mathematical modeling of nuclear power plant structures

Calculated variations in dynamic characteristics and seismic response of nuclear power plant structures caused by different modeling assumptions are quantified. Four different mathematical models were created to describe the same structure, the Zion nuclear power station auxiliary/fuel-handling/diesel-generator complex. The modeling idealizations are a detailed finite element model, a detailed finite element model with masses lumped at selected nodes, a detailed finite element model with the constraint of rigid floors, and an equivalent beam model. Dynamic characteristics and response quantities are determined for the models and compared. Results indicate that large variations in dynamic characteristics and response can be introduced by modeling assumptions when a need exists to reduce the number of dynamic degrees of freedom.     

Recent activities in the field of nuclear materials and nuclear fuels

On the research and development of nuclear materials and fuels, many of outstanding papers have been presented in scientific journals, including the Journal of Nuclear Science and Technology. Some topics have been covered in this summary from the latest activities of nuclear materials and fuels.     

Research needs associated with seismic load on nuclear power plants

A research program aimed at providing the desired level of seismic resistance of nuclear power plants in a cost-effective manner is proposed. The results of such a research program would provide direct measurement of seismic design parameters associated with loadings which closely simulate real strong motion characteristics on major structural and mechanical systems.     

Effects of slip and separation on seismic SSI response of nuclear reactor building

The seismic soil-structure interaction response of a nuclear reactor building requires modeling of the soil-structure interface. It allows slip and separation at the interface that affects the behavior and response of the reactor. The joint elements used to model the soil-structure interface, require incorporation of appropriate joint stiffness so that slip and separation phenomena take place under the warranted conditions. This slip and separation causes change in the response of the structure. This paper duly addresses the related aspects through comparative study of responses and draws important conclusions useful for design of nuclear reactor building.     

An advanced seismic analysis of an NPP powerful turbogenerator on an isolation pedestal

This paper presents a detailed seismic analysis of a powerful high-speed Russian turbine within a Nuclear Power Plant. Although dozens of these turbines have worked reliably since the 1970s worldwide, until the last decade, only simplified structural analyses were available due to the turbines’ complicated overall structure and internal design. The current analysis considers the detailed geometry of the turbine itself and the vibration and seismic isolation system within the turbine's pedestal and the full range of operational, accident and seismic loads like high pressure, outside loads induced by pipelines and so on.To solve the problem of the turbine seismic qualification, the following steps have been taken. The first step was to create detailed finite element models of the turbine's high and low pressure parts and rotor system with bearings. Using such models, corresponding simplified models were developed to be included into the coupled model of the system: “Building–Vibroisolation Pedestal–Turbine” (BVT). The second step was the analysis of that coupled system. Soil–structure interaction was considered using actual soil conditions. Three components of time history acceleration were used to define seismic excitation. As the result of BVT system analysis, a full picture of time history displacements and loads was determined. At the same time, a problem of rotor gaps was solved. In the final step, determined loads were applied to the detailed models of the turbine's parts for seismic qualification of the whole structure.