Seismic response analysis of deeply embedded nuclear reactor buildings considering frequency-dependent soil impedance in time domain

In order to estimate the seismic behavior of deeply embedded nuclear power buildings, it is important to accurately transform the soil impedance in the frequency domain to the impulse response in the time domain. Although the transform is important for some nuclear buildings because they are deeply embedded in the soil, there are few practical and accurate methods at present. The author has proposed practical transform methods. In this paper, seismic response analyses considering frequency-dependent soil impedance in the time domain are shown. First, the formulation of the proposed transform methods is described. Then, the response analysis of a nuclear reactor building deeply embedded in inhomogeneous soil was performed considering the full matrix soil impedance as the example of practical problems. Through these analyses, the validity and efficiency of the methods were confirmed.     

Nonlinear seismic response analysis of an embedded reactor building based on the substructure approach

A practical method to calculate the elasto-plastic seismic response of structures considering the dynamic soil-structure interaction is presented. The substructure technique in the time domain is utilized in the proposed method. A simple soil spring system with the coupling effects which are usually evaluated by the impedance matrix is introduced to consider the soil-structure interaction for embedded structures. As a numerical example, the response of a BWR-MARK II type reactor building embedded in the layered soil is calculated. The accuracy of the present method is verified by comparing its numerical results with exact solutions. The nonlinear behavior and the soil-structure interaction effects on the response of the reactor building are also discussed in detail. It is concluded that the present method is effective for the seismic design considering both the material nonlinearity of the nuclear reactor building and the dynamic soil-structure interaction.     

Earthquake response analysis of reactor structures

The general nature of the principles upon which earthquake resistant design is based is described with particular reference to components and elements of nuclear reactor facilities. Special attention is paid to the response and design criteria of items of equipment or of components that are mounted on or attached to responding elements, and basic procedures are developed to bound the dynamic response of such items.

Consideration is given to vertical as well as horizontal excitation, and the combination of the effects of the various exciations. Suitable approximations are developed for inelastic response estimates.

One section of the paper is devoted to relative motions of points some distance apart, and to bounds for such relative motions.

Recommendations are made for the general criteria governing the design of nuclear facilities, including the basic parameters governing response characteristics and energy absorption.     

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.     

Interaction of ruthenium tetroxide with surfaces of nuclear reactor containment building

During a severe nuclear accident, different fission products will be released from the nuclear fuel and some of them may eventually reach the containment building. Ruthenium is considered to be an important fission product due to the possible formation of volatile oxides. Radiotoxicity and chemical toxicity of the volatile ruthenium compounds present a considerable hazard during a severe nuclear accident.

In this work, experiments regarding behavior of ruthenium tetroxide in the reactor containment were performed. The interactions of ruthenium tetroxide (RuO₄) with zinc, copper, aluminum and epoxy paint in dry and humid atmosphere were examined. SEM/EDX (scanning electron microscope/energy-dispersive X-ray spectroscopy), XPS (X-ray photoelectron spectroscopy) and EXAFS (extended X-ray absorption fine structure) techniques were used to identify the chemical composition of the deposits formed after the interaction of RuO₄ with the different materials. Additionally, distribution of ruthenium between different metals was examined.

Interaction of RuO₄ with the studied samples led to formation of dark, ruthenium-rich deposits. Examination of these deposits showed different chemical speciation of ruthenium on the surface when compared to the deeper layers of deposits. Interaction of RuO₄ with zinc, copper and aluminum resulted to different amounts of the deposited ruthenium on the metals.     

Forced vibration tests and simulation analyses of a nuclear reactor building

Forced vibration tests were carried out at the Hamaoka (BWR) Unit 4 R/B (reactor building) in Japan in April and May of 1992. Fundamental dynamic characteristics of the R/B were obtained, including its interaction with the adjacent T/B (turbine building) and the soil–structure interaction. Results for the preceding R/Bs are compared, and probable causes for fluctuations in the resonance curve around the 1st peak are discussed. Furthermore, simulation analyses of the fundamental dynamic characteristics of the soil–structure system were conducted, using a basic lumped-mass soil–structure model (lattice model), and strong correlation with the measured data was obtained. Other detailed simulation models were employed to investigate the effects of simultaneously induced vertical response and response of the adjacent turbine building on the lateral response of the reactor building.     

Improved response factor methods for seismic fragility of reactor building

A seismic probabilistic risk assessment (PRA) method has been applied to evaluate the safety of nuclear reactor buildings during earthquakes. Improvement was made to two methods (based on linear response and based on non-linear response) of fragility analysis in seismic PRA. The conventional method, which is based on linear response, considers increases of seismic capacity implicitly, using the non-linear behaviour of the structure. We described how to evaluate the capacity increase factor for the linear response method. Secondly, we proposed a method based on the non-linear response and a stratified two-point estimation method which can efficiently evaluate the variability of non-linear responses. We applied the two method to a PWR-type nuclear reactor building and ascertained that these method are useful and effective.     

Interaction of ruthenium tetroxide with iodine-covered surfaces of materials in nuclear reactor containment building

Volatile iodine and ruthenium species are likely to be released from the fuel during a severe nuclear accident. Both iodine and ruthenium are expected to deposit on the surfaces in the containment building of the nuclear power plant. It is assumed that, due to the different release times from the fuel, ruthenium will reach the containment at the time when surfaces are already deposited with iodine species. The influence of ruthenium tetroxide on elemental iodine-covered surfaces in the containment of nuclear power plants was studied in this work. The ability of ruthenium tetroxide to oxidize iodine deposits on zinc, aluminum, copper and epoxy paint at high humidity conditions was evaluated. Quantification of both iodine and ruthenium deposits was done by the means of gamma spectroscopy. The chemical speciation of deposited elements was observed with SEM, XPS and EDX techniques. Experiments showed that ruthenium tetroxide oxidized iodine deposits into the volatile forms of iodine on zinc and aluminum samples and higher iodine oxides in the case of copper and epoxy paint samples. A major increase of ruthenium uptake on iodine-exposed surfaces in comparison to clean surfaces was observed.     

大亚湾核电站反应堆厂房楼层反应谱分析评估

大亚湾核电站核岛厂房的抗震分析遵循技术输出国-法国M310型机组的土建技术规范RCC-G,采用简化的阻抗函数法计算地基岩土的作用.根据大亚湾厂址的地基岩土特点,拟采用更为精确的三维连续半空间边界子结构法来考虑地基岩土的作用,并与原设计进行对比.另外,在原设计中采用多组时程作为地震输入,取各组计算结果的平均值作为设计值的基础(称为"平均"法).在研究中基于相同的时程,拟分别采用"平均"法和更为常用的"包络"法,处理多组时程的响应.基于上述两方面,通过反应堆厂房的地震响应计算,得到核电站系统设备重要的设计基础数据-楼层反应谱(FRS),并将计算的楼层反应谱同设计谱进行比较,从而对设计方法及其结果进行评估,为电站的抗震设计裕量评估和安全管理提供可资参考的结论.     

Earthquake response spectra for seismic design of nuclear power plants in the UK

Earthquake actions for the seismic design of nuclear power plants in the United Kingdom are generally based on spectral shapes anchored to peak ground acceleration (PGA) values obtained from a single predictive equation. Both the spectra and the PGA prediction equation were derived in the 1980s. The technical bases for these formulations of seismic loading are now very dated if compared with the state-of-the-art in this field. Alternative spectral shapes are explored and the options, and the associated benefits and challenges, for generating uniform hazard response spectra instead of fixed shapes anchored to PGA are discussed.     

Seismic response of reactor vessel internals for Korean standard nuclear power plant

This paper presents the development of seismic design criteria for the reactor vessel internals as a part of the standardization programme for the nuclear power plant in Korea. The seismic design loads of the reactor vessel internals are calculated using the reference input motions of reactor vessels taken from Yonggwang nuclear power plant units 3 and 4 which are being constructed in Korea. An overview of analysis related to the basic parameters and methodologies is presented. Also, the response of internal components to the reactor vessel motions is carefully investigated.     

Earthquake response analysis for a BWR nuclear power plant using recorded data

In the dynamic analysis of complicated structures such as nuclear power plants, it is necessary to consider different damping characteristics for each structural element.

The authors have developed a computer program for the dynamic analysis based on the internal viscous damping theory and have recently performed a vibration test and earthquake observations of an actual nuclear power plant. The data resulting from the test and observation were applied to the program and the dynamic response of each part of the plant was computed.

A close agreement was noted between the computed and recorded acceleration-time histories as well as acceleration-response spectra. The authors conclude that their analyzing system might be one of the most reliable methods for the design of the nuclear power plants.     

Response of a NPP reactor building under seismic action with regard to different soil properties

The object of this investigation is the response of a reactor building on seismic action with systematic variation of the soil stiffness. A thin-walled orthotropic containment shell on varying heavy and rigid foundations is regarded as calculation model. The soil stiffness is simulated by means of spring elements for horizontal translation and for rocking motions of the building. By the response spectra method the loads of the containment shell are calculated for a horizontal seismic excitation. The investigation is aimed at determining the influence of differentiated soil stiffnesses on the containment action effects and at recognizing the causes for the occurring effects.The results are thoroughly represented by selected quantities of the building's response, the effects from the soil-structure interaction are discussed and the causes of the effects clearly explained. A possibility is provided for determining critical soil stiffnesses which cause a significant intensification effect.The results of the investigations show that both the soil stiffness and structural configuration of the reactor building, particularly in case of the substructure being heavy and rigid, exert a decisive influence on the loading of the superstructure.     

A study on the performance of a nuclear reactor building floor baffle for particulate debris sedimentation

Nuclear reactor building floor baffle which is a simple structure reducing flow velocity and thus promote particulate debris sedimentation before reaching the emergency sump is proposed and the performance is experimentally studied. In order to account for the particle movement at the baffle upstream, tumbling and lifting velocities of the simulant debris particles are measured in a sedimentation test flume which is designed for simulating reactor building floor waterway. Settling efficiencies are measured using a test flume for various conditions such as water velocity, particle size, and the type and mesh size of the baffle. The tests show that settling efficiencies greater than 80% can be potentially achieved when the baffle is applied to a real plant. Three-dimensional fluid dynamic computations are employed as a supplementary purpose of explaining the mechanism of particle settling by downward flow and resulting forces which are formed and enhance the particle settlement before the baffle on the floor.     

Automation of nuclear reactor controls

Two types of monitoring systems suitable for automatic nuclear reactor control are discussed. In the first system, the detector monitors some given point, such as a preset constant neutron flux. This system is suitable for combining the functions of the period meter and indicator, the power level and criticality of the reactor indicators, and for reactor safety control.In the second system, the reactor power follows detector displacement. Such a system can combine the automatic control functions for criticality, period, and power level of the reactor.If a scheme incorporating monitoring systems of both types is devised, then two variations of complex automation of reactor control may be developed. The first variation is easier to adapt to existing installations. The second variation is more sophisticated, and is suitable, probably, for new installations.The advantages of the proposed schemes are that they consist of simple standard elements, that they make possible servo control, and that they ensure constant operating conditions for the detector and apparatus.     

Engineering initiatives in nuclear reactor regulation

In this paper relevant engineering initiatives that are currently being developed by the United States Nuclear Regulatory Commission (NRC) to enhance regulatory effectiveness are described. The broad issues addressed are: Piping Design and Non-Destructive Examination, Pressurized Thermal Shock, Containment Integrity During Severe Accidents, and Equipment Qualification.