Applications of multi-directional seismic inputs in the design of components

A method is presented to convert the existing component qualification level (acceleration in g's) for two-directional earthquake input to an equivalent qualification level for three-directional earthquake input, and vice-versa. This exact and conservative method is applicable to all simple equipment which uses static acceleration as the basis for design, such as auxiliary pumps, valves, tanks, heat exchangers, filters, and demineralizers.     

Calculation of equivalent static loads and its application

All the forces in the real world act dynamically on structures. Since dynamic loads are extremely difficult to handle in analysis and design, static loads are usually utilized with dynamic factors. Generally, the dynamic factors are determined from design codes or experience. Therefore, static loads may not give accurate solutions in analysis and design and structural engineers often come up with unreliable solutions. Two different methods are proposed for the transformation of dynamic loads into equivalent static loads (ESLs). One is an analytical method for exact ESLs and the other is an approximation method. The exact ESLs are calculated to generate identical response fields such as displacement and stress with those from dynamic loads at a certain time. Some approximation methods are proposed in engineering applications, which generate similar response fields from dynamic loads. They are divided into the displacement-based approach and the stress-based approach. The process is derived and evaluated mathematically. Standard examples are selected and solved by the proposed method and error analyses are conducted. Applications of the method to structural optimization are discussed.     

Realistic seismic qualification using the updated finite element model for in-core components of reactors

It is now mandatory to seismically qualify the safety-related structures and components used in the nuclear power plants. Among several qualification approaches the qualification by the analysis using finite element (FE) method is the most common approach used in practice. However, the estimated dynamic behaviour by FE model of a structure is known to show significant deviations from the dynamic behaviour of the ‘as-installed’ structure in many cases. Considering such limitations, few researchers have advocated re-qualification of such structures after their installation at site to enhance the confidence in qualification vis-à-vis plant safety. For such an exercise, validation of FE model with experimental modal data is important. A validated FE model can be obtained by the model updating methods in conjugation with the in situ experimental modal data. Such a model can then be used for qualification. However, for the reactor in-core components such a modal testing and FE model updating may not be straightforward. Hence, the complication involved in the reliable seismic qualification of in-core components and the advantage of using the FE model updating has been brought out in the paper through an example of a typical in-core component—a perforated horizontal tube recently installed in a nuclear reactor in India.     

Analyses of Pool Swell Tests by Two-Dimensional Hydrodynamic Computer Code

A two-dimensional hydrodynamic computer code SOLA-VOF was examined on the analytical capability for dynamic loads by pool swell in the MARK-I type BWR suppression chamber under LBLOCA (Large Break Loss of Coolant Accident) conditions. Two pool swell tests, (LLL 1/5-scale and EPRI 1/12-scale tests) were selected for this purpose and analyzed by the SOLA-VOF code modified with incorporation of a simple downcomer flow model. In these analyses, it was necessary to take account of three-dimensional effect of pool swell behavior along the chamber axis by use of a method such as spatially weighting function experimentally determined, because a simple two-dimensional calculation by the SOLA-VOF code gave too much conservative evaluation for the impact load on the ring header. Applications of this method gave a good agreement between the calculation and measurement. The vertical loads on the suppression chamber wall were well analyzed by this code. It might be because the local pressure difference caused by the nonuniform pool swelling disappeared owing to pressure integration on the surface of suppression chamber wall.     

蒸汽发生器水室隔板的简化对下部组件等应力的影响

蒸汽发生器内部部件较为复杂,载荷作用下一次侧的力学行为通常会影响管板上部的部件,如支撑板和管束.通过对水室隔板的不同简化建立下部组件的轴对称和三维模型,考察对应力分布及支撑板和管束的影响.研究结果表明:轴对称模型能较为准确地得到应力强度分布,且较为保守.管板在不同模型下的位移差别较大,从而影响管板上部的支撑板以及管束的力学行为.     

Piping system response induced by thermal-hydraulic transient: A review of recent analysis methods and experiments

Complete thermal-hydraulic and structural dynamic response analysis of piping systems subjected to a thermal hydrodynamic transient, such as a safety/relief valve (S/RV) opening, is a complex multi-step process. The four links of this analysis chain are thermal-hydraulic analysis, mechanical loads calculation, structural dynamics analysis, and transient thermal stress analysis. This paper presents summaries of the individual analysis steps, guidelines for the performance of these analyses, and a review of recent experimental versus analytical prediction comparison studies. Finally, research needs are discussed.     

Present and future of equipment qualification for dynamic loads

This paper reviews the existing methods being used to justify equipment under transient dynamic loads of extreme magnitudes. Based on the experience gained during the past ten years in qualifying equipment for such loads, the author discusses future developments needed to obtain a consistent and reliable technology. The potential and the direction for future developments combined analysis test to obtain a unified approach are presented. This study includes qualification by test and combined analysis test.     

NSTX disruption simulations of detailed divertor and passive plate models by vector potential transfer from OPERA global analysis results

The national spherical torus experiment (NSTX) project is planning upgrades to the toroidal field, plasma current and pulse length. This involves the replacement of the centerstack, including the inner legs of the TF, OH, and inner PF coils. A second neutral beam will also be added. The increased performance of the upgrade requires qualification of the remaining components including the vessel, passive plates, and divertor for higher disruption loads. The hardware needing qualification is more complex than is typically accessible by large scale electromagnetic (EM) simulations of the plasma disruptions. The usual method is to include simplified representations of components in the large EM models and attempt to extract forces to apply to more detailed models. This paper describes a more efficient approach of combining comprehensive modeling of the plasma and tokamak conducting structures, using the 2D OPERA code, with much more detailed treatment of individual components using ANSYS electromagnetic and mechanical analysis. This capture local eddy currents and resulting loads in complex details, and allows efficient non-linear, and dynamic structural analyses.     

Pneumatic pressure tests of steel containment models — Recent developments

This paper describes the results of recent pneumatic pressure tests of steel containment models. These tests are part of the Containment Integrity Program whose objective is the qualification of methods for predicting containment response during severe accidents and extreme environments. Sandia National Laboratories is conducting this combined experimental and analytical program for the U.S. Nuclear Regulatory Commission (NRC). The long-range plans for the program include the following three containment loading conditions: static internal pressurization, dynamic internal pressurization, and seismic loadings. Steel, reinforced concrete, and prestressed concrete containment types are being considered.In the present experimental effort, models of steel containment structures are being subjected to static internal pressurization. The first set of models are about the size of hybrid-steel containments. Tests of these models are nearly finished. Testing of a large steel model, about of full size, will complete the static pressure experiments with steel models. Analysis of the models is paralleling the experimental effort.The Containment Integrity Program is being coordinated with other NRC programs on potential leakage of penetrations in containments. The results from all of the programs should provide a basis for predicting the structural and leakage behavior of containments during temperature and internal pressure loadings.     

Steel containment buckling

Two aspects of buckling of a free-standing nuclear steel containment building were investigated in a combined experimental and analytical program. In the first part of the study, the response of a scale model of a containment building to dynamic base excitation is investigated. A simple harmonic signal was used for preliminary studies followed by experiments with scaled earthquake signals as the excitation source. The experiments and accompanying analyses indicate that the scale model response to earthquake-type excitations is very complex and that current analytical methods may require that a dynamic capacity reduction factor be incorporated. The second part of the study quantified the effects of framing at large penetrations on the static buckling capacity of scale model containments. Results show little effect from the framing for the scale models constructed from the polycarbonate, Lexan. However, additional studies with a model constructed of the prototypic steel material are recommended.     

Structural impact stability of commercial nuclear fuel rod

The United States Nuclear Regulatory Commission (USNRC) requires that all licensees evaluate the potential for handling accidents that may occur during the transfer of fuel from the reactor vessel to the spent fuel pool. In this process, a bottom end drop scenario resulting in the release of radiological fissions products is of primary concern. For this reason, the U.S. Code of Federal Regulations requires that each applicant provide an analysis and evaluation of accident conditions that present a risk to public health and safety at the facility of operation. Furthermore, as stated in the USNRC Regulatory Guide 1·183, the number of fuel rods damaged during a handling accident such as that described above should be based on a conservative analysis that considers the most limiting case pertaining to weight, drop height and the compression, torsion and shear stresses on the irradiated fuel rods with the potential damage of adjacent fuel assemblies being considered. It is further recommended by the USNRC that the limiting mode of failure associated with such a postulated handling accident is that of elastic buckling, which should be evaluated through application of Euler’s static load limit. However, as will be shown in this article, the lack of inertial terms and assumed axially continuous compressive load present in Euler’s solution result in an overly conservative design limit when applied to a transient impact analysis. In an effort to illustrate the conservative nature of this recommended design limit, the theories and limitations of closed form static buckling are introduced, followed by a more rigorous treatment of dynamic pulse buckling, and finally, a complete solution via the finite element method. It may be concluded from the results of this investigation that elastic instability during a transient impact event develops only at loads well beyond Euler’s critical load. Such results require that an alternative stress based limit be introduced, which establishes a more reasonable design limit for evaluating the structural integrity of a spent fuel assembly bottom end drop scenario.     

Combined experimental and analytical method for a realistic seismic qualification of equipment

The seismic qualification of equipment/structures are, in general, carried out either exclusively by analysis or exclusively by testing using a shake table. The analytical methods have the risk of the model not being a true reflection of the structure unless very elaborate modelling techniques are used. Even with an elaborate model there are many idealisations made which may not actually be realised in practice. The shake-table testing, avoids the modelling deviations to a large extent, but is also not without drawbacks. The important ones are the cost and the availability of a shake table of the required size and capacity. The shake-table testing is also carried out on the isolated equipment without the piping/structural connections from other components. The present paper suggests a combined experimental and analytical method on the ‘as installed’ equipment as an attractive alternative which overcomes the above drawbacks. In contrast to the existing practice of using the experimental results just to validate the analytical model, the suggested method uses the experimentally obtained dynamic characteristics of the ‘as installed’ equipment to obtain the response to the design seismic load. The paper brings out through an example of a simple storage tank which is too heavy for a shake table, the large deviations in its actual behaviour vis-à-vis an idealised analytical model.     

Structural criteria for extreme dynamic internal pressure loadings of vessels and closure heads

Structural criteria for analytically evaluating the pressure retaining capability of vessels and closure heads subjected to extreme internal transient dynamic loadings are presented. The criteria project against tensile plastic instability and local ductile rupture failure modes. To minimize the number of critical areas that may need more rigorous analytical methods, a screening criterion for limiting the membrane, bending and local stresses is defined. The stresses for this criterion are calculated from either simple and economical elastic dynamic or equivalent static methods. For the critical areas that remain, a strain-based criterion for strains derived from dynamic, inelastic methods is given. To assure that the criteria are properly applied, guidelines are outlined for controlling methods for deriving stresses and strains, for selecting appropriate material properties and for addressing specific dominating parameters that affect the validity of the analysis. The application of the criteria to a complex liquid metal fast breeder reactor vessel and closure head and the subsequent experimental verification of the results by several scale model experiments are summarized.     

Piping dynamic reliability and code rule change recommendations

The conservative nuclear piping design criteria for seismic and dynamic loads have led to piping systems with excessive numbers of snubbers. To improve this undesirable situation, a Piping and Fitting Dynamic Reliability Program was initiated by the Electric Power Research Institute (EPRI) in 1985 with cooperation from the U.S. Nuclear Regulatory Commission (NRC). The objective of the program is to develop improved, realistic, and defensible ASME design rules by taking advantage of the inherent dynamic margins in the nuclear piping system. The research results have demonstrated that piping systems have large reserve dynamic capacity and the dynamic failure mode is due to fatigue or fatigue-ratcheting rather than plastic collapse. Based on such physical evidence, a set of code rule change recommendations is suggested in its preliminary form.     

Use of an unconventional technique for seismic qualification of equipments

There is a great deal of equipment in nuclear power stations which is required to withstand predefined levels of earthquakes. Such equipment is generally qualified analytically or experimentally by shake-table tests. However, some equipment is so complicated that an analytical simulation is very difficult. This equipment could also be so large and heavy physically that shake-table testing may not be possible in many cases. One typical example of such equipment is the Diesel Generator (DG) sets of Nuclear Power Plants (NPP's). For functional qualification of such equipment, the use of railway track unevenness to induce stationary random vibrations is being put forward as an economical and conservative alternative. This article also brings out the feasibility of using such a technique for all difficult to model and/or test equipment both in a passive and an active state.     

Seismic qualification of expansion anchors to Canadian nuclear standards

A large number of unique seismic qualification tests were performed on different types of expansion anchors to determine their seismic capabilities. These tests followed the requirements of the Canadian CSA N287.2 as well as the ASTM E 488 Standards. Expansion anchors ranging in diameter from 1/4 to 11/4 in. were subjected to this seismic qualification testing. Strategically designed and standardised concrete blocks were manufactured to receive each expansion anchor. The anchors were installed according to the anchor manufacturer instructions and the installation was consistent with construction practice. Each anchor type and size was subjected to four different types of tests covering static, dynamic, tension and shear loading. The dynamic cyclic loads applied represented a strong earthquake whose spectrum envelops that of the Design Basis Earthquake (DBE). The seismic testing qualification program involved 300 tests. Results of the testing program are obtained for each type of anchor. Results from these tests confirm that these types of expansion anchors have maintained their tension (pull-out) and shear capabilities following a severe seismic shaking.     

Friction modelling of preloaded tube contact dynamics

Many loosely supported components are subjected to flow-induced vibration leading to localized wear. Life prediction depends on robust and accurate modelling of the nonlinear dynamics as the components interact with their supports. The output of such analysis is the component dynamic response and impact forces, including friction forces during stick–slip motions. Such results are used to determine the normal work rates, which are utilized to predict fretting wear damage. Accurate estimates of these parameters are essential. This paper presents simulations of a loosely supported fuel-channel tube subject to turbulence excitation. The effects of tube/support clearance and preload are investigated. Several friction models, including velocity-limited, spring-damper and force-balance are utilized. A comparison of these models is carried out to investigate their accuracy. The results show good agreement with experimental work rates when a simple iterative procedure to update the friction forces is used.     

Earthquake response of nuclear reactor building deeply embedded in soil

This paper is concerned with experimental and analytical studies to investigate dynamic behavior of deeply embedded structures such as nuclear reactor buildings. The principal points studied are as follows: (1) Examination of stiffness and radiation damping effects according to embedded depth, (2) verification for distributions of earth pressure according to embedded depth, (3) differences of response characteristics during oscillation according to embedded depth, and (4) proposal of an analytical method for seismic design. Experimental studies were performed by two ways: forced vibration test, and earthquake observation against a rigid body model embedded in soil. Three analytical procedures were performed to compare experimental results and to examine the relation between each procedure. Finally, the dynamic behavior for nuclear reactor buildings with different embedded depths were evaluated by an analytical method.     

Modeling of slabs in seismic analysis of nuclear power plant buildings

Safety related nuclear power plant buildings are commonly represented as lumped mass weightless elastic beam stick models to determine their dynamic behavior under seismic ground motions. Implicit in this analysis procedure is the assumption that the floor slabs are rigid. This paper critically evaluates the slab flexibilities in typical power plant buildings and presents a practical approach to include these in the seismic analysis. Vertical as well as horizontal earthquake components are considered. Results presented include amplified floor response spectra for equipment qualification and design forces in floor slabs and the supporting walls. A satisfactory analysis procedure would consist of traditional stick model analysis to obtain overall seismic responses, force distribution by static analysis using suitable methods such as the finite element method and subsystem analysis to evaluate local amplifications, if necessary.