For seismic analyses of linear structural systems including soil-structure systems, the current practice (e.g., the U.S. Nuclear Regulatory Commission (NRC) Standard Review Plan (SRP) (NUREG-0800 [1], “Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition”) and American Society of Civil Engineers (ASCE)/Structural Engineering Institute (SEI) Standard 4-16 [2], “Seismic Analysis of Safety-Related Nuclear Structures”) allows for estimating mean seismic responses by using as few as four or five input time histories. This paper examined whether this practice can achieve a stable mean response by explicitly considering the uncertainty in the Fourier phase spectra of the input time histories and exploring how this uncertainty can affect the coefficient of variation (CV) of the in-structure response spectra (ISRS). ISRS are the response spectra of the seismic response time history at a location in the structure subjected to an input seismic time history. We found that the maximum CV ISRS across all frequencies is around 40% purely due to the uncertainty in the Fourier phase spectra for a typical range of design earthquakes for U.S. nuclear power plants. To estimate a mean ISRS within ±10% of the true mean ISRS, our analyses showed that this level of CV ISRS may require a minimum of 16 input time histories for a confidence level (CL) of 68% and 61 for a CL of 95% for soil-structure systems of low fundamental frequencies. For stiffer systems (for example, with a fundamental frequency of 5 hertz (Hz)), the maximum CV ISRS is about 30%, and thus, the minimum required number of input time histories may be reduced to 9 for a CL of 68% and 35 for a CL of 95%. In summary, the four or five time histories in the current practice may not be sufficient for estimating stable mean responses, especially for soil-structure systems with very low frequencies.

The independent support motion response spectrum method (ISM) is currently used for seismic analysis to calculate the response of multiply supported piping with independent inputs of support excitations. This approach may derive considerable overestimation in the combination of group responses under the absolute sum rule of NUREG-1061 [1]. Then authors have developed an advanced method of the ISM approach named SATH ( S pectrum Method A ssisted by T ime H istory Analysis). In the SATH method, both of floor response spectra and time histories of floor acceleration are used as independent inputs of support excitations. The group responses are summed with correlation coefficients which are calculated by considering each time history of modal response by independent inputs of support excitations. In this paper, the necessity of taking the effects of correlation coefficients for the group responses into account in the ISM approach is examined. The SATH method has advantage to derive a more realistic sum rule of the group responses and applicability for the actual design.

High pressure experiments were done to determine the pressure effect on multiphase induced forces. To that end, a series of water/gas, oil/gas and oil/water/gas vibrations measurements were performed at 10, 25, 45 and 80 bar at different liquid and gas rates in a horizontal, 2″, double Uloop test section installed at the Equinor Porsgrunn site. At higher pressures the vibration amplitude decreased inversely proportional to the pressure. This was measured for both oil and water. Based on the vibration data, the forcing spectrum was reconstructed. In this the same decreased trend was observed, indicating that a changed damping is not the cause of the reduced vibrations. In the forcing spectra the main effect was a higher energy content at lower frequencies. No clear shift in the peak frequency was measured.

Offshore pipelines are generally subjected to variable amplitude (VA) loading in service due to waves or ocean currents. Welded joints often represent the most critical locations for fatigue cracking. Use of the current fatigue design guidance, for example, BS 7608, to assess fatigue performance of the welded joints in such structure may lead to inaccurate estimates depending on the nature of the VA loading spectrum. Further studies on the effect of VA loading spectra on fatigue performance of welded joints are needed. In this research, both uniaxial and 3-point bending fatigue tests were performed on non-load carrying fillet welded plates under VA loading spectra to investigate the effects of mean stress and the type of VA loading spectra. The influence of plate thickness was also investigated. Test results suggest that the spectrum with a high constant maximum tensile stress (cycling-down) could significantly degrade fatigue performance of welded joints, with the damage parameter D only at around 0.5. The severity of this type of loading spectrum depends on the mean stress level and the plate thickness. An analytical model has been developed to predict fatigue crack propagation (FCP) by considering the interaction of stresses in the loading spectrum. The model considers the impact of the mean stress generated by the preceding load on FCP in the subsequent cycles. FCP predicted by the model shows a good agreement with the experimental data.

Flow-induced vibrations of Steam Generator tube bundles are a major concern for the operators and designers of nuclear power plants. In order to predict damages due to such vibrations, predictive calculations are performed, both by designers and by operators, which allow one to evaluate safety margins and thereafter to optimize the SG maintenance policy. In cases when semi analytical models of fluid-dynamic forces are used, together with corresponding dimensionless fluid force coefficients, closures are assessed by experiment. The database of dimensionless coefficients should then cover relevant tube bundle configurations. Within this framework, a test rig which was available at EDF was modified in order to arrange different pitch-to-diameter ratio bundles and different tube diameters. This paper discusses the fluid-induced forces measured on a set of flexible tubes placed in a triangular pitch bundle both in the lift and drag directions. Two-phase flow excitation spectra are presented as well.

In order to better understand the mechanisms of fretting wear damage of guide cards in some Pressurized Water Reactor (PWR) Nuclear Power Plant (NPP), an experimental investigation is undertaken at the Magaly facility in Le Creusot. The test rig consists of a complete Rod Cluster with eleven Guide Cards, submitted to axial flow inside a water tunnel. In order to mimic the effect of fretting wear, the four lower guide cards have enlarged gaps, so that the Control Rods are free to oscillate. The test rig is operated at ambient temperature and pressure, and Plexiglas walls can be arranged along its upper part, and a series of camera records the vibrations of the control rods above and below the guide cards. The vertical flow velocity is in the range of a few m/s. Beam-like pinned-pinned modes at about 5 Hz are observed, and oscillations of several mm of the central rods are measured, which come along with impacts at the higher flow velocities. A simple non-linear calculation reveals that the main effect of the impacts between Control Rods and Guide Cards is an increase of the natural frequency of the rods by about 10%. Furthermore, as the vibration spectra collapse remarkably well with the flow velocity, the experiments prove that turbulent forcing is responsible for the large oscillations of the control rods, no other mechanism being involved.

Turbulent mixing of cold and hot fluids can lead to high-cycle thermal fatigue in T-junctions, but modelling of the thermal load induced to the structure is challenging. The spectrum method has been recently proposed as an efficient modelling method, which uses a theoretical turbulence spectrum to represent the fluctuating thermal load. In this work, further development and application of the method is presented. Estimation of the frequency of the largest-scale temperature fluctuations is considered in particular, as this is an essential input parameter for the model spectrum. The method is applied to a T-junction including a mixer pipe and a high temperature difference that induces significant buoyancy effects. The frequency scale of the fluctuations is determined for the flow case based on temperature spectra obtained from a Computational Fluid Dynamics (CFD) simulation. A formula for the frequency scale is proposed, which can be used in approximating the frequency scale for other flow conditions. Fatigue and crack growth analysis of the T-junction wall are carried out by using the spectrum method, and the results are compared with those obtained with sinusoidal and CFD temperature loads. The results indicate improvement in accuracy over the highly simplified sinusoidal method.

Seismic responses from linear and nonlinear dynamic analyses of reinforced concrete (RC) shear walls are compared to see how the damping ratio and cracking behavior affect the dynamic response of the RC structures used in the nuclear power plant. The nonlinear dynamic analyses are conducted based on the numerical model which is developed to simulate the nonlinear hysteretic behavior of RC structures subjected to in-plane shear. Through comparison of the obtained numerical results with experimental data such as load-displacement relationships and response time-histories, the developed numerical model is validated. The acceleration response spectra from the nonlinear dynamic analysis results of selected RC shear wall and those from linear dynamic analysis with combinations of the damping ratio and concrete stiffness considerations according to the level of earthquake loads and the resultant stresses are addressed.

Waterhammer is the phenomenon which occurs due to rapid valve operation or sudden stop of pumps. When the waterhammer occurs, unbalanced pressure between elbows causes transient load on piping system. In the piping design against the waterhammer, it is necessary to evaluate the strength and the displacement of piping system against the transient load, and required to provide adequate piping supports. In the piping design, dynamic analysis and static analysis with DLF (Dynamic Load Factor) are often conducted to consider dynamic effect of the water hammer load. The piping support often regarded as rigid in the piping system analysis, however, because of support characteristic (flexibility, plastic behavior, sliding friction), the dynamic response of piping system changes from analysis result with rigid support. For this reason, support characteristics shall be considered adequately. Nevertheless, effect of the support characteristics on the piping design has not been discussed sufficiently. In this paper, to clarify the effect of the support characteristics against the waterhammer load, a series of the nonlinear dynamic analyses were conducted. Based on the analysis results, the response spectra and the ductility charts considering the nonlinearities of the piping system were created. the design approach to properly control the displacement of piping system based on the nonlinear response spectra and ductility charts, is proposed, and the dynamic response against waterhammer load in the piping system of LNG loading line is discussed.

In this work, the acoustic effects of horizontal air-water flow through an orifice are investigated experimentally. Single phase flow (air) and two-phase flow (air and water) tests are performed for two sets of orifices. One set of straight edged and one set of upstream rounded orifices. For each set, the diameters of the orifices were 2, 5, and 10mm, with a thickness of 5 mm. The two-phase flow is generated by injecting water at a rate of 0 to 40 g/s to air in a pipe with diameter of 25 mm. The air rate is fixed in the range from 5.8 to 14 g/s, where the upstream pressure varies from 1.5 to 4 bar at ambient temperature. Unsteady pressure fluctuations are recorded at two upstream and two downstream position. The valve noise standard NEN-EN-IEC (60534-8-3, 2011) for dry gas is assessed by means of experimental data in dry conditions at fixed air mass flow rate. Predictions of sound power spectra by means of the standard are found to be more accurate compared to those obtained following Reethof & Ward (1986), also in conditions of a choked orifice. In case of multiphase flow already at very low liquid fractions of much less than 1%, the standard is no longer valid. The frequency spectrum is no longer determined by the jet noise but starts to be dominated by low frequency general multiphase flow. The Strouhal number based on the jet conditions is an order lower than Sr = 0.2 indicating process variations rather than jet noise. Furthermore, at choking conditions the further expansion which occurs in single phase flow is likely different at multiphase flow. For non-choked flow, the standard can be adapted using multiphase mixture properties. This does lead to a good prediction. However at choked conditions, this method fails.

In codes’ provisions and design procedures for acceleration-sensitive nonstructural components, seismic demand is commonly defined by means of floor response spectra expressed in terms of pseudo-acceleration. Depending on the considered analysis method, floor response spectra may be derived from floors’ acceleration histories, based on structural response-history analysis, or calculated using a predictive equation from a given input ground motion spectrum. Methods for estimating floor response spectra that are based on the second alternative are commonly called spectrum-to-spectrum methods. The objective of this paper is to briefly review these methods, and to discuss the main assumptions they are based on. Both predictive equations from selected seismic codes and proposals from the literature are included in the review. A new probability-based method, recently developed by the Authors for generating uniform hazard floor response spectra, namely, floor response spectra whose ordinates are characterized by a given target value of the mean annual frequency of being exceeded, is also described. By using this method floor spectra are determined through closed-form equations, given the mean annual frequency of interest, the damping ratio of the spectra, the modal properties of the structure, and three uniform hazard ground spectra. The method is built on a proposal for a probabilistic seismic demand model that relates the ground spectral acceleration with the floor spectral acceleration, and is able to explicitly account for the ground motion variability of the nonstructural response. Results for a case study consisting of a service frame of a visbreaking unit in an oil refinery are presented to show the good predictive accuracy of the method with respect to exact uniform hazard floor response spectra obtained through a standard probabilistic analysis.

Additively manufactured (AM) austenitic stainless steels are intriguing candidates for the storage of gaseous hydrogen isotopes because complex vessel geometries can be built more easily than by using conventional machining options. Parts built with AM stainless steel tend to have excellent mechanical properties (with tensile strength, ductility, fatigue crack growth, and fracture toughness comparable to or exceeding that of wrought austenitic stainless steel). However, the solidification microstructures produced by AM processing differ substantially from the microstructures of wrought material. Some features may affect permeability, including some amount of porosity and a greater amount of ferrite. Because the diffusivity of hydrogen in ferrite is greater than in austenite (six orders of magnitude at ambient temperature), care must be taken to retain the performance that is taken for granted due to the base alloy chemistry. Furthermore, AM parts tend to have greater dislocation densities and greater amounts of carbon, nitrogen, and oxygen. These features, along with the austenite/ferrite interfaces, may contribute to greater hydrogen trapping. We report the results of our studies of deuterium transport in various austenitic (304L, 316, and 316L) steels produced by AM. Manufacturing by Powder Bed Fusion (PBF) and two different blown powder methods are considered here (Laser Engineered Net Shaping ® (LENS ® ) and a Direct Laser Powder Deposition (DLPD) method with a higher laser power)). The hydrogen permeability (an equilibrium property) changes negligibly (less than a factor of 2), regardless of chemistry and processing method, when tested between 150 and 500 °C. This is despite increases in ferrite content up to FN = 2.7. However, AM materials exhibit greater hydrogen isotope trapping, as measured by permeation transients, thermal desorption spectra, and inert gas fusion measurement. The trapping energies are likely modest (<10 kJ/mol), but may indicate a larger population of trap sites than in conventional 300-series stainless steels.

In nuclear plant piping systems thermal fatigue damage can arise at locations where there is turbulent mixing of different temperature flows. The severity of this phenomenon is difficult to assess via plant instrumentation due to the high frequencies involved. NESC report EUR 22763 EN, published in 2007, defines the “Level 1” screening criterion for the design of austenitic stainless steel mixing tees, based on recorded incidents of fatigue cracking in civil power plants. The experimental data indicates that damage due to High Cycle Thermal Loading (HCTL) is unlikely to occur if the temperature difference between the hot and cold inlet streams is less than 80°C. The “Level 2” approach outlined by NESC provides a methodology for the calculation of a fatigue usage factor based on the assumption of a sinusoidal thermal loading at the most damaging frequency for a given ΔT. Advice is given on selection of heat transfer coefficient, fatigue curves, fatigue strength reduction factors and plasticity correction factors. Experience shows that these methods can be overly pessimistic when compared with plant operational experience. This paper describes a case study using the more detailed NESC “Level 3” evaluation of HCTL at a Pressurised Water Reactor (PWR) mixing tee using a coupled Computational Fluid Dynamics and Finite Element Analysis (CFD/FE) analysis to evaluate the complete load spectra together with the ASME 2010 fatigue S-N curve. The CFD model used is “conjugate”, ie it calculates temperatures in both the fluid and the metal. Large Eddy Simulation (LES) was used to investigate HCTL effects using an appropriate mesh size to accurately predict the rapid fluctuations in metal temperature local to the surface. Metal temperature predictions using conjugate CFD analyses provided the input to finite element analysis, utilising rain-flow techniques, in order to derive fatigue usage factors in the areas of interest. This study found that the severity of HCTL is influenced by various factors such as flow conditions, local geometry including bore match features, integral conical reducers that allow progressive change in pipe radius as well as branch pipe swirl penetration.

Flow-induced vibrations of Steam Generator tube bundles are a major concern for the operators of nuclear power plants. In order to predict damages due to such vibrations, EDF has developed the numerical tool GeViBus, which allows one to evaluate safety margins and thereafter to optimize the SG maintenance policy. The software is based on a semi analytical model of fluid-dynamic forces and dimensionless fluid force coefficients which need to be assessed by experiment. The database of dimensionless coefficients is progressively updated in order to cover most of existing tube bundle configurations. Within this framework, a new test rig was presented in a previous conference with the aim of assessing parallel triangular tube arrangement submitted to a two-phase cross-flow. This paper presents the fluid-induced forces measured on a set of flexible tubes placed in a triangular pitch bundle both in the lift and drag directions. Two-phase flow excitation spectra are presented as well.

High temperature water environments, typical of light water reactor primary coolant, are known to lead to significant environmental enhancement of fatigue crack growth of austenitic stainless steels. For PWR environments. these effects have recently been codified in ASME Code Case N-809. However, just as for the detrimental effect of these environments on fatigue endurance, plant experience indicates that crack growth rates must be significantly lower than predictions based on laboratory data using simple sawtooth waveforms. In order to explain this discrepancy, a significant amount of research has been conducted to quantify factors leading to crack growth rate retardation with sulfur content having been identified as significant in promoting crack growth rate retardation. However, the inherent conservatisms in current analysis techniques may be just as significant in generating the perceived over-conservatism of environmental fatigue crack growth laws such as Code Case N-809. The current work looks at the impact of waveform shape and spectrum loading on the level of environmental enhancement for a given stress intensity factor range and total rise time by considering simplified transients and loading spectra. The observations suggest that simplified definitions of total rise time used in fatigue assessments can lead to large over-estimation of actual fatigue damage. These data form the basis of an analytical methodology being developed by RollsRoyce (presented in a separate paper at this conference) aimed at partitioning damage across the loading cycle in order to remove over-conservatisms in current analytical methodologies.

Equipment installed on the supporting structure responds to the earthquake floor motion, which is strongly amplified by the structural response of the supporting structure. The required yield strength of equipment can be reduced by allowing inelastic deformation of the supporting structure and its equipment. Inelastic behavior of the supporting structure can significantly reduce elastic floor response spectra, especially their peak values. Furthermore, the allowance of inelastic deformation of equipment in strongly amplified spectral region produces a significant reduction in the required yield strength of equipment. In this study, we discussed inelastic behavior of single-degree-of-freedom(SDOF) systems installed on the supporting structures by using constant-ductility floor response spectra. Constant-ductility floor response spectra readily provide the yield strength of a SDOF system necessary to limit the ductility demand imposed by an earthquake floor motion to a specified value. Based on these discussions, we propose simple and conservative equations of the yield strength reduction factor of nonlinear inelastic SDOF systems with the natural period, the ductility factor and the stiffness ratio for earthquake floor motions.

A partially buried fixed-base finite element model of a typical safety-related nuclear structure is analyzed for earthquake loads by the time history method, the response spectrum method, and the equivalent static load method. The spectra-consistent artificial time histories are generated with seed time histories in accordance with Standard Review Plan 3.7.1: Seismic Design Parameters [1] with target spectra based on Regulatory Guide 1.60: Design Response Spectra for Seismic Design of Nuclear Power Plants [2]. The response spectrum analyses are performed with the same target spectra used in generating the artificial time histories. The equivalent static loads are based on the nodal zero period accelerations from the fixed-base time history analyses. The seismic responses in a column in the structure are combined using algebraic sum, square root of the sum of the squares (SRSS), and the 100-40-40 rule in accordance with Regulatory Guide 1.92: Combining modal responses and spatial components in seismic response analysis [3]. The equivalent static load method is applied according to ASCE 4-15: Seismic Analysis of Safety-Related Nuclear Structures [4]. The resulting design forces and required reinforcement for a column in the structure are compared for each method along with the corresponding computational demand.

In the nuclear power plants (NPPs), the base isolators can be used to decrease the seismic force (acceleration, shear force, floor response spectra). During the lifetime of isolation systems, the mechanical properties of rubber bearings will be constantly changed by the vulcanization and degradation of the rubber due to environmental and chemical factors such as the thermal oxidation, ultraviolet irradiation, and ozone. Usually, thermal oxidation is the most significant degradation factors during the lifetime of natural rubber. Thermal oxidation hardens the rubber and results in a significant decrease in performance. It was presented that the stiffness was increased and damping was decreased by the aging of lead rubber bearings (LRB). It can be observed that the horizontal stiffness of rubber bearings increases about 10% after 15 to 40 years. Considering aging effect of bearings at the design stage, AASHTO [1999] suggested an aging factor for the post-yield stiffness in LRBs. The aging of LRB may be proceeded from the surface to internal because the aging of rubber bearings mainly affected by the oxidation. Therefore, the long-term behavior of LRB can different according to the size. In this study, the horizontal stiffness and damping was evaluated by the size of LRB using the analytical model. The LRBs with an outside diameter of 250 mm (D-250) and an outside diameter of 1,500 mm (D-1500) were selected as example model. In this study, Yeo model was used to model the properties of rubber material and ABAQUS was used as the analysis program. Aging depth of LRB was assumed to be 11 mm and 16 mm based on the accelerated thermal oxidation test. From the analysis result, the effective stiffness of LRB of D-250 was increased about 15%∼22% while the effective stiffness of LRB of D-1500 was increased about 4%∼9%. Although the aging depth of LRB was equal, the ratio aging area to total area of LRB was different according to the size of bearings. Therefore, the mechanical properties factors should be proposed considering the size of LRB.

Seismic analysis and design of high-hazard nuclear facilities requires evaluation of soil-structure interaction (SSI) effects on structure and soil response due to earthquake ground motions. The industry-wide methodology of computing SSI response of buildings is through linear SSI techniques using the computer code SASSI. Technical issues were identified by users and regulators (Ref. 1) resulting in the U.S. Department of Energy (DOE) commissioning a large scale, multi-year Validation and Verification (V&V) Project for SASSI (Ref. 2). The project was a highly peer reviewed process that included an esteemed Participatory Peer Review Team, DOE oversight, as well as regulatory and stakeholder input. The project goal was to develop benchmark SSI solutions for the range of SSI problems associated with high-hazard facilities within the DOE complex. As per industry software quality control requirements, an acceptance limit for the benchmark solutions must be provided to define the acceptable accuracy of the results produced by SASSI relative to the benchmark. In order to define this limit, variation in the solution of foundation impedance must be related to an expected level of accuracy in structural design quantities of interest (e.g., response spectra, base shear, etc.). Therefore, extensive parametric analyses were performed for coupled soil-structure systems having a broad range of SSI parameters defined by foundation size, soil properties, building stiffness and mass properties, building height, etc. Based on the results of the parametric analyses, ASCE 4 code guidance, and engineering judgment of CJC&A and the peer review team, an acceptable level of accuracy in computed foundation impedance was determined for SASSI solutions. This supported the successful qualification of SASSI for use in two large-scale DOE projects.

The next generation ground motion prediction equations predict significant high frequency seismic input for rock sites in the Central Eastern United States (CEUS). This high frequency motion is transmitted to basemat supported components and may be transmitted to components supported on elevated slabs. The existing ASCE 4 analysis requirements were initially developed based on seismic motions having lower frequencies, typical of ground motions in the Western United States (WUS). The adequacy of the existing ASCE 4 analysis requirements are examined using high frequency CEUS spectral shapes and the potential error inherent in using the existing approach to computing in structure response spectra is quantified. Modifications to reduce potential error in the existing ASCE 4 criteria are proposed. In structure response spectra are typically generated for a subsystem given the time history response of a building region. The building time history response is based on analyses that use either modal time history superposition, direct integration or complex frequency response analysis of the building and supporting soil. Input to the building analyses consist of either real or synthetic discretized ground motion records. The discretized ground motion records are often based on recorded ground motion seeds and are often limited to a 0.005 second time step. Thus the time step of the seed record often limits the frequency content of the problem. Both the building analyses and in structure response spectra subsystem analysis may interpolate the discretized ground motion records to obtain stable results. This interpolation generates errors that are propagated through the analyses used to calculate in structure response spectra. These errors may result in extraneous high frequency content in the in structure response spectra. Errors are quantified by comparison of time history parameters, Fourier components and in structure response spectra.