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A design seismic motion for design of nuclear power plant have been increased in Japan since Tohoku earthquake in 2011. Consequently, piping systems under the design seismic motion are possibly deformed plastically. Elastic-plastic FEA (Finite Element Analysis) can evaluate elastic-plastic behavior accurately. However, it takes time and cost. In this research the simplified elastic-plastic analysis method has developed. This simplified elastic-plastic analysis method can calculate an equivalent stress and stress components with considering elastic-plastic deformation based on the energy rate density equilibrium which are calculated from linear time history analysis results. Ramberg-Osgood law is defined as true stress-true strain relationship, and the hardening law is isotropic hardening. In order to validate the applicability of the simplified elastic-plastic analysis method to piping systems under seismic motion, linear and non-linear time history analyses were performed for two type of piping systems by using FEA software Abaqus2021, and simplified elastic-plastic analysis method was applied to linear time history analysis results. One of the piping systems consisted of one elbow pipe, two straight pipes and one weight. Nine sinusoidal accelerations with different frequencies were applied to this simple model in in-plane direction. The equivalent stresses at the integration point where the maximum equivalent plastic strain occur in non-linear time history analysis were compared between the simplified elastic-plastic analysis and non-linear time history analysis. At any frequency simplified elastic-plastic analysis method could predict the equivalent stresses calculated by non-linear time history FEA accurately. Another piping system simulated actual piping systems in a nuclear power plant, which include the Tee pipe and several piping supports other than elbow pipes, straight pipes and weights. The random frequency acceleration waves were applied to this model in three direction at the same time. This piping systems took into account the self-eight and the internal pressure. The equivalent stresses and stress components were compared between the simplified elastic-plastic analysis and non-linear time history analysis. As the results, the stresses calculated by the simplified elastic-plastic analysis had good agreement with the results calculated by non-linear time history analysis. Therefore, the applicability of simplified elastic-plastic analysis method to the piping systems was validated.

In previous reports, it was shown that the cumulative absolute velocity (CAV) for each wave of the pipe seismic response corresponds to the vibration stress amplitude. A new seismic fatigue evaluation method was proposed by applying this relationship to Miner’s law of material fatigue damage. As a result, the cumulative fatigue damage can be calculated in proportion to the CAV of each wave to the power of 2.4. In this study, how the new method can be used to evaluate the cumulative fatigue damage quantitatively is considered. The cumulative fatigue damage was evaluated by the rain flow method as well as the new method where the parameters showing the effect of the concentrated mass and the pipe configuration on the vibration stress-velocity relationship were calculated accurately using the precisely analyzed mode function. The two results were found to be in good agreement. It is considered that this new method can give the evaluation result that envelops the pipes of the entire plant by appropriately setting the parameters.

In a previous report, a new method of calculating the approximate seismic cumulative fatigue damage of plant pipelines was developed, in which the sum of the cumulative absolute velocities (CAV) of the pipeline response per cycle was calculated, and the result was applied to the allowable vibration velocity described in the ASME Operation and Maintenance (O/M) code 2012. The new method provided a conservative value of cumulative fatigue damage. In this present study, a parameter showing the effect of a concentrated mass attached to the tip of a cantilever pipe was obtained as a function of the ratio of the concentrated mass to the mass of the cantilever pipe by eigenmode calculation using ABAQUS. In the previous report, the new method was based on the relative response of the pipeline, whereas in this present study, the application of the method was expanded to evaluations using the CAV of the excitation input for each cycle. We conducted the fast forward simulation of a real earthquake to determine the effect of the peak frequency change on cumulative fatigue damage, and we found that the response of cumulative fatigue damage at the peak frequency tends to decrease with increasing peak excitation frequency, which was consistent with the results obtained using the previously reported new method. Both the new method and the newly expended method are based on the ASME O/M code, and the results obtained by these methods suggest that the peak frequency tends to affect general pipelines. In the calculations, when the configuration of the pipeline is fixed and the mode shape does not change, the cumulative fatigue damage was found to decrease with increasing peak frequency of input acceleration. If the mode shape changes with the peak input acceleration frequency, then cumulative fatigue damage is affected. Moreover, if the participation factor has a larger value in a higher mode, the cumulative fatigue damage also has a larger value.

A series of study is presented to develop a prediction method for pipe wall thinning in power plants in order to improve the maintenance management for piping system. In the first report, experiments for flow-accelerated corrosion (FAC) of carbon steel specimens were conducted and basic data of FAC rate were obtained by setting temperature from 50 to 150 ℃ and pH from 7.0 to 9.8 as main parameters. As this second report, the experimental data of FAC rate were compared with the prediction method. Effective mass transfer coefficient correlation was proposed and implemented into the prediction method considering combining effect of local average and turbulent velocity in the near-wall region calculated by computational fluid dynamics (CFD) simulation code. Fairly good agreement was confirmed between experimental and predicted FAC rate profile, quantitatively. Continuously, prediction method was applied to actual power plant piping systems, and some elbow components were chosen for evaluation in detail. Comparison of measured and predicted FAC rate also showed good agreement with data mostly evaluated conservatively in sense of maintenance management. As a whole, presented FAC prediction method including effective mass transfer coefficient was confirmed to predict measured FAC rate data of power plant pipe component with fairly good accuracy and reasonable conservatism, at least for the subjected temperature and pH conditions.

Mass and momentum transfer characteristics in a 90° elbow with a radius to pipe diameter ratio of 1.5 are studied experimentally with the aid of the plaster dissolution method, planar velocity measurement by particle image velocimetry (PIV), and surface flow visualization in the Reynolds number (Re) range of 5 × 104 to 20 × 104. The experimental results indicate that the most significant change in mass transfer distribution occurs on the inner wall of the elbow. The mass transfer coefficient increases along the centerline of the first half of the elbow and decreases in the second half with increasing Reynolds number. The near-wall velocity measurements by PIV show that the flow accelerates on the first half of the inner wall and decelerates on the second half, which contributes to the growth of the turbulent intensities on the second half of the inner wall. The surface flow visualization indicates that the secondary flow is weak on the inner wall of the elbow with higher Reynolds number. These results show that the mass transfer characteristics change on the inner wall of the elbow with increase in the Reynolds number, even when it is larger than Re = 5 × 104.

Although acceleration and cumulative absolute velocity (CAV) are used as seismic indexes, their relationship with the damage mechanism is not yet understood. In this paper, a simplified evaluation method for seismic fatigue damage, which can be used as a seismic index for screening, is derived from the stress amplitude obtained from CAV for one cycle in accordance with the velocity criterion in ASME Operation and Maintenance of Nuclear Power Plants 2012, and the linear cumulative damage due to fatigue can be obtained from the linear cumulative damage rule. To verify the performance of the method, the vibration response of a cantilever pipe is calculated for four earthquake waves, and the cumulative fatigue damage is evaluated using the rain flow method. The result is in good agreement with the value obtained by the method based on the relative response. When the response spectrum obtained by the evaluation method is considered, the value obtained by the evaluation method has a peak at the peak frequency of the ground motion, and the value decreases with increasing natural frequency above the peak frequency. A higher peak frequency of the base leads to a higher value obtained by the evaluation method.

This paper describes the experimental study on the liquid droplet impingement erosion of metal materials to understand the influence of material hardness on the erosion rate. The experiment is carried out using a water spray jet apparatus with a condition of relatively thin liquid film thickness. The metal materials tested are pure aluminum, aluminum alloy, brass, mild steel, carbon steel and stainless steel. The liquid droplets considered are 30 ± 5 μm in volume average diameter of water, which is the same order of droplet diameter in the actual pipeline in nuclear/fossil power plants. In order to understand the influence of material hardness on the liquid droplet impingement erosion, the scanning electron microscope (SEM) observation on the eroded surface and the measurement of erosion rate are carried out in the terminal stage of erosion. The experimental results indicate that the erosion rates are expressed by the droplet velocity, volume flux, Vickers hardness and the liquid film thickness, which are fundamentals of the liquid droplet impingement erosion. The empirical formula shows that the power index for droplet velocity dependency is found to be 7 with a scattering from 5 to 9 depending on the materials, while the power index for Vickers hardness dependency is found as −4.5.

Flow-induced acoustic resonances in piping with closed side branches or T-junctions are one of the causes of severe structural vibrations, which sometimes cause fatigue damage to piping and components in a power plant and many engineering applications. In this paper, on the basis of the results of steam flow experiments and calculations, the effects of the liquid phase on the flow-induced acoustic resonance at closed side branches in the steam flow piping of BWRs are described, and some suggestions for the steam piping design of BWRs are also given. The liquid phase in a steam flow forms droplets or liquid film, which may affect the amplitude, frequency and critical Strouhal number of the resonance. From the results of wet steam experiments and CFD calculations, we have found that in some cases the wetness of the steam flow may decrease the resonant amplitude and change the frequency owing to the interaction of the vortex generation or damping by the existence of the liquid film and droplets. Therefore, for the wet steam piping design of BWR, some suggestions for taking these effects into consideration, under actual BWR steam conditions are described.

In a previous study [1], the seismic behavior of a piping system with local thinning elbow caused by liquid droplet impingement erosion (LDI) was evaluated using hybrid seismic testing method in Central Research Institute of Electric Power Industry (CRIEPI). In those studies, torsional buckling and wrinkles occurred at locally wall thinning elbow under large amplitude seismic motion of eight times the design basis earthquake based on the allowable stress level. It was concluded that buckling should be considered as a failure mode. In this study, the torsion buckling under out-of-plane bending of locally wall thinning elbows could be demonstrated by finite element analyses. Torsion buckling deformation agreed well with test result and the critical buckling deformation was clarified by comparing parameter analyses. The buckling formula was proposed to prevent the torsion buckling of locally wall thinning elbow.

Main steam control valves in power plants are required to operate underwide ranges of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are conducted using rigid and flexible valve head supports to clarify the mechanisms of valve head vibrations that are caused by unsteady flows around the valve. The results obtained using the rigid support without valve head vibration show that the unsteady flow around the valve head causes pressure fluctuations on the valve head surface with random and impulsive wave forms. When using the flexible support, the valve head vibrates near the natural frequency of the valve head support system, and vibrations are excited around the operating conditions where the pressure fluctuation becomes greater when using the rigidly supported valve head. When the valve head vibration increases, the pressure fluctuation becomes periodic with the same frequency as the valve head vibration. The numerical results show that the response of the separated jet lags behind the valve head motion. As a result, the lateral fluid force adds negative damping on the vibration on the valve head.

Flow accelerated corrosion (FAC) is caused by the accelerated dissolution of protective oxide film under the condition of high flow rate and has been one of the most important subjects in fossil and nuclear power plants. The dominant factors of FAC are water chemistry, material, and fluid dynamics. Understanding of the thinning mechanism is very important to estimate the quantitative effects of the dominant factors on FAC. In this study, a novel model of chemical effect on FAC under the steady-state condition was developed in consideration of the diffusion of soluble iron and chromium species, dissolved hydrogen, and dissolved oxygen. The formula to evaluate the critical concentration of dissolved oxygen for FAC suppression was derived. The present model reproduced qualitatively the effect of major environmental parameters on FAC rate. The model could explain the following facts. (1) The FAC rate shows a peak around 413 K. (2) The FAC rate decreases with an increase in Cr content. (3) The FAC rate decreases with an increase in pH. (4) The FAC rate decreases with an increase in dissolved oxygen concentration. (5) The maximum of critical dissolved oxygen concentration is observed around 353 K. (6) The critical dissolved oxygen concentration decreases with an increase in pH. We conclude that the diffusion of soluble species from the saturated layer under the steady-state condition well reproduces the unique FAC behavior with variation of water chemistry parameters.

The boiling water reactor (BWR-3) steam dryer in the Quad Cities (QC) Unit 2 Nuclear Power Plant was damaged by high-cycle fatigue due to acoustic-induced vibration. The cause of the dryer failure was considered as flow-induced acoustic resonance at the stub pipes of the safety relief valve (SRV) in the main steam lines (MSLs). The acoustic resonance was considered to be generated by the interaction between the sound field and an unstable shear layer across the closed side branches of SRVs. We have started a research program on BWR steam dryers to develop methods of evaluating the loading. Moreover, it is necessary to evaluate the dryer integrity of BWR-5 plants, which are the main type of BWR in Japan. In the present study, we conducted 1/10-scale BWR model tests and analysis to investigate the flowinduced acoustic resonance and acoustic characteristics in MSLs. The test apparatus consisted of a steam dryer, a steam dome, and 4 MSLs with 20 SRV stub pipes. Computational fluid dynamics (CFD) analysis was ...

Main steam control valves in power plants are required to operate under wide ranges of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are conducted with rigid and flexible valve head supports in order to clarify mechanisms of valve head vibrations caused by unsteady flows around the valve. Results obtained with the rigid support without valve head vibration show that the unsteady flow around the valve head causes pressure fluctuations on the valve head surface with random and impulsive wave forms. With flexible support, the valve head vibrates at near the natural frequency of the valve head support system and the vibrations are excited around the operation condition where the pressure fluctuation becomes large with rigid support valve head. When the valve head vibration becomes large, the pressure fluctuation becomes periodic with the same frequency of the valve head vibration. The numerical result shows that the response of the separated jet delays behind the valve head motion. As a result, the lateral fluid force fluctuations on the valve head provide negative damping on the vibration.

A steam control valve is used to control the flow from the steam generator to the steam turbine in thermal and nuclear power plants. During startup and shutdown of the plant, the steam control valve is operated under a partial flow conditions. In such conditions, the valve opening is small and the pressure deference across the valve is large. As a result, the flow downstream of the valve is composed of separated unsteady transonic jets. Such flow patterns often cause undesirable large unsteady fluid force on the valve head and downstream pipe system. In the present study, various flow patterns are investigated in order to understand the characteristics of the unsteady flow around the valve. Experiments are carried out with simplified two-dimensional valve models. Two-dimensional unsteady flow simulations are conducted in order to understand the experimental results in detail. Scale effects on the flow characteristics are also examined. Results show three types of oscillating flow pattern and three types of static flow patterns.

Flow visualization tests and fluid force measurements were conducted in a water tunnel to investigate flow around and fluid forces on a cross-shaped tube (CST) in a cross-flow. The following results were obtained. Flow around a CST is classified into two patterns. One is perfect-separation flow for small incidence angle and the other is reattached flow for large incidence angle. The critical incidence angle where the flow pattern changes from the former to the latter is about 7.5°∼15°. At the critical angle, the mean drag and lift coefficients reach their minimum while the Strouhal number attains to the maximum. The amplitude of the fluctuating lift decreases as the incidence angle increases. These features of flow around and fluid forces on a CST are quite similar to those of a square prism. These results imply that one-degree-of-freedom galloping and vortex-induced vibration possibly occur for small incidence angle while ILEV (impingement of leading edge vortices) vibration might be caused for large incidence angle when a CST is elastically supported in a cross-flow.

A steam control valve is used to control the steam flow from a steam generator to a turbine in power plants. The flow field in the steam control valve is usually complex and transonic. A flowinduced vibration can occur in the valve and may cause unexpected shutdown. An experimental study is conducted to clarify the characteristics of the flow in the steam control valve. In the present study, scale effects on the condition of occurrence of various flow patterns are investigated. Experimental results have shown various flow patterns including new patterns which have not been reported. Additionally, even under the same operating condition, different flow patterns have been observed under a certain range of condition. This effect was found to be comparable with the scale effect.

A steam control valve is used to control the steam flow from a steam generator to a turbine in power plants. The flow field in the steam control valve is usually complicated and transonic. A flow-induced vibration can occur in the valve and may cause unexpected shutdown of the plant. An experimental study is conducted to clarify the characteristics of the flow in the steam control valve. In the present study, scale effects on the condition of occurrence of various flow patterns are investigated. Experimental results show various flow patterns including new patterns which have not been reported. Additionally, even under the same operation condition, different flow patterns are observed under a certain range of condition. This effect is found to be comparable with the scale effect.

A steam control valve is used to control a steam flow from a steam generator to a turbine in power plants. The flow field in the steam control valve is usually transonic and complex. A flow-induced vibration can occur in the valve and its downstream pipe. Such vibrations may cause an unexpected shutdown of the power plant. An experimental and numerical study has been conducted to clarify the characteristics of the vibration. The amplitude of the pressure fluctuation in the pipe has been influenced by the flow fluctuation near the valve. Additionally, it has been found that the amplitude of pressure fluctuation in pipe can be reduced by increasing a pipe cross section area downstream of the valve.

Leakage-flow-induced vibration for a relatively short gap is studied analytically to provide useful information to design structures that include a leakage flow. The relationship between the analysis of a one-dimensional system and that of an annular gap is explained first. Then, the mechanism of flutter-type instability is reproduced from previous study after correcting an error. Finally, the self-excited vibration potential of an engineering system is shown from sample calculations. It is shown that an axial flow becomes dominant in the short-gap approximation, and in this case, the analysis of a one-dimensional flow can be expanded to that of an annular flow. The result that negative damping can occur in the case of a divergent passage owing to the delay induced by fluid inertia was obtained from a previous study. It was suggested analytically that the damping ratio could become negative and its absolute value could become more than 10% in a system that is frequently encountered in a plant, if the natural frequency decreases. The value could be sufficient to generate self-excited vibration.Copyright © 2015 by ASME

The local fluid excitation force acting on a rotated square tube bundle having transverse pitch-to-diameter ratio of T∕D=3.1, in a single-phase cross-flow was measured, and the normalized power spectral density (NPSD) and correlation length in the axial direction of a tube were examined. The fluid excitation force acting on the interior tube was from three to ten times larger than that acting on the upstream tube. The fluid force was almost fully developed after the third row. NPSD of the fluid excitation force could be almost plotted on a single universal curve. Regarding the lift direction, there was a peak in NPSD at fD∕u∼0.3 caused by vortex shedding. Regarding the drag direction, there could be another peak in NPSD around twice the vortex shedding frequency. In the region of fD∕u>0.5, where the effect of the vortex shedding was assumed to be small in the lift direction, the correlation length of the lift direction was ∼1.1D. NPSD was a little larger than previous results for tube bundles of relatively small pitch to diameter ratios summarized by Axisa, Antunes, and Villard (1990, J. Fluid Struct., 4, pp. 321–341).

Under certain opening conditions (partial opening) of a steam control valve, the piping system in a power plant occasionally experiences large vibrations. To understand the valve instability that is responsible for such vibrations, detailed experiments and CFD calculations were performed. As a result of these investigations, it was found that under the middle-opening (partial opening) condition, a complex three-dimensional (3D) flow structure (valve-attached flow) sets up in the valve region leading to a high pressure region on a part of the valve body. As this region rotates circumferentially, it causes a cyclic asymmetric side load on the valve body, which is considered to be the cause of the vibrations.

The SIRIUS-N facility was designed and constructed for highly accurate simulation of core-wide and regional instabilities of a natural circulation BWR. A real-time simulation was performed in the digital controller for modal point kinetics of reactor neutronics and fuel-rod conduction on the basis of measured void fractions in reactor core sections of the thermal-hydraulic loop. Stability experiments were conducted for a wide range of thermal-hydraulic conditions, power distributions, and fuel rod time constants, including the nominal operating conditions of a typical natural circulation BWR. The results show that there is a sufficiently wide stability margin under nominal operating conditions, even when void-reactivity feedback is taken into account. The stability experiments were extended to include a hypothetical parameter range (double-void reactivity coefficient and inlet core subcooling increased by a factor of 3.6) in order to identify instability phenomena. The regional instability was clearly demonstrated with the SIRIUS-N facility, when the fuel rod time constant matches the oscillation period of density wave oscillations.

Experiments were conducted to investigate two-phase flow instabilities due to flashing in a boiling natural circulation loop with a chimney at low pressure. The SIRIUS-N facility was designed to have non-dimensional values nearly equal to those of typical natural circulation boiling water reactor (BWR). The observed instability is suggested to be flashing-induced density wave oscillations, since the oscillation period correlated well with the passing time of single-phase liquid in the chimney section regardless of system pressure, heat flux, and inlet subcooling. Stability maps were obtained in reference to the inlet subcooling and the heat flux at the system pressures of 0.1, 0.2, 0.35, and 0.5 MPa. The flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarged with increasing system pressure. Thus, the stability margin becomes larger in a startup process of a reactor by pressurizing the reactor sufficiently before heating according to the stability map.

Experiments were conducted to investigate two-phase flow instabilities in a boiling natural circulation loop with a chimney at high pressure. The SIRIUS-N facility was designed to have non-dimensional values which are nearly equal to those of a typical natural circulation BWR. The observed oscillations are found to be density wave oscillations, since the void fractions in the chimney inlet and exit are out of phase. They belong to the Type-I category, since they occur at low flow qualities, according to the Fukuda—Kobori's classification. Moreover, the oscillation period correlates well with the passing time of bubbles in the chimney section regardless of the system pressure, the heat flux, and the inlet subcooling. Two distinct phenomena are found in relation between the oscillation period and liquid passing time in the chimney, indicating that the driving mechanisms of the instabilities are different between low and high pressures. Stability maps were obtained in reference to the inlet subcooling and th...

A steam control valve causes vibration of the piping when the opening is in the middle condition. For the rationalization of maintenance and management in the plant, the valve should he improved, but it is difficult to understand the flow characteristics in detail by experiment because the flow around the valve is complex 3D stlacture and becomes supersonic (M>1). In this report, a high-precision 3D CFD code for compressible flow is developed. After validating the code by comparing with experiments, the detailed flow characteristics around the valve are studied. At only the middle opening condition, a circumferentially propagating spike-type pressure fluctuation caused by asymmetric flow around valve can be seen. As this fluctuation causes cyclic side load on the valve body, it is thought to be the cause of vibration.

Air water two phase flow experiment was conducted to examine the basic flow characteristics of a centrifugal gas liquid separator. Vertical transparent test section, which is 4 m in height, was used to imitate the scale of a BWR separator. Flow rate conditions of gas and liquid were fixed at 0.1 m3/s and 0.033 m3/s, respectively. Radial distributions of two-phase flow characteristics, such as void fraction, gas velocity and bubble chord length, were measured by traversing dual optical void probes in the test section, horizontally. The flow in the standpipe reached to quasi-developed state within the height to diameter aspect ratio H/D=10, which in turn can mean the maximum value for an ideal height design of a standpipe. The liquid film in the barrel showed a maximum thickness at 0.5 to 1 m in height from the swirler exit, which was a common result for three different standpipe length conditions, qualitatively and quantitatively. The empirical database obtained in this study would contribute practically to the validation of numerical analyses for an actual separator in a plant, and would also be academically useful for further investigations of two-phase flow in large-diameter pipes.