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This paper provides investigation on method of an elasto-plastic analysis for practical seismic design of nuclear piping systems, which are excited by multi-direction input. The Japan Society of Mechanical Engineers (JSME) established a task group to develop an elasto-plastic analysis method for nuclear piping systems, and prepared a case example code proposal for JSME.[1],[2] Our studies in past (ASME PVP2016-63186[3] and ASME PVP2017-65341[4]) were implemented on tests with unidirectional excitation using simple piping systems. In order to examine the applicability of the proposed case example code for JSME in piping of actual systems, it is necessary to examine cases in which there is multidirectional input excitation in piping systems in scales comparable to those of the piping in actual systems. We therefore conducted an analytical examination on demonstration of “the ultimate strength of piping system,” which was implemented at NUPEC. [5] We confirmed in the results of analytical examination that the strain range could be calculated at precision nearly equivalent to our examinations in the past, and that the draft code case was applicable. However, we also found a problem which needs to be solved. In addition, we were able to confirm that the local damping increase caused by the elasto-plastic behavior of the elbow which was subject to examination in this study was 1% or larger.

This paper provides investigation on method of elasto-plastic analysis for practical seismic design of nuclear piping system made of austenitic stainless steel. Our policy for the evaluation is that material properties used in the benchmark analyses are based on Japanese standard in nuclear design. The result of the first phase of this benchmark analysis intended for carbon steel piping systems has been provided in ASME PVP2016-63186[1]. In secondary benchmark analysis, analytical investigations focused on austenitic stainless steel piping were conducted. These analysis objects are two vibrating tests (model 1: piping containing an elbow, model 2:piping containing a tee). The elasto-plastic characteristic based on bilinear plasticity model based on the draft code case of JSME (Japan Society of Mechanical Engineers) was used in analyses. Additionally, analyses using the elasto-plastic characteristic which made yield point and 2nd modulus as a parameter were also carried out. For the model 1, the maximum strain estimated by elasto-plastic analysis using the elasto-plastic characteristics of stainless steel material determined as proposed by the draft code case of JSME agreed well, but on the safe side, with the experiment. However, this is not the case for the model 2: the maximum strain estimated using the same elasto-plastic characteristics was underestimated compared with the experiment. However, for both the models 1 and 2, the elasto-plastic behavior of the piping systems estimated by this analysis was approximately the same between the analysis with the material’s elasto-plastic characteristics approximated by bilinear plasticity modeling proposed by the draft code case of JSME and that approximated by the same bilinear plasticity modeling but with different setting of yield point and 2nd modulus values of the material. A possible cause of the underestimate that occurred in the model 2 is, according to the shape data of the tee, that the wall thickness of the tee is so large that its connection with the main and branch pipes has a large, step-like change in thickness, as opposed to the model that was designed without referencing the shape data. Another possible cause of this underestimate is coarse meshes of elements of the model. In order to improve analytical accuracy, it is necessary to add a method for modeling tee joints to the draft code case. To this end, a database of the shape of tee joints should be developed in cooperation with the manufacturers so that an optimum modeling method can be developed. As mentioned above, according to the result of elasto-plastic analyses for the model 1 (piping containing an elbow) and model 2 (piping containing a tee), it is necessary to develop a modeling method for tee joints. It is likely possible to directly use the elasto-plastic characteristics of carbon steel for the purpose of analyzing a piping of stainless steel.

This report proposes an elasto-plastic analysis method to be used for practical aseismic designing of nuclear piping systems. JSME (the Japan Society of Mechanical Engineers) initiated a task to establish an elasto-plastic analysis method for nuclear piping systems. During this task, benchmark analysis was conducted in order to examine the elastic-plasticity analytical method, in which our company decided to participate. Our policy for evaluation was that the material characteristics to be used in benchmark analysis were based on the standards for nuclear designs in Japan. As a consequence, we prepared a method to accurately simulate the vibration test on piping systems. The recommended elasto-plastic analysis method is thus specified as follows: 1) The elasto-plastic analysis method comprised of dynamic analysis on piping system modeled using beam elements and static analysis of the deforming elbow which was modeled using shell elements. 2) Bi-linear was applied as the elasto-plastic characteristics. The yield point was the standardized yield point times 1.2, and the second gradient was 1/100 the Young’s modulus. Kinematic hardening law was used as the hardening law. 3) Rain flow method and fatigue curve of an existing research were used to evaluate the fatigue life for the strain range obtained by elasto-plastic analysis.

In Japan, seismic assessments considered aging phenomena have been conducted as part of activities for aging measures of nuclear power plants. In the activities, it is found that FAC (Flow Accelerated Corrosion) of piping components made of carbon steel impact on seismic integrity because of decreasing a cross section of piping. Therefore, many destructive tests of piping have been conducted in Japan and the other countries. At first, factors on which did not focus in the past destructive test have been extracted in this investigation. displacement controlled cyclic tests of piping components which focused on extracted factors have been conducted additionally in order to provide enough test data for past destructive test data of piping components. Moreover, fatigue curve for practical evaluation of piping components made of carbon steel has been settled after obtained test data were put in order including past destructive test data of piping components from a point of fatigue.

Structural strengths of the piping and components in NPPs have been designed with seismic margin. They are classified seismically S, B and C class in terms of the influence rate to nuclear safety. For the highest seismic class (Class S) equipment, it is clarified that they have enough seismic margins against design seismic conditions by shaking table tests or numerical simulations. However, for the lower seismic class (Class B and C) equipment, their seismic margins have not been clarified quantitatively. In this paper, in order to evaluate seismic robustness of the lower seismic class equipment with no clarification of seismic margin, seismic influences of the lower seismic class equipment in NPPs damaged by actual large earthquakes have been surveyed and sorted as a database, and the integrity of the lower class equipment have been discussed. Seismic effects on 24 plants damaged by the recent large 6-earthquakes are surveyed, sorted as a database, and investigated. As a result, a total of 29 cases of function deterioration or loss were observed. Considering the total number of components and piping, the frequency of those cases in class B and C components and piping was low. And also, as it is found there are a few cases of degradation or loss of function in the equipment installed on the bedrock or in the buildings.

Small and precise actuators for multi-degrees of freedom (DOF) motion is needed to construct a dexterous robot arms and manipulators. Ultrasonic motors are suitable for the multi-DOF actuation of the robot arms and manipulators because of its characteristics such as its output power per unit volume, high stationary limiting torque, high maximum operating torque, simple design, silence, and high controllability. In this study we develop a three-DOF ultrasonic motor. It is found that the three-DOF ultrasonic motor can be constructed when natural frequencies of two second bending modes and a first longitudinal mode of a bar-shaped stator correspond. A spherical rotor in contact with the stator head rotates around three perpendicular axes. Geometry and structure of the ultrasonic motor in detail is determined using finite element analysis. Measured natural frequencies and natural modes of bending and longitudinal vibrations of the stator agree well with calculated ones. The rotational speed of the spherical rotor around three perpendicular axes are measured.

As seismic motion level for seismic resistant designing has been increased, usual tolerance is no longer enough in the existing domestic PWR and BWR plants. In this study, the authors had an eye on devices supported by four legs in the existing domestic PWR and BWR plants, and established a reasonable earthquake-proof evaluating safety method compared to the original method (JEAG4601.that is earthquake resistant design code for nuclear plants in Japan). This was done by establishing an analytical method which premises elasto-plastic deformation of the support legs that do not associated with device’s function maintenance. At first, the support leg’s configuration of the devices supported by four legs in the existing plants was organized, and for the typical legs, the elasto-plastic behavior of support leg itself was verified by element test. Then the authors have established an analytical method that would simulate with result in element test. Secondly, main configuration (including support leg configuration) of the practical device was organized, and for the typical devices the elasto-plastic behavior of the practical test object was verified by seismic excitation test. Then, the authors have established an analytical method that would simulate with result in seismic excitation test. By implementing the established analytical method to a few practical devices supported by four legs in the existing domestic PWR and BWR plants, this analytical method was a reasonable method compared to the original standard.

The present design damping constants for nuclear power plant (NPP)’s piping system in Japan were developed through discussion among expert researchers, electric utilities and power plant manufactures. They are standardized in “Technical guidelines for seismic design of Nuclear Power Plants” (JEAG 4601-1991 Supplemental Edition). But some of the damping constants are too conservative because of a lack of experimental data. To improve this excessive conservatism, piping systems supported by U-bolts were chosen and U-bolt support element test and piping model excitation test were performed to obtain proper damping constants. The damping mechanism consists of damping due to piping materials, damping due to fluid interaction, damping due to plastic deformation of piping and supports, and damping due to friction and collision between piping and supports. Because the damping due to friction and collision was considered to be dominant, we focused our effort on formulating these phenomena by a physical model. The validity of damping estimation method was confirmed by comparing data that was obtained from the elemental tests and the actual scale piping model test. New design damping constants were decided from the damping estimations for piping systems in an actual plant. From now on, we will use the new design damping constants for U-bolt support piping systems, which were proposed from this study, as a standard in the Japanese piping seismic design.Copyright © 2002 by ASME