Analysis of JNES Seismic Tests on Degraded Piping

Abstract

Nuclear power plant safety under seismic conditions is an important consideration. The piping systems may have some defects caused by fatigue, stress corrosion cracking, etc., in aged plants. These cracks may not only affect the seismic response, but may also grow and break through causing loss of coolant. Therefore, an evaluation method needs to be developed to predict crack growth behavior under seismic excitation. This paper describes efforts conducted to analyze and better understand a series of degraded pipe tests under seismic loading that was conducted by Japan Nuclear Energy Safety Organization (JNES). A special “cracked-pipe element” (CPE) concept, where the element represented the global moment-rotation response due to the crack, was developed. This approach was developed to simplify the dynamic finite element analysis. In this paper, model validation was conducted by comparisons with a series of pipe tests with circumferential through-wall and surface cracks under different excitation conditions. These analyses showed that reasonably accurate predictions could be made using the ABAQUS connector element to model the complete transition of a circumferential surface crack to a through-wall crack under cyclic dynamic loading. The JNES combined-component test was analyzed in detail. The combined-component test had three crack locations and multiple applied simulated-seismic block loadings. Comparisons were also made between the ABAQUS FE analyses results to the measured displacements in the experiment. Good agreement was obtained and it was confirmed that the simplified modeling is applicable to a seismic analysis for a cracked pipe on the basis of fracture mechanics. Pipe system leakage did occur in the JNES tests. The analytical predictions using the CPE approach did not predict leakage, suggesting that cyclic ductile tearing with large-scale plasticity was not the crack growth mode for the acceleration excitations considered here. Hence, the leakage was caused by low-cycle fatigue with small-scale yielding. The procedure used to make predictions of low-cycle fatigue crack growth with small-scale yielding was based on the Dowling ΔJ procedure, which is an extension of linear-elastic fatigue crack growth methodology into the nonlinear plasticity regime. The predicted moments from the CPE approach were used using a cycle-by-cycle crack growth procedure. The predictions compare quite well with the experimental measurements.

Seismic Capacity Test of Nuclear Piping System using Multi-platform Shake Table

“…Seismic Capacity Test of Nuclear Piping System using Multi-platform Shake Table 정진환 1) ･ 계만수 2) ･ 서영득 1) ･ 최형석 3) * ･ 김민규 부산대학교 사회환경시스템공학부, 2) MIDAS IT 중국지사, 3) 부산대학교 지진방재연구센터, 4) 한국원자력연구원 1) Department of Civil and Environmental Engineering, Pusan National University, 2) MIDAS IT, Beijing, China, 3) Pusan National University Seismic Simulation Test…”

Assessing Equivalent Viscous Damping Using Piping System Test Results

“…If a FRF function determined from test data has many points around a mode, Equation (3) can be repeated for a number of frequencies near the damped natural frequency, leading to a least-square SDOF method: (4) Solution to this least square equation automatically removes the high frequency noise as shown in Figure 9. This method is fast and suitable for lightly damped modes that are well separated from each other.…”

“…Test data from the JNES degraded piping system tests were used to estimate the equivalent viscous damping values [4]. The purpose of this effort was to obtain realistic damping estimates directly from the test data and then to use these damping estimates in analytical prediction of the dynamic response of the subject degraded piping system.…”

“…In the dynamic analyses of runs C030 to C035 using ABAQUS, 2% damping at 5 Hz and 50 Hz was used in the Rayleigh damping model, while the entire piping system was allowed to behave plastically in order to account for the increase in damping due to plasticity (e.g. 0.4% for C030) [4,7]. …”

“…This evaluation included nonlinear time history analyses and response spectrum analyses of the test piping systems. Most of the results of this evaluation have been documented in two NUREG/CR reports [3,4]. Damping assessment was one of the important areas during the evaluation process.…”