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Gyeonggi, South Korea

Cho S.G.,JACE KOREA Company | Kim D.,Kunsan National University | Chaudhary S.,Malaviya National Institute of Technology, Jaipur
Nuclear Engineering and Design

An analytical model is required when the analysis method or the combined analysis and modal testing method is used as an aid to the seismic qualification (SQ) of equipment cabinets within nuclear power industry. This study proposes a simplified and computationally efficient model to represent the nonlinear dynamic behavior of the cabinet during earthquake. The presented model accounts for the softening behavior of the cabinets by incorporating the Duffing's type of restoring force. The characteristic of nonlinear restoring force for the finite element model (FEM) is based on the relationship of stress-strain of the element. Experiments have also been performed on an actual cabinet of nuclear power plant (NPP) to validate the model. The softening or reduction in dynamic stiffness of cabinets with increase in the excitation levels is observed in the experiments. It is also found that material yielding is not a significant source of the nonlinear behavior of the cabinet. The results obtained from the analysis using the proposed model are found to be in good agreement with the experimental results. The proposed model is expected to be useful for the prediction of seismic behavior of cabinets, particularly during the operation, owing to less computational effort required, accurate prediction of softening and no requirement of tests. © 2011 Elsevier B.V. All rights reserved. Source

Ali A.,Kunsan National University | Hayah N.A.,Kunsan National University | Kim D.,Kunsan National University | Cho S.G.,JACE KOREA Company
Nuclear Engineering and Technology

The probabilistic seismic performance of a standard Korean nuclear power plant (NPP) with an idealized isolation is investigated in the present work. A probabilistic seismic hazard analysis (PSHA) of the Wolsong site on the Korean peninsula is performed by considering peak ground acceleration (PGA) as an earthquake intensity measure. A procedure is reported on the categorization and selection of two sets of ground motions of the Tohoku earthquake, i.e. long-period and common as Set A and Set B respectively, for the nonlinear time history response analysis of the base-isolated NPP. Limit state values as multiples of the displacement responses of the NPP base isolation are considered for the fragility estimation. The seismic risk of the NPP is further assessed by incorporation of the rate of frequency exceedance and conditional failure probability curves. Furthermore, this framework attempts to show the unacceptable performance of the isolated NPP in terms of the probabilistic distribution and annual probability of limit states. The comparative results for long and common ground motions are discussed to contribute to the future safety of nuclear facilities against drastic events like Tohoku. © 2014, Korean Nuclear Society. All rights reserved. Source

Vu T.D.,Kunsan National University | Vu T.D.,JACE KOREA Company | Kim Kor D.,Kunsan National University | Kim Kor D.,JACE KOREA Company
Nuclear Technology

The application of base isolation to nuclear systems has been limited to date for some important reasons, including the deprivation of sufficient data for the longterm operation of such isolation devices and the lack of specific standards. Moreover, it is difficult to provide seismic protection in the vertical direction and to qualify the large-scale isolators up to experimentally large deformations in real dynamic conditions. The effect of aging on isolators is therefore one of the issues to be considered for the safety and reliability of base-isolated nuclear power plants (NPPs). Accounting for the variations of the postaging parameters of the isolators on the structural performance of the plant, this study proposes a simplified and efficient method to update the analytical model of baseisolated structures based on mean-iterative neural networks (MINNs). A bilinear model with a zero length element was built to represent the characteristics of lead-rubber bearings for their numerical analysis. Analytical model updating by MINNs has been successfully performed, and the observed results are found to be in good agreement with those obtained from experiments. Additionally, it is observed that the stiffening or hardening with time in the shear properties of isolation devices affects the seismic performance of the base-isolated structure. Seismic design over the service life span of NPP structures should take these aging effects of the isolators into account. Source

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