Abstract
The main purpose of this study is to investigate the feasibility of the seismic fragility analysis (FA) with the strain-based failure modes for the nuclear metal components retaining pressure boundary. Through this study, it is expected that we can find analytical ways to enhance the high confidence of low probability of failure (HCLPF) capacity potentially contained in the conservative seismic design criteria required for the nuclear metal components. Another goal is to investigate the feasibility of the seismic FA to be used as an alternative seismic design rule for beyond-design-basis earthquakes. To do this, the general procedures of the seismic FA using the inelastic seismic analysis for the nuclear metal components are investigated. Their procedures are described in detail by the exampled calculations for the surge line nozzles connecting hot leg piping and the pressurizer, known as one of the seismic fragile components in NSSS (Nuclear Steam Supply System). To define the seismic failure modes for the seismic FA, the seismic strain-based design criteria, with two seismic acceptance criteria against the ductile fracture failure mode and fatigue-induced failure mode, are used in order to reduce the conservatism contained in the conventional stress-based seismic design criteria. In the exampled calculation of the inelastic seismic strain response beyond an elastic regime, precise inelastic seismic analyses with Chaboche’s kinematic and Voce isotropic hardening material models are used. From the results of the seismic FA by the probabilistic approach for the exampled target component, it is confirmed that the approach of the strain-based seismic FA can extract the maximum seismic capacity of the nuclear metal components with more accurate inelastic seismic analysis minimizing the number of variables for the components.
Highlights
There have been many efforts to develop advanced seismic design rules that enable accommodating the increasing of the design-basis earthquake (i.e., SSE: Safe Shutdown Earthquake) for the nuclear facility components, especially for piping systems [1,2,3,4]
This paper presents the overall procedure of the inelastic analysis-based seismic fragility analysis (FA) for the safety-related nuclear components
When the ground input motions of the NUREG/CR-0098 uniform response spectrum (URS) are used for the system seismic analysis, a single set of three directional ground artificial time histories to match URS as complying with the provisions of RG 1.122 [26] to account for uncertainty can be used for the system seismic analysis to generate the median in-structure seismic input motions at subsystem support locations
Summary
There have been many efforts to develop advanced seismic design rules that enable accommodating the increasing of the design-basis earthquake (i.e., SSE: Safe Shutdown Earthquake) for the nuclear facility components, especially for piping systems [1,2,3,4]. The general procedures of seismic FA with the probabilistic approach are investigated with the previously developed strain-based seismic design criteria [17] for the nuclear metal components retaining high-pressure boundary, which inevitably require an inelastic seismic time history analysis. To validate the detailed procedures for HCLPF capacity calculations, example calculations for the surge line nozzles connecting hot leg piping and the pressurizer, known as one of the seismic fragile components in NSSS (Nuclear Steam Supply System), are performed. Such calculations follow the basic FA variables generically recommended in EPRI/TR-103959 [15]
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