An efficient simplified elastic–plastic analysis procedure using engineering formulae for strain-based fatigue assessment of nuclear safety class 1 piping system subjected to severe seismic loads

Highlights

• The study proposed an efficient simplified elastic-plastic analysis procedure using engineering formulae for performing strain-based fatigue design of nuclear safety class 1 piping system during severe seismic loads.

• The proposed procedure was applied to a pressurizer surge line system.

• The total strain amplitudes, fatigue assessment results, and total calculation time were compared between the proposed analysis procedure and the detailed dynamic finite element elastic-plastic analysis.

• The comparison confirmed that the proposed simplified elastic-plastic analysis procedure can efficiently derive reasonable and conservative results.

Abstract

This paper proposes an efficient simplified elastic–plastic analysis procedure using engineering formulas to perform strain-based fatigue design considering the plasticity that occurs in nuclear safety class 1 piping systems during severe seismic loads, and then presents the application results of the procedure to a pressurizer surge line system. The total strain amplitudes, fatigue assessment results, and total calculation time were compared between the proposed procedure and the detailed dynamic finite element elastic-plastic analysis. The comparison confirmed that the proposed analysis procedure can efficiently derive reasonable and conservative results for nuclear safety class 1 piping systems under severe seismic loads.

Introduction

For service level D load conditions including safe shutdown earthquake (SSE), which is one of design basis earthquakes (DBEs), nuclear power plant operators shall demonstrate that structural integrity of nuclear safety class 1 piping system, which plays a role of reactor coolant pressure boundary (RCPB), is maintained according to the procedures presented in the design technical basis such as the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessels (B&PV) Code, Sec.III, Div.1, Subsec. NB [1].

It has been reported that some severe earthquakes at levels exceeding the design basis have occurred at nuclear power plants [2], [3]. These earthquakes are called beyond design basis earthquake (BDBE) because loads exceeding the design basis are applied. They can cause incremental plastic strain and failures due to plastic collapse or low cycle fatigue. In response, regulatory agencies such as the United States Nuclear Regulatory Commission (USNRC) and the Japanese National Security Council (NSC) have published guidelines for seismic design or re-evaluation taking into account BDBE and plasticity [4], [5].

Therefore, when constructing new nuclear power plants, BDBE should be considered at the design stage, and in the case of existing operating nuclear power plants, structural integrity against BDBE should be re-evaluated. The seismic assessment procedures presented in design technical basis such as the ASME B&PV Code, Sec. III, Div.1, Subsec.NB are based on stress and do not consider fatigue as one of the potential failure mechanisms. Due to excessive conservatism of the stress-based assessment procedures, it is too difficult to ensure structural integrity of nuclear safety class 1 component for BDBE using such procedures. Therefore, recently, several studies [6], [7], [8], [9], [10], [11], [12], [13] have been performed to evaluate structural integrity of nuclear safety class 1 components under BDBE through strain-based procedures instead of stress-based procedures. Unlike the design basis that does not require fatigue assessment for severe seismic loads, several experimental studies [7], [11], [13] have confirmed that fatigue is a major failure mechanism in nuclear components subjected to severe seismic loads such as SSE and BDBE. Nakamura et al. [8], [12] performed an inelastic benchmark parameter analysis of pipe elbows and piping systems made of carbon steel and austenitic stainless steel to investigate the reliability of finite element (FE) elastic-plastic seismic analysis. Based on these inelastic benchmark analysis results, the Japan Society of Mechanical Engineers (JSME) developed the JSME Nuclear Code Case NN-CC-008, which embodied a seismic design assessment methodology for nuclear piping using an elastic-plastic seismic response analysis method and strain-based fatigue acceptance criteria [9]. The ASME B&PV Code committee is enacting a code case that proposes a strain-based acceptance criterion for nuclear safety class piping system subjected to reversing dynamic loads that does not require coupling with non-reversing dynamic loads [6].

Under severe seismic loads, nuclear safety class 1 piping system is subjected to high loads of ultra low-cycle fatigue (ULCF) or LCF loads. These loads can induce plastic strain in the piping system, and proportion of the plastic strain to total strain will dominate. Therefore, it is required to accurately evaluate plastic strain for reliable strain-based fatigue assessment for nuclear safety class 1 piping system subjected to severe seismic loads. Detailed FE elastic-plastic seismic analysis is required for accurate calculation of plastic strain. However, the strain calculated through the FE elastic-plastic analysis is very sensitive to the finite element (FE) analysis variables as well as the FE elastic-plastic analysis requires more input data, a much larger storage space, and a longer calculation time than elastic analysis. Due to these shortcomings of the elastic-plastic analysis, nuclear industries generally perform elastic stress analysis for various transients and then evaluate fatigue life using the elastic analysis results. However, the elastic stress analysis cannot take into account the plastic strain. Therefore, the ASME B&PV Code Committee devised and proposed a simplified elastic-plastic analysis procedure that increases the stress amplitude by multiplying the elastic analysis results by penalty factor. Prerequisite for fatigue analysis presented in the ASME B&PV Code, Sec.III, Div.1, Subsec.NB is that the range of primary plus secondary stress intensity should not exceed three times the design stress intensity [14]. If the stress intensity range exceeds these limits, the Code provides a simplified elastic-plastic analysis approach for fatigue assessment [15], [16], [17]. This simplified elastic-plastic analysis procedure can easily calculate alternating stress intensities considering plastic effect using the penalty factors and the elastic stress analysis results. However, the Code does not prescribe that the simplified elastic-plastic analysis procedure is applicable to the service level D limits [10], [16]. Kim and Kim proposed a strain-based simplified elastic-plastic analysis procedure for fatigue evaluation based on Design-by-Analysis (DBA) of nuclear safety class 1 components subjected to severe earthquake loads [10]. Nuclear safety class1 components such as a reactor pressure vessel and a reactor coolant pump casing should perform DBA fatigue assessment [18]. On the other hand, it is not suitable to apply DBA to nuclear safety class1 piping system because the scope of the analysis target is very wide and its shape is relatively simple. Therefore, for the nuclear safety class 1 piping system, the Design-by-Formulae (DBF) procedure is applied to perform fatigue assessment by easily calculating various stress intensities with various formulae [20]. For this reason, it is too difficult and impractical to apply the strain-based simplified elastic-plastic analysis procedure [10] based on DBA to nuclear safety class 1 piping system. Therefore, it is necessary to propose a simplified elastic-plastic analysis procedure based on DBF that can be applied to nuclear safety class1 piping system.

This study developed a strain-based simplified elastic-plastic analysis procedure for DBF fatigue assessment that can be efficiently applied to nuclear safety class 1 piping system subjected to severe seismic loads, and then confirmed applicability and validity by applying the procedure to the actual piping system, pressurizer surge line. The previous simplified elastic-plastic analysis procedures and penalty factors presented in the technical design basis [15] were reviewed, and then the formulae used to structural integrity assessment of nuclear safety class 1 piping system [18] were analyzed. Based on the review and analysis results, and considering stress categories classified in the design formulae for seismic loads, a strain-based simplified elastic-plastic analysis procedure was proposed to perform DBF fatigue assessment of nuclear safety class 1 piping system subjected to severe seismic loads. In order to derive the reference total strain amplitude necessary to confirm validity and conservatism of the proposed analysis procedure, the pressurizer surge line, which is simultaneously subjected to combined loads such as deadweight, internal pressure, thermal load, severe seismic inertia load, and severe seismic anchor motion, was selected as an analysis target, and then detailed dynamic FE elastic-plastic seismic analysis was performed for the target piping system. FE elastic stress analysis for each load was individually implemented for the target piping system to calculate input data of the formulae presented in the proposed analysis procedure. Substituting each analysis result into the formulae of the proposed analysis procedure, total strain amplitude was derived. Fatigue damage of the target piping system subject to the combined load including the severe seismic load was assessed by using the derived total strain amplitude and the strain-based fatigue evaluation procedure of the ASME B&PV Code, Sec. III, Div.1, Subsec.NB, Article NB-3600 [19]. To confirm the validity, conservatism and usefulness of the proposed simplified elastic-plastic analysis procedure, the total strain amplitude calculated by applying the proposed analysis procedure, the fatigue assessment result, and the total calculation time during the elastic analysis were compared with the results derived from the detailed FE elastic-plastic seismic analysis.