Development of a Fatigue Design Curve for Austenitic Stainless Steels in LWR Environments: A Review

The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components and specifies fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Existing fatigue strain–vs.–life (ε–N) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. This paper reviews the existing fatigue ε–N data for austenitic stainless steels in LWR coolant environments. The effects of key material, loading, and environmental parameters, such as steel type, strain amplitude, strain rate, temperature, dissolved oxygen level in water, and flow rate, on the fatigue lives of these steels are summarized. Statistical models are presented for estimating the fatigue ε–N curves for austenitic stainless steels as a function of the material, loading, and environmental parameters. Two methods for incorporating environmental effects into the ASME Code fatigue evaluations are presented. Data available in the literature have been reviewed to evaluate the conservatism in the existing ASME Code fatigue design curves.

Making Sense of Section XI Visual Examination Requirements

This paper will review the development of the visual examination requirements of Section XI of the ASME Boiler and Pressure Vessel Code. The original visual requirements were ‘one visual exam fits all’ – to detect physical damage, physical displacement, and evidence of leakage. Resolution requirements were those of Section V of the Code. These requirements evolved over the next 20 years to become several specific types of requirements, each with specific resolution, illumination, and proximity restraints. Review indicated that these separate visual rules are now converging. This paper provides recommendations for revisions to Section XI that consolidate and simplify these requirements.

CF/SCC Interaction on Structural Materials for LWR in High Temperature Water

A test apparatus was developed to study the interaction between corrosion fatigue (CF) and stress corrosion cracking (SCC) in high-temperature water simulated boiling water reactor environment. Tests were conducted using 1/2T-CT samples of both low alloy and sensitized stainless steels under 3 different types of loading at 0.2–8 ppm in dissolved oxygen concentrations at 563 K in water. Type 1 was a normal cyclic loading test of constant amplitude, Type 2 a monotonic constant loading rate test, and Type 3 a combination of Type 1 + Type 2 loading modes. In the low alloy steel, no striking interaction was observed between CF and SCC, whereas in the case of Type 3 loading condition crack growth rates of the sensitized stainless steel were as much as 3 times higher than those for Type 1 + Type 2. The mechanism of the CF and SCC interaction is discussed.

Background to Recent Revision of the Section III Seismic Piping Rules

The new rules for seismic piping design in Section III that were developed and included in the requirements in 1994 Addenda of the ASME Boiler and Pressure Vessel Code (B&PV Code) generated considerable discussion within the industry and from the United States Nuclear Regulatory Commission, (USNRC). The USNRC initiated a review of the results of the previous EPRI/NRC experimental program and the Japanese industry started its own experimental program. To accommodate and address developments resulting from these efforts, the ASME, B&PV Code established a Special Working Group (SWG) to continue the review and study of the questions and information generated. This paper reports on the efforts of this SWG which resulted in refinements of the revised rules. These refinements have been accepted for inclusion in Section III of the ASME, B&PV Code.

The Use of the ASME PRA Standard in Pressure Vessel and Piping Work

After several years of intense labor by many industry people, ASME is about to issue its newly approved PRA standard. This standard is for probabilistic risk assessment (PRA) for nuclear power plant applications. It is not a standard on how to build a PRA model; although, that could be inferred from the standard’s technical requirements. This Standard sets forth requirements for PRAs used to support risk-informed decisions related to design, licensing, procurement, construction, operation, and maintenance. It also prescribes a method for applying these requirements depending the degree to which risk information is needed and credited.

Simulation of Excessive Deformation of Piping Due to Seismic and Weight Loads

The ASME Code for seismic design of piping system was revised in 1994 based on evaluation of pipe component test results submitted by the Piping and Fitting Dynamic Reliability Program (PFDRP). PFDRP indicated piping component failure to result in most cases from fatigue with ratchet. Excessive progressive deformation was noted by this program to ultimately occur in test #37, #39 and #40 piping models. This mode of failure was considered due to superposition of bending moment arising from vertical load produced by weight and horizontal seismic inertia force. To clarify the conditions leading to such failure, elastic-plastic dynamic analysis using the general-purpose FEM Code was carried out on the test#37 piping model of PFDRP. The analytical model and method were verified as effective means of study by comparison of analytical results with those obtained experimentally. The parameter determinations were made under condition of variation in dead weight stress and excitation and its dominant frequency. 1994 ASME seismic stress limits were shown effective for preventing excessive progressive deformation in tests #37, #39 and #40.

The European Approach to Design by Analysis

This paper discusses the background of a new approach to check the admissibility of pressure vessels via the Direct Route (DR) to Design by Analysis (DBA), and the various design checks required, in general and in detail. Emphasis is on the various tools and procedures used in applications, and on typical applications.

Finite Element Analysis and Design of a Horizontal Tank on Saddle Supports

The objective of this paper is to design and analyze a horizontal tank on saddle supports. The horizontal vessel is to store various chemicals used in today’s industry. The over all dimensions of the horizontal vessel are determined from the capacity of the stored chemicals. These dimensions are first determined. The design function is performed using the ASME Code Sec VIII Div 1. The horizontal tank design is broken up into (a) shell design, (b) two elliptical heads and (c) two saddle supports. The designed dimensions are used to recalculate the stresses for the horizontal vessel. The dimensioned horizontal vessel with saddle supports and the saddle support structure is modeled using STAAD III finite element software. The stresses from the finite element software are compared with the stresses obtained from calculated stresses by ASME Code Sec VIII Div 1 and L. P. Zick’s analysis printed in 1951. The difference in the stress value is explained. This paper’s main objective is to compare the code design to the finite element analysis. The design is found to be safe for the specific configuration considered.

Evaluation of Thermal Stress Ratchet in Plastic FEA

Alternative stress evaluation criteria suitable for Finite Element Analysis (FEA) proposed by Okamoto et al. [1] have been studied by the Committee on Three Dimensional Finite Element Stress Evaluation (C-TDF) in Japan. Thermal stress ratchet criteria in plastic FEA are now under consideration. Two criteria are proposed: evaluating variations in plastic strain increments and evaluating variations in the width of elastic core. To verify the validity of these criteria, calculations were performed for several typical models in C-TDF [2]. This paper shows the results of a simple cylinder model. Cyclic plastic analyses were performed applying sustained internal pressure and alternating linear temperature distribution through the wall. Analyses were performed with various load ranges to evaluate the precise ratchet limit and its behavior across the limit. Both pressure and thermal stress were given parameters. In the analyses, Elastic-Perfectly-Plastic (EPP) material was used and also strain hardening material for comparison. The ratchet limit in the Code [3] is based on Miller’s theoretical analysis [4] for a cylinder assuming a uni-axial stress state, whereas real vessels are in multi-axial stress state. By our calculations, we also examined the ratchet limit in real vessels. The results show that for the cylinder in a multi-axial stress state, the ratchet limit rises 1.2 times the ratchet limit by the Code. The evaluation results show that variations in equivalent plastic strain increments can be used for ratchet criterion and ratcheting can be assessed by confirming the presence of elastic core in the second cycle.

A Parametric Study of the Seismic Margins of Piping Components

This paper presents an analytical study of frequency effects on seismic margins of piping components. The study is based on response data obtained as part of a joint Electric Power Research Institute (EPRI) and NRC Piping and Fitting Dynamic Reliability (PFDR) Program. The majority of the PFDR component tests were performed using a narrow-banded earthquake excitation input that was tuned to have a frequency slightly lower than the fundamental frequency of the test components. However, the natural frequency of a piping system in an actual plant may vary over a wide range. Therefore the seismic margins at off-resonance conditions are of importance. Two seismic margin definitions are examined. The primary objective of this study was to extrapolate the PFDR test margins to other frequency regions and to investigate the effects of various parameter changes on the margins.