Technical Basis of Proposed New Acceptance Standards for Class 1, 2 and 3 Piping

Acceptance Standards in Section XI of the ASME Boiler and Pressure Vessel Code have an important role as the first step in the flaw evaluation procedure. When a flaw size is within the allowable flaw size in the Acceptance Standard, the flaw is acceptable and analytical evaluation is not required. Although ASME Section XI has Acceptance Standards for Class 1 piping in IWB-3500, there are no Acceptance Standards for Class 2 and 3 piping. Furthermore, the development of the current Acceptance Standards for Class 1 piping was based on flaw detectability by ultrasonic inspection and consideration of fracture mechanics. In this paper, the development of proposed new Acceptance Standards for Class 2 and 3 piping, as well as for Class 1 piping, is described. The development methodology is based on a fracture mechanics approach. For Class 1 piping with high fracture toughness, the allowable flaw sizes were determined by limit load solution. For Class 1 piping, the intent was to maintain overall consistency with the current Acceptance Standards. Proposed Acceptance Standards for Class 2 and 3 austenitic piping were also developed by the methodology used to develop the proposed new Acceptance Standards for Class 1 piping. Allowable flaw sizes for both surface flaws and subsurface flaws for preservice and inservice examinations were developed.

Fatigue Strength Reduction Factors for Welded Joints: Another View

The paper distinguishes between FSRFs that are used for two different purposes. One is to serve as a guideline for an initial estimate of the fatigue strength of a welded joint. That is the purpose of the FSRFs that are given in the ASME B&PV Code and various accompanying documents. If that estimate renders the fatigue strength inadequate, an FSRF can be sought that is limited to the joint under consideration. The paper shows how such FSRFs can be determined from fatigue test data. In order to make it possible to read the allowable cycles from the same design fatigue curve as that used for the FSRFs of the guidelines, a Langer curve [defined by equation (2) in the paper] is used to curve fit the data. The appropriate FSRF is obtained by minimizing the standard deviation between this curve and the data. The procedure is illustrated for girth butt-welded pipes. The illustration shows that for the data used in the analysis, a constant FSRF is applicable to less than one million cycles but not to the high-cycle regime.

Experimental Study of Ratcheting for SUS304 Stainless Steel and A1070 Aluminum Alloy

This paper studies the cyclic ratcheting for two materials under multiaxial stress state. The two materials are SUS304 austenitic stainless steel and A1070 pure aluminum. The former material is known as a material that gives strong additional hardening and the latter material shows little additional hardening under nonproportional cyclic loading. The ratcheting behavior under 12 stress-strain waveforms was extensively studied using hollow cylinder specimen. Ratcheting strain depended on the material and stress-strain waveform. Anisotropic ratcheting was found in A1070 but isotropic ratcheting was observed in SUS304 steel.

Field Experience With Code Case N-659: Ultrasonic Examination in Lieu of Radiography

Code Case N-659 Revision 0 was approved in 2002 to allow ultrasonic examination (UT) an alternative to radiography (RT) for nuclear power plant components and transport containers under Section III of the ASME Code. The Nuclear Regulatory Commission has not approved N-659 and its subsequent revisions (currently N-659-2) for general use, but they have been used on a case-by-case basis mainly where logistic problems or component configuration have prevented the use of radiography. Like the parallel Code Case 2235 for non-nuclear applications under Section I and Section VIII, Code Case N-659 requires automated, computerized ultrasonic systems and capability demonstration on a flawed sample as a prerequisite for using UT in lieu of RT. Automated ultrasonic examination can be significantly more expensive than radiography, so a cost-benefit evaluation is a key factor in the decision to use the Code Case. In addition, the flaw sample set has recently become an issue and a topic of negotiation with the NRC for application of the Case. A flaw sample set for a recent radioactive material transport cask fabrication project was successfully negotiated with the NRC. The Code Case N-659 approach has been used effectively to overcome barriers to Code required radiography. Examples are examination of welds in an assembled heat exchanger and in a radioactive material transport cask assembly where internal shielding prevented radiography of the weld. Future development of Code Case N-659 will address sample set considerations and application-specific Code Cases, such as for storage and transport containers, will be developed where NRC concerns have been fully addressed and regulatory approval can be obtained on a generic basis.

Application of Draft Regulatory Guide DG-1144 Guidelines for Environmental Fatigue Evaluation to a BWR Feedwater Piping System

Recently published Draft Regulatory Guide DG-1144 by the NRC provides guidance for use in determining the acceptable fatigue life of ASME pressure boundary components, with consideration of the light water reactor (LWR) environment. The analytical expressions and further details are provided in NUREG/CR-6909. In this paper, the environmental fatigue rules are applied to a BWR feedwater line. The piping material is carbon steel (SA333, Gr. 6) and the feedwater nozzle material is low alloy steel (SA508 Class 2). The transients used in the evaluation are based on the thermal cycle diagram of the piping. The calculated fatigue usage factors including the environmental effects are compared with those obtained using the current ASME Code rules. In both cases the cumulative fatigue usage factors are shown to be less than 1.0.

Sample Environmental Fatigue Calculations Associated With the Review of Draft Regulatory Guide DG-1144

Draft Regulatory Guide DG-1144 “Guidelines for Evaluating Fatigue Analyses Incorporating the Life Reduction of Metal Components Due to the Effects of the Light-Water Reactor Environment for New Reactors”, July 2006 [1], and Associated Basis Draft Document NUREG/CR-6909 (ANL-06/08), “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials”, July 2006 [6] provided methods for addressing environmentally assisted fatigue (EAF) in all new nuclear plant designs. In these documents, a new model was proposed that more accurately accounts for actual plant conditions. The new model includes an EAF correction factor, Fen, which is different from Fen methods previously and currently being considered for adoption into the ASME Code. The Fen methods proposed in DG-1144 are also different than the Fen methods utilized by license renewal applicants, as required by the Generic Aging Lessons Learned (GALL) report [2], as documented in NUREG/CR-5704 [4] (for stainless steel) and NUREG/CR-6583 [3] (for carbon and low alloy steels).

Evaluation of Controlling Transient Ramp Times Using Piping Methodologies When Considering Environmental Fatigue (Fen) Effects

As a part of the 2006 ASME Code support being provided by the Materials Reliability Program (MRP) Fatigue Issue Task Group (ITG), and later the Technical Support Committee (TSC), it is desired to develop a solution that establishes the most severe transient for design purposes when environmental fatigue rules are considered. This problem does not have an obvious answer, since the environmental fatigue multiplier (e.g., Fen) expressions depend on the strain rate during a transient. The strain rate in a thermal transient is dependent on the ramp time of the temperature change. Classically, fatigue analysis has been performed by conservatively considering that temperature changes are instantaneous (e.g., ramp time = zero). This results in maximizing the stress response. But, all other things being equal, Fen effects are minimum at instantaneous changes. Previous work performed by the MRP to support 2005 ASME Code activities has investigated how Fen × Usage varies with ramp time and has concluded that Fen × Usage maximizes at a small but definitely non-zero ramp time [1]. The implications of non-zero transient ramp times are that difficulties arise in both specifying ahead of time and qualifying the component for appropriate ramp times and, at the same time, not creating a situation whereby plant operations are required to proceed at a specified pace to remain design compliant. Therefore, it is desirable to have the qualifying fatigue analyses cover all conceivable ramp times such that the operator neither has to: (a) be limited to a minimum pace, nor (b) confirm through observations that the pace is at least as fast as assumed in the design. This paper summarizes qualifying fatigue analyses that have been performed using piping methodologies to define bounding ramp times for a variety of piping geometry and material configurations. The intent of these analyses is to provide the component designer with a set of parametric tools that can be used to easily design components without the need for iterative fatigue analyses to determine the bounding conditions when Fen rules are considered. In addition, the tool developed to perform the parametric analyses is available for future use by the designer should more specific analyses be required.

Thermal Fatigue Testing and Analysis of a Thick Perforated Ring

Thermal fatigue cracking has been observed for thick perforated spacer rings used as part of a thermal fatigue test loop operating at Bechtel Bettis, Inc. The perforated rings are used for instrumentation access to the fluid flow at the test specimen inlet and outlet, and are subject to alternating hot and cold forced flow, low oxygenated water every three minutes so that rapid changes in water temperature impart a thermal shock event to the inner wall of the rings. Thermal and structural three dimensional elastic and elastic-plastic finite element analyses (FEA) were conducted for the ring and the results used to predict fatigue crack initiation using strain-based fatigue-life algorithms. Predicted cycles-to-crack initiation agreed well with the observed cracking when alternating shear strain intensity analogous to the Tresca stress was used. This analysis qualifies the use of FEA for thermal fatigue assessments of complicated three-dimensional components.

Influence of Load Ratio, Rise Time and Waveform on the Corrosion Fatigue Crack Growth of Austenitic Stainless Steel in a PWR Primary Coolant Environment

The rate of growth of flaws in reactor circuit components by fatigue is usually determined using the reference crack growth curves in Section XI of the ASME Boiler and Pressure Vessel Code. These curves describe the rate of crack propagation per cycle (da/dN) as a function of the applied stress intensity factor range (ΔK). No reference curves for water-wetted defects in austenitic stainless steels are currently available. This paper describes the results of testing of austenitic stainless steel and weld metal in simulated PWR primary coolant over a range of temperatures and mechanical loading conditions. Previous data presented by the authors on wrought stainless steel demonstrated that crack growth rates can be significantly enhanced by the PWR primary environment at temperatures between 150°C and 300°C. The current study extends these data to weld metal and also investigates the impact of other loading waveforms (e.g. trapezoidal loading) on the degree of environmental enhancement. The environmental enhancement increases significantly with reducing loading frequency and decreases with decreasing water temperature. The environmental influence on fatigue is shown to be independent of load ratio over the range R = 0.1 to R = 0.8. The level of enhancement is frequently smaller at very high R ratio (≥0.85) with the enhanced rates of fatigue frequently being unsustained at these high load ratios. There is a strong correlation between the rise time and the level of enhancement of crack growth rate over inert crack growth rates at all temperatures tested. Weld metal has been shown to exhibit similar behavior to wrought material over the whole temperature range studied although the apparent rates of enhancement relative to average inert crack growth rates are lower than found for wrought material. For complex loading waveforms (e.g. trapezoidal loading with hold periods at maximum or minimum load) it is possible predict the level of enhancement on the basis of the test data generated using simpler saw tooth loading regimes.

Confirmation Method of Calibration by Visual Calibration Gauge on Surface Testing

In 2002, Japanese Industrial standard JIS Z 2340 [1] has issued to provide the instruction of new methodology to calibrate the observing conditions of surface testing such as liquid penetrant and magnetic particle testing applying visual calibration gauges. The visual calibration gauges are the transparent plates, on which the line pairs printed to be used to confirm the resolution of visual testing observing view. The concept of line pair has been used to evaluate the resolutions of optical instruments, and the line pair value is a numerical value that shows how many line pairs can be distinguished as separate lines within 1 millimeter. In this paper, the background of the development and the general outlines of visual calibration gauges are introduced at first. And then applications of the gauges and detail process to calibrate the surface testing observing view conditions are also described.