Probabilistic Risk Assessment of Aging Layered Pressure Vessels

The National Aeronautics and Space Administration (NASA) operates approximately 300 aging layered pressure vessels that were designed and manufactured prior to ASME Boiler and Pressure Vessel (B&PV) code requirements. In order to make decisions regarding the continued fitness-for-service of these non-code carbon steel vessels, it is necessary to perform a relative risk of failure assessment for each vessel. However, risk assessment of these vessels is confounded by uncertainties and variabilities related to the use of proprietary materials in fabrication, missing construction records, geometric discontinuities, weld residual stresses, and complex service stress gradients in and around the welds. Therefore, a probabilistic framework that can capture these uncertainties and variabilities has been developed to assess the fracture risk of flaws in regions of interest, such as longitudinal and circumferential welds, using the NESSUS® probabilistic modeling software and NASGRO® fracture mechanics software. In this study, the probabilistic framework was used to predict variability in the stress intensity factor associated with different reference flaws located in the head-to-shell circumferential welds of a 4-layer and 14-layer pressure vessel. The probabilistic studies predict variability in flaw behavior and the important uncertain parameters for each reference flaw location.

Technical Basis for ASME Code Section XI Nonmandatory Appendix C Update

The ASME Boiler and Pressure Vessel (B&PV) Code Section XI Appendix C provides analytical procedures, criteria, and evaluation methodologies used to determine acceptability for continued service for a specified evaluation time period of flawed pipe. However, Appendix C applicability to subsurface flaws and flaws located on external pipe surfaces is unclear. Appendix C as currently written suggests surface flaws are (only) on the inner pipe diameter. It is recognized that flaw solutions specific to different combinations of the type of flaw, location on component, and failure mode may not be currently available. There are also inconsistencies in the equations for determining fracture toughness for ferritic piping between circumferential and axial-oriented flaws, and the allowable applied hoop stress definitions. Furthermore, there is recent work on several topics in Appendix C that necessitate updating Appendix C. Topics include stress intensity factor (SIF) solutions for circumferential and axial through-wall flaws in cylinders, and the method of combination of bending moments and torsion for elastic-plastic fracture mode and limit load analyses when the torsion stress does not exceed 0.2 times the flow stress. This paper summarizes the proposed ASME Code Section XI Appendix C revisions that will be incorporated in the 2017 edition of the Code. The impact of revising stress intensity factor solutions for circumferential and axial through-wall cracks in cylinders is also presented. In addition to technical changes, several errata are also suggested to be corrected.

Phased Array Ultrasonic Testing of Thin DP Steel Electron Beam Weld Joints

Duplex steels are an important group of materials which are generally used for applications where resistance to corrosion, or high strength and creep resistance at elevated temperatures, are required. They are used extensively in nuclear plant. Although ultrasonic inspection methods have been routinely used in industry for some three decades, it is well known that cast or welded austenitic components can be difficult, or even impossible, to examine ultrasonically. Development of ultrasonic techniques is therefore in progress in several countries to provide improvements which are being sought on safety and economic grounds.The aim of this paper is a brief description of the relevant metallurgical characteristics given before a consideration of the physical properties of the weld metal and the current theoretical models used to describe ultrasound propagation in it. The paper deals with the steps taken to improve the capabilities of ultrasonic inspection and includes a discussion of the problems of flaw location and sizing.

Closed-Form Relations for Stress Intensity Factor Influence Coefficients for Axial Outside Surface Flaws in Cylinders for Appendix A of ASME Section XI

Linear elastic fracture mechanics based flaw evaluation procedures in Section XI of the ASME Boiler and Pressure Vessel Code require calculation of the stress intensity factor. Article A-3000 of Appendix A in ASME Section XI prescribes a method to calculate the stress intensity factor for a surface or subsurface flaw by making use of the flaw location stress distribution obtained in the absence of the flaw. The 2015 Edition of ASME Section XI implemented a number of significant improvements in Article A-3000, including closed-form equations for calculating stress intensity factor influence coefficients for circumferential flaws on the inside surface of cylinders. Closed-form equations for stress intensity factor influence coefficients for axial flaws on the inside surface of cylinders have also been developed. Ongoing improvement efforts for Article A-3000 include development of closed-form relations for the stress intensity factor coefficients for flaws on the outside surface of cylinders. The development of closed-form relations for stress intensity factor coefficients for axial flaws on the outside surface of cylinders is described in this paper.

Technical Basis for Application of Weight Function Method for Calculation of Stress Intensity Factor for Surface Flaws Proposed for ASME Section XI Appendix A

Analytical evaluation procedures for determining the acceptability of flaws detected during in-service inspection of nuclear power plant components are provided in Section XI of the ASME Boiler and Pressure Vessel Code. Linear elastic fracture mechanics based evaluation procedures in ASME Section XI require calculation of the stress intensity factor. A method for calculating the stress intensity factor is provided in Appendix A of ASME Section XI. This method consists of a two-step process. In the first step, the stress distribution, as calculated in the absence of the flaw, is obtained at the flaw location. For a surface flaw, the stress distribution at the flaw location is then fitted to a third-order polynomial equation. In the second step, the fitted polynomial representation of the stress distribution is used with standardized influence coefficients to calculate the stress intensity factor. An alternate method for calculation of the stress intensity factor for a surface flaw that makes explicit use of the universal weight function method and does not require a polynomial fit to the actual stress distribution is proposed in this paper for implementation into Appendix A of ASME Section XI. Universal weight function coefficients are determined from standardized influence coefficients through closed-form equations. Closed-form equations for calculation of the stress intensity factor are provided. The technical basis and verification for this alternate method for calculation of the stress intensity factor are described in this paper.

Technical Basis for Proposed Weight Function Method for Calculation of Stress Intensity Factor for Surface Flaws in ASME Section XI Appendix A

Analytical evaluation procedures for determining the acceptability of flaws detected during in-service inspection of nuclear power plant components are provided in Section XI of the ASME Boiler and Pressure Vessel Code. Linear elastic fracture mechanics based evaluation procedures in ASME Section XI require calculation of the stress intensity factor. A method for calculating the stress intensity factor is provided in Appendix A of ASME Section XI. This method consists of a two-step process. In the first step, the stress distribution, as calculated in the absence of the flaw, is obtained at the flaw location. For a surface flaw, the stress distribution at the flaw location is then fitted to a third-order polynomial equation. In the second step, the fitted polynomial representation of the stress distribution is used with standardized influence coefficients to calculate the stress intensity factor. An alternate method for calculation of the stress intensity factor for a surface flaw that makes explicit use of the Universal Weight Function Method and does not require a polynomial fit to the actual stress distribution is proposed in this paper for implementation into Appendix A of ASME Section XI. Universal Weight Function coefficients are determined from standardized influence coefficients through closed-form equations. Closed-form equations for calculation of the stress intensity factor are provided. The technical basis and verification for this alternate method for calculation of the stress intensity factor are described in this paper.

Statistical Assessment of Effect of Hydride Non-Ratcheting Conditions on Delayed Hydride Cracking Initiation in CANDU Pressure Tubes

In-service flaws in CANDU Zr-Nb pressure tubes are evaluated for initiation of delayed hydride cracking (DHC) according to Canadian nuclear standards. A flaw may be under hydride ratcheting conditions or under hydride non-ratcheting conditions, depending on the hydrogen concentration at the flaw location. Under hydride ratcheting conditions (higher hydrogen concentrations), flaw-tip hydrides do not completely dissolve at normal operating temperature and incrementally accumulate with each reactor heatup/cooldown cycle. On the contrary, under hydride non-ratcheting conditions (lower hydrogen concentrations), the flaw-tip hydrides completely dissolve at normal operating temperature, so that no incremental hydride accumulation occurs with each reactor heatup/cooldown cycle. Experiments have demonstrated that the resistance to DHC initiation under hydride non-ratcheting conditions is substantially higher than that under hydride ratcheting conditions. A single-valued lower-bound hydride non-ratcheting factor had been proposed to conservatively quantify this behaviour. However, such a factor does not account for the experimentally observed variation in the relative increase in the threshold stress for DHC initiation under hydride non-ratcheting conditions with respect to that under hydride ratcheting conditions. In this paper, this problem has been addressed statistically by evaluating the probability of DHC initiation at the in-service flaws. The predicted probability of DHC initiation has been found to increase with increasing the applied stress and the stress concentration at the flaw tip and to decrease with increasing the threshold stress intensity factor for DHC initiation. All these trends are consistent with our fundamental understanding of the DHC initiation phenomenon. Also, the predicted probability of DHC initiation is higher for hydride ratcheting conditions than for hydride non-ratcheting conditions. On the basis of this analysis, the hydride non-ratcheting factor has been represented as a distributed parameter, suitable for use in the probabilistic assessments of delayed hydride cracking initiation.