Analysis of the failure performance of internally pressurised piping with surface flaws

Because of frequent failures of the Atucha PWR moderator circuit branch piping, which is made of stainless steel type AISI 347 (DIN 1.4550), studies have been made, involving the application of several fracture mechanics criteria, in order to determine the conditions of leak-before-break (LBB) and the critical crack length of the piping.These studies lead to the conclusion that, for straight pipe of outside diameter 219 mm and 16 mm wall thickness, with a circumferential flaw and with the principal stress being in bending, the LBB criteria are satisfied, the critical crack length being of the order of 400 mm.A better mechanical finishing and heat treatment was suggested in order to improve the resistance to crack initiation.     

Tensile and fracture properties evaluation of PHT system piping material of PHWR

The aim of this paper is to report the tensile and fracture properties of SA333 Gr.6 carbon steel material which is used for the primary heat transport (PHT) system piping of the Indian pressurized heavy water reactor (PHWR). Tensile and J integral tests have been carried out on specimens machined from the base material as well as weldments of actual PHT pipes. The effects of test temperature and specimen orientation on the material properties have been discussed.     

Evaluation of local thinned pressurized elbows

Erosion, corrosion or mechanical damage may cause local thinned areas (LTA) in piping and degrade its integrity. Acceptability criteria for LTA are available for straight pipes. However, for elbows, there are few studies. In this paper, the finite element method (FEM) is used to calculate the collapse load of a local thinned pressurized elbow and to analyse the factors influencing the collapse load. Based on the solution of the collapse load of LTA straight pipe, the stress level in an elbow without defects, and finite element analysis results, an analytical solution of the collapse load of LTA pressurized elbow is presented. A method to assess the acceptability of LTA in an elbow is given and compared with FEM results.     

Improved integrity assessment equations of pipe bends

Pipe bend or elbow is one of the important components in any piping system. Accurate integrity assessment of these pipe bends is very important for reliable operation of all types of plants including nuclear plants. While considerable research has been done in the last few decades to develop accurate integrity assessment procedures of straight pipe with or without cracks, similar efforts were missing for pipe bend or elbow. Reactor Safety Division, Bhabha Atomic Research Centre in collaboration with MPA, University of Stuttgart had embarked upon a comprehensive component integrity test program (CITP) in around 1998 to develop improved integrity assessment methods of piping components in general and elbow in particular. As a part of this program, detailed analytical, numerical and experimental investigations for so many years have generated large number of new equations for improved integrity assessment of elbows. Mainly three aspects of the integrity assessment procedure are focused – development of improved plastic collapse moment equations, proposing new elastic–plastic J-integral and crack opening displacement (COD) estimation schemes to simplify leak-before-break (LBB) analysis and presenting new eta and gamma expressions to evaluate J–R curve from test data. All these newly proposed equations have been validated with the findings of the test data, generated as a part of the CITP. A reasonably good to excellent matching between predictions of the newly proposed equations and test results have been observed in all the cases. The present paper enumerates these research findings in a consolidated yet brief manner.     

Leak-before-break application in US light water reactor balance-of-plant piping

This paper describes the criteria and methodology for a leak-before-break (LBB) program for high energy balance-of-plant (BOP) nuclear piping in the United States. LBB, the analytical demonstration that high toughness piping will leak detectably before catastrophic failure, can be applied to any operational or pre-operational light water reactor plant to minimize pipe rupture hardware and to discount pipe rupture dynamic effects.

The general methodology described herein, encompasses applicable US NRC regulatory requirements and incorporates experience gained in the licensing process of actual LBB programs. First, candidate piping systems must be carefully screened to verify that they are not subject to failure by phenomena that would adversely affect the accurate evaluation of flaws. Next, pipe stresses, material properties, and leak detection capabilities are gathered for the fracture mechanics and fluid mechanics analyses. At the piping locations which have the least favorable combination of material properties and stress, a crack is postulated which is of sufficient size that the resulting leakage will be detected by installed leak detection systems. Finally, LBB is demonstrated if the postulated crack remains stable even if a seismic event takes place before the crack is discovered and repaired. An LBB example is presented in this paper for a generic pressurizer surge line, and reflects the consideration of flow stratification on LBB analyses.     

A computer model for probabilistic leak-rate analysis of nuclear piping and piping welds

This paper describes the development of a computer code entitled PSQUIRT for probabilistic evaluations of leak rate in nuclear piping. It is based on (1) the Henry-Fauske model of two-phase flow for thermal-hydraulic analysis and (2) an estimation model for elastic-plastic fracture-mechanics analysis. In both analyses, uncertainties arise due to the incomplete knowledge of the crackmorphology variables and statistical scatter of the pipe material properties. The relevant parameters required to conduct these analyses were modeled as random variables. Henceforth, the above thermal-hydraulic and fracture-mechanics models were put in a probabilistic format to allow statistical variability of input and determination of their effects in pipe fracture and leak-before-break (LBB) evaluations. A standard Monte Carlo simulation technique was used to perform the probabilistic analysis. Numerical examples are presented to illustrate the capabilities of the PSQUIRT code. Probabilistic analyses were performed by PSQUIRT for a stainless steel and a carbon steel pipe. Histograms were developed for leakage rate and flaw size in these pipes for LBB applications. The results suggest that the variability of leak rate can be significant due to statistical scatter of crack-morphology parameters. Using these histograms, the subsequent fracture stability of a leaking crack, actural or hypothetical, can be evaluated by either a deterministic or a probabilistic method.     

Fracture investigations on piping system having large through-wall circumferential crack

In level-3 Leak-Before-Break (LBB) analysis, stability of a postulated through-wall circumferential crack is demonstrated by simplified fracture mechanics calculations. Detailed experimental studies, conducted by the authors, have revealed that the conventional assessment procedure used to demonstrate LBB is too conservative. There is a large factor of safety due to system indeterminacy. It was observed that the critical load of a cracked piping system (with even a large through-wall circumferential crack of about 120°) is of the order of 75–90% of the collapse load of the uncracked piping system. Reduction in load carrying capacity is even less for a piping system having an off-centre crack. This article discusses the above-mentioned aspects in detail. Detailed 3-D elastic-plastic finite element analyses of some of these tests were performed. The suitability of these numerical results to predict crack initiation load in light of the experimental data is discussed.     

Development of USNRC standard review plan 3.6.3 for leak-before-break applications to nuclear power plants

In the United States, it is now permissible to eliminate the dynamic effects of postulated high energy pipe ruptures from the design basis of nuclear power plants using ‘Leak-Before-Break’ (LBB) technology. To provide review guidance for the implementation of LBB, a new Standard Review Plan (SRP) 3.6.3 was issued for public comment. Based upon public comments received and advances in fracture mechanics application, further development of SRP 3.6.3 is in progress.

SRP 3.6.3 will outline the review procedures and acceptance criteria for LBB licensing applications. A deterministic fracture mechanics evaluation accounting for material toughness will be required. Margins on load, crack size, and leakage will be specified and the load combination methods and leakage detection sensitivity will be described. Piping particularly susceptible to failure from potential degradation mechanisms will be excluded from the application of LBB. The design basis of containment, emergency core cooling systems, and environmental qualification of equipment in the context of LBB applicability will be clarified.     

Ductile fracture properties for assessing leak-before-break issues in ferritic weldments

A Leak-Before-Break (LBB) approach is being used by Ontario Hydro's Darlington nuclear generating station as a design alternative to pipe rupture restraint hardware on the large diameter piping of the primary heat transport system. The J-resistance curves of four different ferritic weldments, fabricated by either the submerged arc weld (SAW) or shielded metal arc weld (SMAW) process, were determined as part of this program.

Results indicated that the as-welded and post-weld heat treated (PWHT) welds were susceptible to varying degrees of static or dynamic strain aging at 200 and 250°C. Dynamic strain aging effects were most significant for as-welded welds, as evidenced by sudden load drops on the load-displacement curves and ductile crack jumping. The effect of loading displacement rate and PWHT on toughness was assessed and related to the weld's tensile properties and susceptibility to dynamic strain aging. Implications of strain aging to LBB assessments are discussed.     

Status of stainless steel pipe inspection in the US

This paper summarizes recent progress made in the inspection of stainless steel piping. Topics covered include the training of personnel for both detection and sizing of intergranular stress corrosion cracking (IGSCC) found in Boiling Water Reactor piping. The inspection of the weld overlay used to repair pipe joints with IGSCC and inspection of centrifugally cast stainless steel pipe is also addressed. A summary of advanced systems that have satisfied the NRC performance demonstration requirements is also given.     

The interaction of pressure, in-plane moment and torque loadings on piping elbows

This study concerns the load interaction behaviour of 90° smooth piping elbows with circular cross-section and long straight tangent pipes. The finite element method is used for stress analysis of elbows having a wide range of bend and pipe factors. The main aim of the study is to establish the first yield interaction behaviour when an elbow is subjected to a combination loading of in-plane bending, torsion and internal pressure. The study shows that load interaction is influenced by pipe factor, bend radius and load coupling effect, with thinner elbows being affected to a larger degree.     

Leak-before-break verification test and evaluations of crack growth and fracture criterion for carbon steel piping

A proving test on the integrity of carbon steel piping in light water reactors (LWRs) was planned by the Nuclear Power Engineering Test Center (NUPEC) as a four-year verification test program; it was completed at the end of March 1989. The objective of this proving test was to demonstrate the validity of the Leak-Before-Break (LBB) concept for high quality carbon steel piping under actual plant conditions.

This paper briefly describes the results of material property tests, fracture behavior tests, LBB verification tests, numerical analyses of pipe fracture behavior and evaluation of flaw growth and fracture criterion. From these results, LBB has been verified and a fracture criterion has been developed for carbon steel piping in LWRs.     

Net-section limit moments and approximate J estimates for circumferential cracks at the interface between elbows and pipes

This paper firstly presents net-section limit moments for circumferential through-wall and part-through surface cracks at the interface between elbows and attached straight pipes under in-plane bending. Closed-form solutions are proposed based on fitting results from small strain FE limit analyses using elastic–perfectly plastic materials. Net-section limit moments for circumferential cracks at the interface between elbows and attached straight pipes are found to be close to those for cracks in the centre of elbows, implying that the location of the circumferential crack within an elbow has a minimal effect on the net-section limit moment. Accordingly it is also found that the assumption that the crack locates in a straight pipe could significantly overestimate the net-section limit load (and thus maximum load-carrying capacity) of the cracked component. Based on the proposed net-section limit moment, a method to estimate elastic–plastic J based on the reference stress approach is proposed for circumferential cracks at the interface between elbows and attached straight pipes under in-plane bending.     

Allowable sizes of axial flaws in pressurized pipes made of moderate toughness materials

Failure stresses for axially part-through flawed pipes made of moderately tough materials are predicted by several fracture mechanics. However, allowable flaw sizes using these fracture mechanics cannot be simply described because there are many effective parameters such as pipe diameter, wall thickness, material properties, etc. To establish codes and standards to evaluate flaws for piping of light water reactors, we determine unified allowable sizes for axial flaws in pipes subjected to internal pressure from J-integral based fracture mechanics. The allowable sizes are simply tabulated using a single parameter which consists of pipe geometry and material properties.     

Leak-before-break: An integrated approach for high energy piping

An integrated approach that involves system design, thermal hydraulics, materials, and fracture mechanics analyses to assure that pipe failure is highly unlikely is described. This approach is based on a leak-before-break (LBB) premise and includes through-wall flaw detectable leakage, through-wall flaw stability, and part-through-wall flaw fatigue crack propagation calculations. A successful application of LBB can reduce the amount of excessive pipe rupture restraint hardware. Assuring LBB not only reduces initial construction, future maintenance, and radiation exposure costs, but the overall safety and integrity of the plant are improved. This last benefit comes about from gaining additional insight into the piping systems and their capabilities. Details of the LBB methodology are presented here with specific examples for two pressurized water reactor lines (one inside containment fabricated of stainless steel, and the other outside containment made from ferritic steel). The application of this approach at Beaver Valley Power Station—Unit 2 indicates that pipe rupture hardware is not necessary for stainless steel lines inside containment as small as 6-in (152 mm) nominal pipe size that have passed screening criteria designed to eliminate potential problem systems (such as the feedwater system). Similarly, some ferritic steel lines as small as 3-in (76 mm) diameter (outside containment) can qualify for pipe rupture hardware elimination.     

Development of criteria for protection against pipe breaks in LWR plants

A proving test for the structural integrity of safety-related carbon steel piping components in light water reactor plants was conducted in NUPEC as a four-year project, in which the applicability of the Leak-Before-Break (LBB) concept to protect against a postulated pipe break was reviewed in parallel with the clarification of fracture behavior. The comprehensive review of LBB applications consists of applicable piping systems, premise for evaluations, procedure and evaluation findings. The review concluded that present practice for design, fabrication, installation and operation can ensure structural integrity and moreover postulated that instantaneous pipe break as a basis for structural design is unrealistic if certain conditions are met. Fatigue is the only failure mechanism to be considered to affect the piping system.     

Evaluation of leak-before-break assessment methodology for pipes with a circumferential through-wall crack. Part II: J-integral estimation

Evaluation of the J-integral plays a central part in evaluation of the critical crack length for unstable fracture for piping systems. Simplified evaluation methods for the J-integral for a circumferential through-wall crack in pipes subjected to axial and bending loading or their combination is reviewed in this paper. Use of the LBB.ENG2 method and a similar approach based on the η-factor concept were found to result in significant underestimation of the J-integral for small and medium crack angles. On the other hand, the reference stress method based on the solutions for stress intensity factor and limit load recommended in the companion paper (Part I) provides solutions which agree well with the available non-linear finite-element solutions and can be utilized as a powerful tool for J-integral evaluation for arbitrary materials, not restricted to simple power-law hardening.     

Crack resistance of austenitic pipes with circumferential through-wall cracks

For a monotonously increasing load the correct evaluation of the crack resistance properties of a structure is essential for safety analyses. Considerable attention has been given to the through-wall case, this is generally believed to be the controlling case with regard to complete pipe failure. The maximum load conditions for circumferential crack growth in pipes under displacement-controlled loading has been determined. The need for crack resistance curves, measured on circumferentially through-wall cracked straight pipes of austenitic stainless steel 316 L under bending, is emphasized by the limitation in the data range from small specimens and by the differences in the fracture mechanics procedures. To address these issues and to improve calculational methods a joint fracture mechanics programme is being performed by Electricité de France, Novatome and Siemens-Interatom. The working programme contains experimental and theoretical investigations on the applicability of small-specimen data to real structures.     

Research on the leak-rate characteristics of leak-before-break (LBB) in pressurized water reactor (PWR)

Leak rate calculation is the foundation of Leak-Before-Break (LBB) technology application in Pressurized Water Reactor (PWR). In the paper, a leak rate Mathcad calculation code with different critical flow mathematical assumptions was completed. The code calculation results were contrasted from the published experimental data. The compared results show that the code calculation results are coincident with experimental data. However, the theoretical results are greater than experimental data with different crack L/D, stagnation pressures and subcooled temperatures in case of ignoring friction effect. While the crack friction effect is considered, the calculated results are well in accordance with the experimental data. Also, the different pressure drops are obtained and studied with variations of important parameters in detail. It demonstrates that the friction effect is a significant factor and must be considered in the crack leak rate calculation. The Mathcad code can be used to calculate the crack leak rate and provide application foundation of LBB in PWR pipe system.     

Determination of the elastic-plastic fracture mechanics Z-factor for alloy 182 weld metal flaws

One of the ways that the ASME Section XI code incorporates elastic-plastic fracture mechanics (EPFM) in the Section XI Appendix C flaw evaluation procedures for circumferential cracks is through a parameter called Z-factor. This parameter allows the simpler limit-load (or Net-Section-Collapse) solutions to be used with a multiplier from EPFM analyses. This paper shows how 3-D finite element (FE) analyses were employed to investigate the sensitivity of the crack-driving force as a function of crack location (i.e., crack in the center of weld, or closer to the stainless or low alloy steel sides) in an Alloy 182 dissimilar metal weld (DMW), and how an appropriate (or equivalent) stress-strain curve was determined for use in the J-estimation schemes. The J-estimation schemes are then used to cover a wider range of variables, i.e., pipe diameters, cracks lengths, and also incorporate crack growth by ductile tearing. The Z-factor equations as a function of pipe diameter were calculated using the LBB.ENG2 J-estimation scheme along with the most conservative equivalent stress-strain curve from the FE analyses. The proposed Z-factor approach was then validated against an Alloy 182 DMW full-scale pipe test that had a circumferential through-wall crack in the fusion line. The predicted EPFM maximum load showed excellent agreement with the experimental result. Furthermore, it was shown that the proposed Z-factor equation is not sensitive to the location of the crack.