Leak-before-break Research Articles

Combination of two-fluid model and delayed equilibrium model for the critical flow in a slit

Accurately predicting the mass flux, pressure profile, and velocity profile of the critical flow in a slit is essential for analyzing the breaking process of the liquid phase and calculating the aerosol source term for leak-before-break (LBB) monitoring and Loss of Coolant Accident (LOCA) risk analysis. A new critical flow model combining Two-fluid Model (TFM) and Delayed Equilibrium Model (DEM) is built to get accurate profiles while avoiding the same phase velocity in DEM and the arbitrary critical flow criterion in TFM. The new model is verified using past experiments of the critical flow in a slit. It proves to be accurate in mass flux but not in critical pressure, with maximum relative errors of around 25% in mass flux and around 80% in critical pressure. The new model is optimized for higher accuracy in critical pressure. The empirical equation of saturated phase mass flow rate fraction gradient is optimized by conducting approximate pressure profile calculation and regression analysis. The maximum relative error decreases little while the ratio of critical pressure relative errors lying in the range of ±40% increases after optimization. In contrast, the difference in the average abstract relative error of pressure between original TFM-DEM and DEM is much larger, for the maximum relative error of mass flux and critical pressure are around 25% and 110%. The comparison between the original and optimized TFM-DEM proves that the new critical flow model is accurate in mass flux and can be optimized to raise pressure calculation accuracy. The comparison between the original TFM-DEM and DEM proves that the phase velocity difference is the major source of accuracy improvement in the pressure profile.

Structural integrity assessment of CANDU pressure tubes using Sobol indices for global sensitivity analysis

CANDU11CANDU is a registered trademark of Atomic Energy of Canada Limited. reactors are a channel-type design where up to 480 zirconium alloy pressure tubes (PTs) act as the reactor pressure boundary. Fitness-for-service standards such as CSA N285.8 (CSA Group, 2015) have been established to prescribe the requirements of pressure tube evaluations to maintain their integrity throughout their lifetime. Probabilistic leak-before-break (LBB) assessments can be used in evaluations to demonstrate that the likelihood of a flaw growing past the stability point while undetected (or before a safe shutdown can be achieved) is below regulatory limits. The assessments typically require a ranking of model parameters based on their influence. The ranking process traditionally uses expert judgment and local sensitivity analysis methods. There has been recent interest in implementing global methods for LBB assessments as they can manage large numbers of variables and extensive sampling spaces. However, their adoption is hindered by the computational cost of the models involved.The current study proposes a method for performing global sensitivity parameter rankings during probabilistic LBB assessments. The study is the continuation of the CANDU nuclear power plant (NPP) pressure tube (PT) case study in El Bouzidi et al., (2024) where a probabilistic approach was implemented in RAVEN (Rabiti and Alfonsi, 2019) to estimate whether the probability of unstable crack growth in PTs is within acceptable limits. This follow-up work proposes surrogate modeling using a high-dimensional model representation to derive Sobol indices and establish a parameter influence ranking. The methodology is demonstrated in a Monte Carlo simulation campaign of a pressure tube inlet rolled joint. The implemented approach successfully identified and quantified critical parameters of the model while efficiently ranking and quantifying the interactions between input variables.

Effects of microalloying element addition on mechanical properties of SA508 Gr.1A low-alloy steels

SA508 Gr.1A low-alloy steel is being considered as a candidate material for main steam line piping in nuclear power plants. Therefore, improving its strength and toughness is essential for enhancing the leak-before-break (LBB) margin. In this study, six types of model alloys were fabricated by varying the contents of microalloying elements (C, Cu, B, Ti, and Nb) to enhance the mechanical properties of the specimens. The addition of a few ppm of B led to the formation of a fine-grained low-temperature transformation microstructure, resulting in the highest strength among the model alloys. However, the addition of Nb and Ti increased the formation of coarse ferrite, significantly decreasing the strength of the alloys. Reducing the C content while adding a small amount of B simultaneously maintained strength and enhanced toughness. Furthermore, the LBB margins of model alloys and commercial steel were evaluated to validate the influence of varying microalloying content. The model alloys exhibited a substantial increase in yield strength and fracture resistance, resulting in a more than 10% increase in the LBB margin. Notably, the LBB margin of the alloy with 15 ppm B was 1.39, approximately 25% higher than that of commercial steels.

Fracture investigation at 300oC on carbon-steel and austenitic stainless-steel pipes of NPPs under cyclic loads

The Leak-Before-Break (LBB) of high enthalpy piping of Indian NPPs require fracture stability demonstration of pipelines with postulated cracks under design basis earthquake loads. Earlier large number of cyclic fracture and corresponding monotonic fracture tests on large sized pipe components were conducted at room temperature. In current program, monotonic/cyclic fracture experiments are extended to reactor operating temperature (300 oC). The aim of this study is to demonstrate the structural integrity of SA333Gr6 piping and Stainless Steel (SS) 304LN welds under monotonic/cyclic loading which simulates extreme earthquake events and operating temperatures. Total six pipes/pipe welds of carbon steel (8-inch NB ~219 mm outer diameter and 19 mm thick) and SS304LN (12-inch NB~324 mm and outer diameter ~25 mm thick) with through-wall cracks have been tested under four-point bending. Two monotonic fracture tests have been conducted at 300 oC under monotonic loadings. Different experimental results like LLD (Load Line Displacement), Total Load, CMOD (crack mouth opening displacement), Crack Growth values have been obtained. Based on this data the load carrying capacity are obtained for both 8-inch and 12-inch cracked pipes. The cyclic fracture tests have also been conducted at 300 oC under fully reversing cyclic loading on four pipes. Three 8-inch SA333Gr6 pipes have been tested under different magnitude of peak load values. Similarly, one 12-inch SS pipe is also tested under cyclic loading. For carbon steel pipes, the reduction in load capacity is slightly higher under cyclic loadings at 300 oC compared to that at room temperature. However, for stainless steel pipes the load reduction factor at 300 oC is in good agreement with that at room temperature.

Leak-before-Break (LBB)-Based Safety Verification of Reverse Cyclic Loading for 316L Stainless Steel: A Study Using Flat ESG Specimens

The leak-before-break design concept is based on J-R curves, which are obtained by J-R tests on various types of specimens and are known to be dependent on the cyclic load history. The J-R curves of standard specimens suggested by the American Society for Testing Materials are determined based on quasi-static tensile loading. However, seismic loading induces a reverse cyclic loading that alternately applies a tensile and a compressive load to nuclear plant piping. Therefore, it is very important to obtain the fracture toughness characteristics under reverse cyclic loading for the integrity estimation of nuclear plant piping. The objective of this paper is to study the effects of reverse cyclic loading on the fracture toughness characteristics of SA312 TP316L stainless steel, which is a nuclear plant piping material. J-R tests on a flat, equivalent stress gradient specimen with varying incremental displacement were carried out. The test results were reviewed by comparing the J-R test results under quasi-static loading. In addition, the safety margin of the nuclear plant piping was evaluated using a crack driving force diagram method. For the SA312 TP316L stainless steel, the results showed that the J-R curves were decreased with a decrease in the incremental displacement. When the incremental displacement was set to 0.25 mm, the unstable crack growth point value was about 73.0% of those for the quasi-static loading conditions.

Hydrogen-related degradation of fracture properties and altered fracture behavior of Cr–Mo steel used in hydrogen stationary vessels

This study comprehensively analyzes the effect of high-pressure hydrogen gas on the elastic-plastic fracture toughness, fatigue crack growth rate (FCGR) properties, and the corresponding fracture behavior of Cr–Mo steel in hydrogen stationary vessels. The fracture toughness value under 99 MPa hydrogen pressure significantly reduced by approximately one-third of the value tested in ambient air while FCGRs tested under hydrogen environments at a specified ΔK exhibited an approximate two-order magnitude increase compared to those observed in ambient air. The hydrogen-assisted fracture mechanism, encompassing crack initiation, propagation, and crack arrested, was meticulously examined. The distinct variations in fracture modes with changes in the ΔK range can be attributed to the synergistic interaction of HELP and HEDE mechanisms. Leak before break (LBB) behavior was not satisfied for the hydrogen vessel due to a significant reduction in the fracture toughness of materials when exposed to 99 MPa H2.

Coolant leak rate through fatigue cracked pipe bend: An experimental and design standard based investigation

Leak-Before-Break (LBB) requirement plays a vital role in demonstrating the structural integrity of sodium-cooled Fast Breeder Reactor (FBR) piping system. Crack size at penetration (2CS), crack size that corresponds to detectable coolant leak rate (2CL), and unstable crack size (2CG) are the primary crack parameters associated with the LBB design of FBR piping system. The detectable leak rate through a piping system is directly related to the differential pressure (ΔP) and crack size. The design pressure of FBR piping systems is relatively low, resulting in a high value of CL. Considering the need for an optimum LBB design of the FBR piping system, an experimental approach is adopted to arrive at a realistic estimate of CL. Based on the experiments, a power-law type correlation can be derived to predict the value of CL as a function of ΔP. The evidence from this study points towards the conservatism present in the current FBR LBB design philosophy as per RCC-MRx A16. These observations have several implications for achieving an optimum LBB design criterion for an FBR piping system.

Characteristics of allowable axial cracks for pressurized pipes governed by limit load criteria

When axial cracks were detected in pressurized pipes, failure stresses for high-toughness ductile pipes are estimated using the Limit Load Criteria. The allowable stresses for the cracked pipes are derived from the combination of the failure stresses and safety factors. Then, the allowable sizes of crack depths and lengths are determined from the allowable stresses. From the comparison of the allowable and failure stresses for through-wall cracks, the allowable cracks are not uniform. The allowable crack sizes can be separated into different characteristics of leak-before-break (LBB) and crack growth stability. Inspectors and users should pay special attention to allowable cracks with non-LBB and crack growth instability characteristics to prevent unexpected failure, particularly for thin-wall pipes. The boundaries of allowable crack depths and lengths that require special attention can be expressed by appropriate equations.

Role of welding residual stress in stainless steel piping with application of the leak-before-break technology

The predicted leakage rate of piping circumferential through-wall cracks (CTWC) under various loading levels is a critical factor for the application of leak-before-break (LBB) technology. In current engineering approaches, the effect of welding residual stress has not been carefully taken into account. In this paper, both numerical analyses and comparative verification are adopted to examine the influence of typical welding residual stress field on the crack opening displacement for austenitic stainless steel piping with representative geometric dimensions and in situ measured material performance curves. An in-depth investigation is carried out to reveal the effect of the residual stress on morphological parameters of the CTWC flow channel. In the case where the residual internal stress of welding has the greatest effect, the Henry-Fauske model is employed to analyze the flow medium passing through the CTWC and the corresponding leak rate for a typical nuclear power plant. The results indicate that the welding residual stress leads to a substantial change in the crack opening displacement and crack morphology parameters. Both the current GE/EPRI method and the NUREG/CR-6837 modification recommended by the American Electric Power Research Institute underestimate the effect of this phenomenon, resulting in a higher prediction of the medium leakage rate. A similar situation is most likely to occur for the cases of short cracks in thin-walled piping and long cracks in thick-walled piping. Additionally, the obtained results reveal that the welding residual stress causes the whole crack surface to open negatively, and the axial line of the long crack on the thick-walled piping is close to a conic curve under specific conditions.

Improved Method for Fracture Toughness Testing Using Compliance Method for High Toughness Materials

Abstract In order to ensure the integrity of structures such as gas turbines and nuclear power plants, the materials used should have excellent toughness. Especially in the case of nuclear piping materials applied to leak before break (LBB) design, high toughness materials are used to meet the stringent fracture toughness criteria and integrity must be verified through static J–R curve testing using the compliance method, one of the measurement techniques for fracture toughness. The measured and estimated values for the crack extension length during the test should also match, within a certain tolerance. However, in the case of materials with high toughness, rotation of the specimen becomes significant, because the test is performed until the crack open displacement (COD) is relatively large to ensure sufficient crack extension. In this case, it is not easy to satisfy these conditions due to the rotational effect on the specimen. Even though ASTM E1820 suggests a method for correcting the crack length for the rotational effect on these specimens, it has been found that there are substantial differences for high toughness materials. To resolve this problem, a new crack length correction formula considering a large rotation effect is proposed. Through analysis of the data from J–R curve testing with this proposed method, it is confirmed that the accuracy of crack extension length estimation is improved compared to the existing method. The proposed method can well explain the variation of crack extension length due to rotation and is suitable as a correction equation for rotation of compact tension specimens.

Allowable stresses related to crack growth stability and leak-before-break for circumferentially cracked pipes governed by limit loads: Part I, Pipes subjected to tensile loading

When cracks are found in high toughness ductile pipes governed by limit loads at in-service inspections, and the applied stresses at the cracked locations are less than the allowable stresses, the cracks are allowable. The allowable stresses are generally determined in combination with failure stresses and safety factors. Failure of pipes with circumferential part-through cracks subjected to tensile loading occurs in a two-step sequence: first there is the onset of crack penetration, and then there is the break after the crack penetration. The Local Approach of Limit Load Criteria, which is based on the net-section stress approach, can estimate both crack penetration and break stresses for high-toughness ductile pipes. Using the Local Approach of Limit Load Criteria, allowable tensile stresses based on crack penetration stresses are determined by means of safety factors for plant service level conditions. The allowable stresses decrease with increasing crack angles and depths. The characters of the allowable tensile stresses are found to vary with changing sizes of crack angles and depths. These different characters involve the presence/absence of two parameters: leak-before-break (LBB) and crack growth stability. The crack size boundary on LBB vs non-LBB can be expressed by appropriate equations containing crack depths and angles. In addition, the boundaries of crack growth stability/instability are also expressed by crack depths and angles. These boundaries are taken into consideration during non-destructive inspection and crack growth evaluation.

Allowable stresses related to crack growth stability and leak-before-break for circumferentially cracked pipes governed by limit loads: Part II, Pipes subjected to bending moment

If cracks are detected in power plant pipes and the cracks are allowable, the plants can continue operating without the need for repair/replacement. The allowability of cracks is determined by means of allowable stresses, based on failure stresses. Failure bending stress at crack penetration is estimated by applying the Local Approach of Limit Load Criteria, which is based on the net-section stress approach. Using the stress at crack penetration, the allowable bending stress can be calculated for a pipe with a circumferential part-through crack. The allowable bending stress decreases with increasing crack depth and angle, as expected. However, the allowable bending stress can have one of three characters, depending on crack size: i) leak-before-break (LBB) and crack growth stability, ii) non-LBB and crack growth stability, and iii) non-LBB and crack growth instability. Character iii) represents a severe crack, and special attention needs to be paid to such a crack, even if it is allowable. Frequent non-destructive inspection and precise calculation of subcritical crack growth are recommended in order to prevent unexpected failure, especially for small-diameter thin-walled pipes. The boundaries of leak-before-break and crack growth stability can be successfully expressed by appropriate equations.

Asymptotic Crack Size of a Prototype Sized Pipe Bend and Comparison With A16 Master Curve

AbstractIn the design of a fast breeder reactor (FBR) piping system, asymptotic crack size (2Cs) is traditionally estimated using the leak-before-break (LBB) master curve given in RCC MRx A16. The current approach is seen not to be sufficiently efficient in the LBB demonstration of FBR piping systems. Numerical crack growth studies have received much attention in the past decade. Deployment of fracture mechanics based numerical crack growth studies on prototype-sized pipe geometries generates considerable interest in estimating asymptotic crack size. This paper outlines the advantages of numerical finite element based crack growth studies to estimate the Cs on a prototype-sized pipe bend. The numerical procedure has resulted in an accurate and more realistic estimate of Cs than the traditional LBB Master curve as per A16. Considering the economic advantages of a relatively lower value of Cs for LBB demonstration, it is recommended to adopt the numerical method to estimate Cs for the FBR piping system.

The Huge Missing Factor in Leak-Before-Break Analysis: How a Circumferential Through-Wall-Crack in a Pipe System Changes the Flexibility and Reduces the Applied Moments

Abstract In typical leak-before-break (LBB) analyses in the nuclear industry, the uncracked piping normal operating forces and moments are applied in a cracked-pipe analytical procedure to determine normal leakage, and the combined forces and moments under normal operating condition and safe shutdown earthquake seismic loading are used in a fracture analysis to predict margins on “failure.” The International Piping Integrity Research Program (IPIRG) performed in 1990–1998 provided some insights to typical LBB behaviors where pipe system tests were conducted with simulated seismic loadings. The test results showed a large margin on LBB, which was also recognized in 2011 when the Argentinian Atucha II plant was analyzed using a robust full FE model. It was found that when circumferential through-wall cracks were put in the highest stressed locations, the applied moment dropped for both normal operating and N + safe shutdown earthquake (SSE) loading as the crack length increased. The through-wall crack size for causing a double-ended guillotine break (DEGB) was greater than 90% of the circumference. Similar results were also found for a petrochemical pipe system where thermal expansion stresses are much higher than the primary stresses. Even with very low toughness materials, the critical crack size leading to DEGB was greater than 80% of the circumference. The implication of this work is that pragmatically there is much higher margin for DEGB failure in nuclear plant operation, and efforts would be better focused on the potential for a small-break loss-of-coolant accident (SB-LOCA).

Numerical simulation of micro-crack leakage on steam generator heat transfer tube

The pressure relief rate and loss rate of coolant, which affect largely safety of fission reactor systems, are determined by the flashing process inside cracked passages. Information about flashing phenomenon is of great significance for the leakage online monitoring system of SG (Steam Generator), which ensures the normal operation of SG and safety of the reactor system during early stage of tube rupture accidents. In this research, flashing flow inside a thin micro-crack in heat transfer tube of SG has been studied using ANSYS-FLUENT. The classic cavitation model and evaporation–condensation model, in combination with both the mixture two-phase flow model and Eulerian two-fluid model, are adopted to simulate flashing phenomenon. The real geometry and operation conditions of AP1000 nuclear system are adopted to reflect the reality leakage process in SG. Two types of micro-cracks including axial crack and circumferential crack, which both could happen in the reality operations, are considered. The CFD (Computational Fluid Dynamic) results gained from this research have been compared with test data, and good agreement is demonstrated. The results show that the evaporation–condensation model under Eulerian two-fluid model framework behaves superior to the classic cavitation models in simulating the flashing phenomenon after evaluating accuracy of leakage rate and rationality of thermal hydraulic parameters distribution. In addition, two-phase choking flow phenomenon is obtained by decreasing backpressure of cracked tubes. The simulation in this research reveals the flashing process in short and micro-crack and the results could be good reference for leakage prediction of SG under LBB (Leak Before Break) operations to improve operation performance of SG and safety of the whole nuclear power system.

Comparison of safety margin in LBB design of nuclear pipes based on various types of fracture resistance test specimens

The geometry of cracked pipe, loading condition and crack size all can have a strong effect on its resistance against crack-tip plastic deformation and crack growth. Usually, specimens listed in test standards (e.g. CT specimen) are applied to evaluate the fracture resistance (J-R) curves, which may cause excessive safety margin in leak-before-break (LBB) design of nuclear pipes. In this study, standard (CT) specimen and constraint designed specimens, including curved CT and compact pipe (CP) specimens were applied to measure the J-R curves of nuclear pipes. Finite element analysis (FEA) was applied to derive J-equations to calculate J-R curves for newly designed specimens. Constraint effects in these specimens and full-scale pipes under various loading conditions were compared according to crack-tip plastic deformation level. Then, safety margins in LBB design using these specimens were quantitatively analyzed by constructing the critical crack length lines. Both the tests and FEA results verified the validity of these constraint designed specimens for reducing conservatism.

BORONIZING MITIGATE FAILURE MECHANISMS WITH EXTENDED LIFE IN OIL/GAS STEEL PIPING UNDER ACOUSTIC INDUCED VIBRATION (AIV)

Oil and gas piping systems suffer from acoustically induced vibration, AIV, that end with hearing losses and consuming personnel’s lives. However, Leak-Before-Break (LBB) methodology was developed to mitigate failure, the primary piping systems exhibit an extremely low probability of rupture (xLPR). For understanding of piping failure mechanisms under AIV, 3-point bending tests are conducted to study structural integrity in oil/gas piping (ASTM A333 Carbon Steel GR.6 Seamless). Besides, for ultra-fast surface hardening and strengthening, a non-traditional powder pack boronizing technique is introduced at 950oC for 10 and 30 minutes. Mechanical characterization as well as microstructural analysis are conducted for continuous and incremental bending tests. In addition, FE modeling, ANSYS, is conducted on actual “case study” of oil/gas piping to study the changes in dynamic characteristics for both as-received (AR) and boronized steel Under AIV(173 dB). Failure mechanisms is established in AR sample with clear banding and early localized micro-plasticity at 0.2 flexural stress (sf). Void coalescence induce crack initiation at 0.3 sf. Crack propagation (at 0.5 sf), crack evolution (at 0.7sf) and catastrophic failure are captured. Boronizing with the diffusion of Boron into the outer surface dominate functional-graded materials (FGM), with surface and bulk hardening. Surface hardening via boronizing mitigate fracture mechanisms with no clear micro-plasticity, but with clear twinning and banding elimination. In addition, to investigate the sustainability of piping and the effect of boronizing, under AIV, FE modeling is introduced with two different steel piping systems (AR with strength 580 MPa, and boronized steel with strength 1390 MPa). Failure mitigation is introduced with the increase of endurance limit as well as increase of piping life. تعاني أنظمة أنابيب النفط والغاز من الاهتزازات الناتجة عن الصوت ، AIV ، والتي تنتهي بفقدان السمع واستهلاک حياة العاملين. ومع ذلک ، تم تطوير منهجية التسرب قبل الکسر (LBB) للتخفيف من الفشل ، حيث تظهر أنظمة الأنابيب الأولية احتمالية منخفضة للغاية للتمزق . (xLPR) لفهم آليات فشل الأنابيب بموجب AIV ، تم إجراء اختبارات الانحناء من 3 نقاط لدراسة السلامة الهيکلية في أنابيب النفط / الغاز المستخدمة (ASTM A333 Carbon Steel GR.6 Seamless) . إلى جانب ذلک ، من أجل تغطية وتقوية السطح بسرعة فائقة ، تم تقديم تقنية التغطية بالبورون غير التقليدية عند درجة حرارة 950 درجة مئوية لمدة 10 و 30 دقيقة. تم إجراء التوصيف الميکانيکي وکذلک تحليل البنية المجهرية لاختبارات الأنابيب –الإنحناءات المستمرة والمتقطعة. بالإضافة إلى ذلک ، يتم إجراء نمذجة FE ،بإستخدام ANSYS ، على حالة فعلية لأنابيب النفط / الغاز لدراسة التغيرات في الخصائص الديناميکية لکل من الفولاذ المغطي بالبورون المستلم (AR) والفولاذ البورونى تحت AIV (173 ديسيبل). تم دراسة آليات الإنهيار في عينة AR مع النطاقات الواضحة واللدونة الدقيقة الموضعية المبکرة عند 0.2% إجهاد الانحناء . (sf) يؤدي الاندماج الفارغ إلى بدء الشروخ عند %0.3 . sfتم التقاط انتشار الشروخ (عند %0.5 sf) ، وتم إکتشاف تطور الشقوق (عند %0.7 sf) ثم الفشل الکارثي المفاجئ. تؤدي التغطية بالبورون إلى انتشار البورون علي السطح الخارجي تدريجيا ويؤدي إلى تکوين مواد مرکبة- المواد المتدرجة الوظيفية (FGM) ، مع تصلب السطح والکتل. تصلب السطح الخارجي يؤدي إلى تخفيف آليات الکسر وزيادة التصلد بالبورون مع عدم وجود لدونة دقيقة واضحة ، ولکن مع إزالة التوأمة والنطاقات الواضحة (Banding). بالإضافة إلى ذلک ، للتحقق من استدامة الأنابيب وتأثير البورون ، في إطار AIV ، تم تقديم نمذجة FE مع نظامين مختلفين للأنابيب الفولاذية (AR) بقوة 580 ميجا باسکال ، والفولاذ بالبورون بقوة 1390 ميجا باسکال). يتم تقديم تخفيف الأعطال من خلال زيادة حد التحمل بالإضافة إلى زيادة عمر الأنابيب) .

Effect of cooling rate on mechanical properties of SA508 Gr.1A steels for main steam line piping in nuclear power plants

To improve mechanical properties of SA508 Gr1.A steels for application of leak-before-break (LBB) design in main steam line piping in nuclear power plants, the effect of cooling rate on microstructure formation, tensile properties, Charpy impact properties, and J-R fracture toughness of SA508 Gr.1A steels was analyzed. As cooling rate increased, coarse ferrite/pearlite microstructure changed to fine bainite and martensite, and coarse grain boundary carbides disappeared and fine carbides precipitated inside grains. When the cooling rate is faster, both strength and elongation at 286 °C improved owing to the formation of fine low-temperature-transformation phases. The increase in cooling rate also have positive effect to the ductile-brittle transition temperature (DBTT) and J-R fracture toughness at 286 °C. From the fractograph analysis of J-R tested specimens, it is revealed that large voids formed at the coarse grain boundary carbides and crack propagated along the ferrite/pearlite interface. The increase in the cooling rate suppressed the formation of coarse carbides and ferrite/pearlite and promoted the formation of bainite and martensite having fine effective grain size, which improved the strength and toughness of the SA508 Gr.1A steels.

Evaluation of LBB characteristics of candidate materials for main steam line piping in Korea nuclear power plants

Recently, efforts have been made to apply the leak-before-break (LBB) design concept to the main steam line (MSL) piping in secondary systems of Korea standard nuclear power plants (KSNPs). In this study, the microstructures, mechanical properties, and consequential LBB margins of five candidate materials (SA335 Gr.P11/P22, SA508 Gr.3 Cl.2, SA508 Gr.3 Cl.1, and SA508 Gr.1A steels) were evaluated, and strong/weak points of candidate materials for applying MSL piping were analyzed through the comprehensive analysis including microstructure, mechanical properties, LBB margin, usage history, economics, and so on. SA508 Gr.1A steels, which has been used extensively as a piping material in NPPs and has a low fabrication cost, had a good combination of yield strength and fracture toughness. They had LBB margin of 1.11–1.21. SA508 Gr.3 Cl.1 steel had an excellent combination of yield strength and fracture toughness with an LBB margin of 1.64. SA508 Gr.3 Cl.2 steel had the highest LBB margin (1.84) because it had the highest strength; however, it had low fracture toughness and low upper shelf energy (USE). The effect of yield strength on the LBB margin was considerably higher than the effect of fracture toughness. SA508 Gr.3 Steels have high LBB margin owing to the high yield strength, but they have never been used for piping materials in NPPs before. SA335 Gr.P11 and P22 had the LBB margin of 1.1 and 1.23, respectively. However, the elongation and J-R fracture toughness of SA335 Gr.P11 and P22 decreased significantly at 286 °C owing to dynamic strain aging (DSA).

A further study on fracture resistance evaluation of nuclear pipes using CP specimens

The leak-before-break (LBB) concept has been widely used to design the piping system in nuclear power plant, which is based on the fracture resistance (J-R) curves of cracked nuclear pipes. However, accurate fracture resistance evaluation of nuclear pipe is always a tough task. Full-scale nuclear pipe tests are costly and difficulty, and the specimens listed in test standards (e.g. ASTM E1820) are usually too conservative. Thus, compact pipe (CP) specimen has been developed to evaluate the J-R curves of nuclear pipes with high accuracy but much lower cost than full-scale pipe tests. But, the practical application of CP specimen still asks for more theoretical and experimental research. In this study, the assembly of CP specimen was modified to improve the assembly convenience and ensure a more uniform loading condition. Instead of load-line displacement (LLD), crack mouth opening displacement (CMOD) based J-formulation was proposed to determine the J-R curves of modified CP specimens considering more reliable evaluation of CMOD. In addition, the differences of constraint effect among CT specimen, CP specimens and full-scale pipes are distinguished using equivalent plastic strain (εp) isoline regions, which agrees quite well with the results of fracture resistance tests.