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1.
Stress corrosion cracking (SCC) examination of Inconel 600 steam generator tubing has continued at Brookhaven National Laboratory, using U-bends, constant load and slow extension rate tests, leading to Arrhenius plots of failure times versus inverse temperature for crack initiation and propagation. Effect of applied load can be expressed as log-log curves for failure times versus stress. Variations in environment and cold work are included in all the experiments. Microstructure and composition of oxide films on Inconel 600 surfaces were examined after exposure to pure water at 365°C, and stripping with the bromine-methanol method. Results are consistent with a mechanism of transient creep, film rupture and a mass-transport-limited anodic process.  相似文献   

2.
The Palisades nuclear plant has developed a comprehensive inspection program to support safe, reliable, and cost-effective operation of all Alloy 600 nozzles and safe ends in the primary coolant system (PCS). As a part of the Palisades Alloy 600 Project, an inspection prioritization scheme was developed to help the plant focus its resources on high-risk components and plan appropriate inspection activities for the other components. The inspection prioritization scheme is based on the susceptibility of the components to primary water stress corrosion cracking (PWSCC), component failure consequences, component leak detectability and component radiation exposure. The scheme provides a simple, systematic and technical base for selecting Alloy 600 components for inspection. The scheme, however, could be used to develop an inspection schedule or to select the highest priority components for mitigation or replacement.  相似文献   

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The effect of compressive residual stress on the primary water stress corrosion cracking behavior was investigated, based on the J-1 and J-2 nuclear power plant data. The following analyses were performed such as: (i) Weibull slope; (ii) crack growth rate; (iii) average crack length; (iv) crack length distribution. Alloy 600 TT exhibits strong heat to heat variations in its sensitivity to PWSCC. Crack growth rate was retarded after shot-peening. The compressive residual stress induced by shot-peening was more effective on new, short cracks, than on existing, long cracks. However, whether the ‘new’ cracks were initiated after peening is an unresolved issue, due to the present ECT sensitivity limit.  相似文献   

5.
Slow strain rate stress corrosion tests have been performed on specimens cut from four separate heats of Alloy 600 steam generator tubing. The material was tested in the mill-annealed and thermally-stabilised conditions and after various low temperature ageing treatments. Only limited cracking was observed, even for tests at 340°C, but the initiation of intergranular cracking was easier on the inner than on the outer surfaces on the tubing. Polarization data has been obtained in high-temperature water and in saturated boric acid and saturated lithium hydroxide at the atmospheric boiling points, and slow strain tests were performed at controlled potentials in these environments. Again, only very short cracks formed during the slow strain rate tests which were performed at a strain rate of about 10−6 s−1. The data is discussed in terms of the probable crack tip strain rates that would exist in these tests and at other strain rates. It is argued that if cracking occurs, the main role of very slow strain rate tests is to provide time for initiation and crack growth, so that cyclic loading or intermittent loading long term tests are likely to be more successful in sustaining crack growth in this alloy.  相似文献   

6.
Nb can improve the resistance of Ni-based Hastelloy N alloy to Te-induced intergranular embrittlement. First-principles calculations are performed to research this mechanism by simulating the Ni(111) surface and the ∑ 5(012) grain boundary. The calculated adsorption energy suggests that Te atoms prefer diffusing along the grain boundary to forming the surface-reaction layer with Nb on surface of the Ni alloy. First-principles ten- sile tests show that the Nb segregation can enhance the cohesion of grain boundary. The strong Nb-Ni bonding can prevent the Te migration into the inside of the alloy. According to the Rice-Wang model, the strengthen- ing/embrittling energies of Nb and Te are calculated, along with their mechanical and chemical components. The chemical bonds and electronic structures are analyzed to uncover the physical origin of the different effects of Te and Nb. Our work sheds lights on the effect of Nb additive on the Te-induced intergranular embrittlement in Hastelloy N alloy on the atomic and electronic level.  相似文献   

7.
Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr2O3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.  相似文献   

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Iodine-induced stress corrosion cracking (I-SCC) is a recognized factor for fuel-element failure in the operation of nuclear reactors requiring the implementation of mitigation measures. I-SCC is believed to depend on certain factors such as iodine concentration, oxide layer type and thickness on the fuel sheath, irradiation history, metallurgical parameters related to sheath like texture and microstructure, and the mechanical properties of zirconium alloys. This work details the development of a thermodynamics and mechanistic treatment accounting for the iodine chemistry and kinetics in the fuel-to-sheath gap and its influence on I-SCC phenomena. The governing transport equations for the model are solved with a finite-element technique using the COMSOL Multiphysics® commercial software platform. Based on this analysis, this study also proposes potential remedies for I-SCC.  相似文献   

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The stress corrosion behavior in the presence of iodine vapor or chlorine gas of unirradiated, recrystallized and stress-relieved Zircaloy-4 as a function of hoop stress, test temperature and contaminant concentration has been investigated using internally pressurized tube specimens. Comparison with similarly tested control specimens (no halogen) shows that at 360°C and 400°C as little as 0.005 to 0.01 mg halogen per cm2 Zircaloy surface is sufficient to reduce failure time for hoop stresses above 21 to 24 kpsi. Failure time appears to be essentially independent of iodine up to 0.08 mg/cm2 . At equivalent stress ratios (σapplied/σYS) with iodine (i) the higher temperature results in shorter failure times over most of the stress range investigated and (ii) stress-relieved material generally failed before recrystallized material. Failures were typically pinholes except at high stresses where stress rupture occurred. Scanning electron microscopy revealed intergranular failure in recrystallized specimens and the presence of fluted facets in both materials.  相似文献   

12.
Type 308 stainless steel weld metal as an internal cladding of reactor pressure vessels for boiling water reactors is subject to postweld heat treatment during fabrication and can suffer sensitization depending on carbon and ferrite contents. This sensitization can be avoided by using niobium-added Type 308 weld metal (specified as Type 308 NbL) which was developed for one-layer overlay welding application. In the present study, stress corrosion cracking (SCC) behavior of heat-treated Types 308 and 308NbL weld metals in oxygenated high temperature pure water was evaluated by slow strain rate test and U-bend tests with and without crevice. Every test showed that Type 308NbL weld metals were highly resistant to SCC compared to ordinary Type 308 weld metals. In single U-bend test, one-layer overlay weld metals of Type 308NbL produced by electroslag welding process using wide strip electrodes were crack free over 23,000 h. The U-bend test data of ordinary Type 308 weld metals were successfully analyzed by an SCC reaction model. Using this analysis, the SCC life margin for Type 308NbL over ordinary Type 308 weld metals, expressed as a ratio of respective times to SCC initiation, was estimated to be about 36.  相似文献   

13.
The effect of shot peening on the primary stress corrosion cracking behavior of thermally treated Alloy 600 steam generator tubes in an operating pressurized water reactor (PWR) plant was analyzed based on pulled tube examinations and in-service inspection eddy current test (ISI-ECT) data. The evaluation was focused on the shape of crack, evolution of the number of new cracks and cracked tube fraction, and variation of crack length and the corresponding eddy current amplitude before and after shot peening. The shape of the crack was changed from a half-elliptical type before shot peening, to an elliptical one with bulging after peening. It was concluded that the shot peening was not effective for retarding both crack initiation and growth for this plant.  相似文献   

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The influence of ageing heat treatment on alloy A-286 microstructure and stress corrosion cracking behaviour in simulated Pressurized Water Reactor (PWR) primary water has been investigated. A-286 microstructure was characterized by transmission electron microscopy for ageing heat treatments at 670 °C and 720 °C for durations ranging from 5 h to 100 h. Spherical γ′ phase with mean diameters ranging from 4.6 to 9.6 nm and densities ranging from 8.5 × 1022 m−3 to 2 × 1023 m−3 were measured. Results suggest that both the γ′ phase mean diameter and density quickly saturate with time for ageing heat treatment at 720 °C while the γ′ mean diameter increases significantly up to 100 h for ageing heat treatment at 670 °C. Grain boundary η phase precipitates were systematically observed for ageing heat treatment at 720 °C even for short ageing periods. In contrast, no grain boundary η phase precipitates were observed for ageing heat treatments at 670 °C except after 100 h. Hardening by γ′ precipitation was well described by the dispersed barrier hardening model with a γ′ barrier strength of 0.23. Stress corrosion cracking behaviour of A-286 was investigated by means of constant elongation rate tensile tests at 1.5 × 10−7 s−1 in simulated PWR primary water at 320 °C and 360 °C. In all cases, initiation was transgranular while propagation was intergranular. Grain boundary η phase precipitates were found to have no significant effect on stress corrosion cracking. In contrast, yield strength and to a lesser extent temperature were found to have significant influences on A-286 susceptibility to stress corrosion cracking.  相似文献   

16.
Caution when applying eddy current inversion to stress corrosion cracking   总被引:1,自引:0,他引:1  
This study evaluates the applicability of computer-aided eddy current inversion techniques to the profile evaluation of stress corrosion cracking in Inconel welds. Welded plate specimens, which model head penetration welds of pressurized water reactors, are fabricated; notches and stress corrosion cracks are artificially introduced into the weld metal of the specimens. Eddy current inspections are performed using a uniform eddy current probe driven at frequencies of 10 and 40 kHz. Since weld noise is observed uniformly along the weld line, a simple signal processing is applied to eliminate it. First, the artificial notches are reconstructed and good agreements between reconstructed and true profiles are provided, which demonstrates that the computer-aided eddy current inversion technique can deal with defects in welds. Then, numerical simulations are performed to evaluate the profiles of the stress corrosion cracks. In the numerical simulations, the stress corrosion cracks are modeled as a conductive region with a fixed width of 0.3 mm. The cross-sectional profiles of the cracks are reconstructed from measured eddy current signals directly above and along a crack. Although eddy current signals calculated from the reconstructed profiles agree well with measured ones, the true profiles revealed by destructive testing are found to be very different from the reconstructed ones. Whereas the most plausible reason for the difference is the unexpectedly volumetric profile of the stress corrosion cracks, this study has revealed that computer-aided eddy current inversion techniques that have been used to consider cracks in thin structures would not at this point be directly applicable to those in thick structures. It is also important to know in advance those crack features that can adversely impact accurate crack sizing including whether a detected crack is volumetric or not, namely there are many parallel cracks in a cluster or not.  相似文献   

17.
This study discusses the equivalent conductivity, the equivalent width, and the equivalent resistance of stress corrosion cracks from the viewpoint of eddy current testing. Four artificial stress corrosion cracks were prepared for this study, and their eddy current signals were gathered using two absolute pancake probes and two differential type plus point probes. Then their numerical models were evaluated using finite element simulations on the basis of the measured eddy current signals and their profiles revealed by destructive tests. The results of this study revealed that whereas the equivalent conductivity and the equivalent width depend on the exciting frequency utilized, the equivalent resistance of a crack has much less dependency, which agrees well with an earlier report. This study also revealed that the resistance of a crack depends on probe utilized. Larger probes tend to lead to smaller crack resistance. Pancake type probes tend to lead to larger crack resistance than plus point probes. Analyzing the results together with earlier reports indicates that cracks with a large equivalent conductivity tend to have large equivalent width, and supports the validity of assuming the minimum resistance of a stress corrosion crack whereas considering the conductivity and the width individually would not be viable.  相似文献   

18.
A fracture mechanics approach to interpreting iodine-vapor stress-corrosion cracking in unirradiated Zircaloy-4 tubing is presented in which crack velocities are related to the fourth power on the stress intensity factor, KI. The crack growth power law on KI is shown to predict well the time-to-failure in internally pressurized Zircaloy-4 tubing at 360 and 400°C reported by Busby, Tucker and McCauley. The temperature dependency on iodine stress corrosion cracking in Zircaloy can be described by an Arrhenius-type equation in which the activation energy Q for recrystallized and cold-reduced Zircaloy was determined to be 42.9 and 35.9 kcal/mole, respectively. It is concluded that the geometry of the initial surface flaw, through its attendant elastic stress field, is directly responsible in controlling the SCC time-to-failure, cold working having a relatively small effect on increasing the susceptibility to SCC. The effects of neutron flux on iodine stress corrosion cracking of Zircaloy-4 tubing in-reactor are still unknown.  相似文献   

19.
Intergranular stress corrosion cracking of Boiling Water Reactor piping has been a prominent problem since 1974 and has reduced plant availability as a result of the requirements (1) for inspection of piping susceptible to cracking and (2) for repair or replacement of piping that has cracked. In 1979, a Boiling Water Reactor Owners Group was formed to expand upon and, where possible, accelerate research into the causes of the problem and its remedies. In this paper, the research program of this international group is described and the program's results to date and expectations are highlighted. Interactions with regulatory groups in the United States of America and reactions of the regulatory groups to the research program are also presented.  相似文献   

20.
The stress corrosion cracking (SCC) rate of reactor internals of boiling water reactors (BWR) continues to increase with on-line operating years. The recent occurrences of cracking in the weld heat affected zones of high carbon stainless steel core shrouds correlate with the years of operation and the water chemistry history. Recently, cracking has also been found in shrouds that were constructed of low carbon or stabilized stainless steels. While these steels are more resistant to intergranular stress corrosion cracking (IGSCC) in the as-fabricated condition, this field experience establishes that the conditions under which the materials will crack in core structures are attributable to the combined effects of high residual stresses, associated with the shroud construction, the presence of a more aggressive, oxidizing environment in the core and to microstructural changes in the material. These changes result from the manufacturing process as well as thermal exposure during operation. Studies of materials that have cracked in the field, as well as high temperature material studies in the laboratory, are being performed to understand the mechanisms. The use of highly oxidizing, high purity water environments is integral to reproducing the conditions for cracking. The status of the laboratory efforts to gain understanding and to verify the mechanisms are presented. Modeling of IGSCC is also a key tool used to understand the cracking behavior of the low carbon stainless steels. The PLEDGE (Plant Life Extension Diagnosis by GE) model is used to support the hypotheses that tie together the role of the different contributing elements: residual stress, core water chemistry and microstructural features. The crack growth modeling is also used to evaluate the benefits of different strategies to manage and mitigate cracking of reactor internals such as hydrogen water chemistry.  相似文献   

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