Corrosion fatigue behavior of low alloy steels under simulated BWR coolant conditions

The corrosion fatigue crack growth behavior of A533 and A508 low alloy steels under simulated boiling water reactor (BWR) coolant conditions was studied. Corrosion fatigue crack growth rates of A533B3 and A508 cl. 3 steels were significantly affected by the steel sulfur content, loading frequency and dissolved oxygen content of water environments. The data points outside the bound of Eason’s model could be attributed to the low frequency, higher steel sulfur content and high dissolved oxygen in water environments. The sulfur dissolved in the water environment from the higher-sulfur steels was sufficiently concentrated to acidify the crack tip chemistry even in the hydrogen water chemistry (HWC). Therefore, nitrogenated or HWC water showed little or no beneficiary effect on the high-sulfur steels. For the steel specimens of the same sulfur level, their corrosion fatigue crack growth rates were comparable in different orientations, which could be related to the exposure of fresh sulfides to the water environment. The percentages of sulfides per unit area, by quantitative metallography, were comparable for the steel specimens of both orientations. When the steel sulfur content was decreased to a critical sulfur content 0.005 wt.%, the crack growth rates decreased remarkably.     

Modeling of fatigue crack growth rate for ferritic steels in light water reactor environments

This paper presents the methodology and results of an effort to analyze and model a large set of fatigue crack propagation data on A508 Class 2 and Class 3 and A533B pressure vessel steel in light water reactor (LWR) environments. The data were from a variety of laboratories worldwide, in most cases contributed to the EPRI Database on Environmentally Assisted Cracking (EDEAC). The data were analyzed in a consistent manner using the computer code FATDAC, which minimizes the scatter arising from numerical differentiation during fatigue data reduction. The models were developed in the time domain, then converted to the more conventional da/dN versus ΔK form. Two modes of corrosion fatigue crack growth behavior were identified and modeled, one about a factor of two faster than the rates in air and independent of loading rate and the other up to two orders of magnitude faster and strongly dependent on loading rate. Variables such as material sulfur content, sulfide inclusion morphology, water chemistry, R-ratio, load rise time, stress intensity range, temperature, electrochemical potential, and flow velocity affect both the probability of observing the highly enhanced crack growth rates and the rates themselves. Representative crack propagation models are developed and presented in this paper together with supporting data.     

Development of thermal fatigue testing apparatus with BWR water environment and thermal fatigue strength of austenitic stainless steels

A thermal fatigue testing apparatus was developed in order to clarify the fatigue behavior in BWR environment. Pressurized high and low temperature pure water were alternately supplied into an autoclave with a small cylindrical specimen. Then a fatigue specimen was subjected to homogeneous thermal stress through the wall thickness. Fatigue crack initiation behavior was observed with the replication method and compared with the mechanical fatigue strength performed in air and high temperature water. The thermal fatigue strength of type 304 and 316 nuclear grade (316NG) stainless steels agreed closely with the mechanical fatigue strength, when transforming the nominal stress amplitude to the fictitious stress amplitude by using the mean value of strain amplitudes for room temperature and 288°C.     

The FEM based calculation of crack-tip strain rate for determining the crack growth rate of 304 stainless steel in BWR environments

This research calculates the crack growth rate (CGR) of the cooling pipe in the nuclear power plant. The crack is caused by the tension left in welding as well as by the coolant chemicals. There is, by far, no satisfactory deterministic model that can predict the crack growth rate. Although Macdonald's CEFM model is more close to a deterministic one, it requires accurate calculation of the crack-tip strain rate before the crack growth rate can be obtained by using Faraday's equation. The empirical equation used by the CEFM model for estimating the crack-tip strain rate is a drawback in making it a totally deterministic model. This research is set to make up this critical defect by applying non-linear finite element method to calculate the crack-tip strain rate. According to the metallic property, there is a plastic zone forming around the crack-tip. This research found when applying finite element method, the number of the elements (namely the mesh density) in the vicinity of the crack-tip has significant effects on the crack-tip strain obtained. Therefore, this research pioneered by using different mesh density in the vicinity of the crack-tip for the calculation of the crack-tip strain, the convergence of which is used for selecting the appropriate mesh density. The selected mesh density is then used to determine the dimension of the plastic zone, in which the crack-tip strain rate can be calculated. When the FEM results are compared with the experimental data, the nonlinear finite element method combined with CEFM shows a very satisfactory prediction capability.     

Interpretation of the influences of irradiation upon fatigue crack propagation in austenitic stainless steels

An interpretation of the influences of neutron irradiation upon fatigue crack propagation in austenitic stainless steels is given. The approach has been to extend a previously developed rationalisation of the effects of various test and materials variables upon fatigue crack propagation in unirradiated stainless steels to include irradiated stainless steels.Irradiation has diverse influences upon the rate of fatigue crack propagation depending on the exact irradiation and test conditions. It has been shown that, by considering the underlying mechanisms of failure, some confidence is established in trends in data in a subject where information is very scarce and difficult to obtain.     

Crack initiation and growth under creep and fatigue loading of an austenitic stainless steel

Both the initiation and the propagation of macroscopic cracks have been studied in a creep ductile 316L type stainless steel at 575–650°C using various fracture mechanics specimens and a wide range of test conditions including steady load at constant or varying temperatures, varying loads at constant temperature. It is shown that, even for isothermal tests, the C^* parameter is unable to provide unique correlations for all the stages of both creep crack initiation and growth. A unique correlation nevertheless exists between C^* and the time to initiation, T_i. Large differences – either conservative or not – from a simplified linear damage cumulation rule are found when the tests are performed at two successive temperatures or two loads. Very detrimental effects of creep-fatigue loadings are shown.A simplified global approach to creep crack initiation under isothermal conditions, based on reference stress and length concept is developed. A local approach to creep cracking, in which an intergranular physical damage law determined experimentally on notched bars, and stress-strain fields obtained by analytical results is shown to provide crack growth results in good agreement with experiment.     

Effects of loading factors on environmental fatigue behavior of low-alloy pressure vessel steels in simulated BWR water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in simulated boiling water reactor (BWR) water. Much attention was paid to the effects of loading factors on fatigue life and environmentally assisted cracking (EAC) behavior, in which strain rate, strain waveform and strain amplitude were taken into account. The fatigue resistance and EAC behavior of the steels in simulated BWR water were found to be closely dependent on the strain rate, strain waveform and strain amplitude applied. The above fatigue behavior may be attributed to loading-factor-induced change in dominant EAC processes in high temperature water environments. Related EAC mechanisms are also discussed.     

Fracture behavior of austenitic stainless steels irradiated in PWR

Fracture behavior of cold-worked 316 stainless steels irradiated up to 73 dpa in a pressurized water reactor was investigated by impact testing at −196, 30 and 150 °C, and by conventional tensile and slow tensile testing at 30 and 320 °C. In impact tests, brittle IG mode was dominant at −196 °C at doses higher than 11 dpa accompanying significant decrease in absorbed energy. The mixed IG mode, which was characterized by isolated grain facets in ductile dimples, appeared at 30 and 150 °C whereas the fracture occurred macroscopically in a ductile manner. The sensitivity to IG or mixed IG mode was more pronounced for higher dose and lower test temperature. In uniaxial tensile tests, IG mode at a slow strain rate appeared only at 320 °C whereas mixed IG mode appeared at both 30 and 320 °C at a fast strain rate. A compilation of the results and literature data suggested that IG fracture exists in two different conditions, low-temperature high-strain-rate (LTHR) and high-temperature low-strain-rate (HTLR) conditions. These two conditions for IG fracture likely correspond to two different deformation modes, twining and channeling.     

Experimental study of jet discharging test results under BWR and PWR loss of coolant accident conditions

This report describes the results of the jet discharging experiments conducted at the Japan Atomic Energy Research Institute. The tests were done under BWR and PWR Loss of Coolant Accident conditions using 4 inch, 6 inch and 8 inch test pipes, and varying distance between the pipe exit and the target plate.Simple and practical experimental formulae to estimate the maximum pressure on the target plate and maximum pressure distribution are given. Further, relations between pipe reaction thrust forces and jet impingement forces are described.     

Prediction of void fraction for PWR and BWR conditions with the sub-channel code F-COBRA-TF

The present paper discusses a comprehensive extension of the already substantial validation of the sub-channel code F-COBRA-TF with respect to void fraction prediction. In contrast to many established sub-channel codes, F-COBRA-TF allows a two-fluid/three-field representation of two-phase flow. Due to this modeling F-COBRA-TF, does not need to apply classical void correlations predicting void fraction according to quality. Instead, void fraction is a direct result of the basic transport equations for mass, momentum, and energy by using flow regime dependent interfacial friction correlations.Experimental data from open literature (tube boiling measurements in the sub-cooled region), from the OECD/NRC PSBT benchmark, and from in-house tests in AREVA's KATHY loop were used for validating F-COBRA-TF. In summary, it can be stated that all measurements could be recalculated with F-COBRA-TF with overall good agreement. Especially having in mind that no special code tuning concerning certain flow geometries or ranges of flow conditions had been performed, the obtained results look very convincing. Both, BWR and PWR conditions were simulated with exactly the same model set.     

A study on the evolution of crack networks under thermal fatigue loading

The crack network is a typical cracking morphology caused by thermal fatigue loading. It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow deeply. In this study, the mechanism of evolution of crack network and its influence on crack growth was examined by numerical calculation. First, the stress field near two interacting cracks was investigated. It was shown that there are stress-concentration and stress-shielding zones around interacting cracks, and that cracks can form a network under the bi-axial stress condition. Secondly, a Monte Carlo simulation was developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network. The local stress field formed by pre-existing cracks was evaluated by the body force method and its role in the initiation and growth of cracks was considered. The simulation could simulate the evolution of the crack network and change in number of cracks observed in the experiments. It was revealed that reduction in the stress intensity factor due to stress feature in the depth direction under high cycle thermal fatigue loading plays an important role in the evolution of the crack network and that mechanical interaction between cracks in the network affects initiation rather than growth of cracks. The crack network appears only when the crack growth in the depth direction is interrupted. It was concluded that the emergence of the crack network is preferable for the structural integrity of cracked components.     

The frequency effect on the fatigue crack growth rate of 304 stainless steel

Under cyclic loading condition, the fatigue crack growth (FCG) rate governed by stress intensity factor and stress ratio is well known; Walker’s equation, Forman’s equation and Elber’s equation are typical formulae to describe the fatigue crack growth rate. However, the loading frequency effect on the fatigue crack growth rate has yet to be explored. Recently, studies have focused on the loading frequency effect on some visco-elastic materials, and have provided a clearer understanding of the frequency effect on the fatigue crack growth rate. In a physical sense, knowledge about the loading frequency effect on the fatigue crack growth rate for 304 stainless steel is still lacking. James conducted a lot of experiments, and through data analysis, he concluded an evaluation equation which is based upon the experimental illustration. In this study, the physical properties of the material are used to illustrate the modification of fatigue crack growth rate, and a new formula which is based upon the modified Forman’s equation, is provided.     

Thermal fatigue strength of type 304 stainless steel in simulated BWR environment

Thin-walled cylindrical carbon steel specimens were thermally fatigued in a pressurized autoclave. Since high and low temperature pure water were alternately supplied into the autoclave, the specimens were subjected to homogeneous thermal stress through the wall thickness. The thermal fatigue life was defined as the number of cycles to crack penetration to the inside of the cylindrical specimen. The thermal fatigue strength was compared with the mechanical fatigue strength performed in air and in high temperature water. Even if taking account of the Higuchi-Iida formula, which considers the effects of strain rate, dissolved oxygen concentration and water temperature on fatigue life, the thermal fatigue lives of carbon steel were found to be slightly shorter than the mechanical fatigue lives.     

Temperature increase on target due to jet impingement under BWR/PWR loss of coolant accident conditions

This report describes the temperature increase on the target plate after jet impingement on it from a ruptured pipe under BWR/PWR Loss of Coolant Accident Conditions. From test results it is shown that the temperature on the target can be conservatively estimated by taking it equal to the saturated temperature corresponding to the pressure on the target, which is given by the steam table. An experimental formula is presented to estimate the maximum temperature increase on the target.     

Life prediction on SCC of solution annealed stainless steels under laboratory conditions

The stress corrosion cracking (SCC) behavior of austenitic stainless steels, types 304 and 316 has been investigated in acidic solutions to verify whether or not a parameter for prediction of time to failure can be detected as functions of applied stress and environmental factors (temperature, concentration, pH, anion species) by using a constant load. The results show that the steady state elongation rate becomes a parameter for predicting time to failure at a time within 10–20% of time to failure irrespective of the above factors. The steady state elongation rate is also found to become a parameter for the assessment of SCC susceptibility. On the basis of the results obtained, a SCC mechanism is discussed in terms of corrosion current density at crack tips, time to failure and length of crack propagation. The SCC mechanism proposed can be applied to both active path dissolution model and film rupture model.     

Assessment of the interchannel mixing model with a subchannel analysis code for BWR and PWR conditions

The influence of the interchannel mixing model employed in a traditional subchannel analysis code was investigated in this study, specifically on the analysis of the enthalpy distribution and critical heat flux (CHF) in rod bundles in BWR and PWR conditions. The equal-volume-exchange turbulent mixing and void drift model (EVVD) was embodied to the COBRA-IV-I code. An optimized model of the void drift coefficient has been devised in this study as the result of the assessment with the two-phase flow distribution data for the general electric (GE) 9-rod and Ispra 16-rod test bundles. The influence of the subchannel analysis model on the analysis of CHF was examined by evaluating the CHF test data in rod bundles representing PWR and BWR conditions. The CHFR margins of typical light water nuclear reactor (LWR) cores were evaluated by considering the influence on the local parameter CHF correlation and the hot channel analysis result. It appeared that the interchannel mixing model has an important effect upon the analysis of CHFR margin for BWR conditions.     

Effect of corrosion potential on the corrosion fatigue crack growth behaviour of low-alloy steels in high-temperature water

The low-frequency corrosion fatigue (CF) crack growth behaviour of different low-alloy reactor pressure vessel steels was characterized under simulated boiling water reactor conditions by cyclic fatigue tests with pre-cracked fracture mechanics specimens. The experiments were performed in the temperature range of 240-288 °C with different loading parameters at different electrochemical corrosion potentials (ECPs). Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by SEM were used to quantify the cracking response. In this paper the effect of ECP on the CF crack growth behaviour is discussed and compared with the crack growth model of General Electric (GE). The ECP mainly affected the transition from fast (‘high-sulphur’) to slow (‘low-sulphur’) CF crack growth, which appeared as critical frequencies ν_crit = f(ΔK, R, ECP) and ΔK-thresholds ΔK_EAC = f(ν, R, ECP) in the cycle-based form and as a critical air fatigue crack growth rate da/dt_Air,crit in the time-domain form. The critical crack growth rates, frequencies, and ΔK_EAC-thresholds were shifted to lower values with increasing ECP. The CF crack growth rates of all materials were conservatively covered by the ‘high-sulphur’ CF line of the GE-model for all investigated temperatures and frequencies. Under most system conditions, the model seems to reasonably well predict the experimentally observed parameter trends. Only under highly oxidizing conditions (ECP ? 0 mV_SHE) and slow strain rates/low loading frequencies the GE-model does not conservatively cover the experimentally gathered crack growth rate data. Based on the GE-model and the observed cracking behaviour a simple time-domain superposition-model could be used to develop improved reference CF crack growth curves for codes.