Initial Hydrogen Concentration Research Articles

Analysis of Characteristics of Spent Fuels on Long-Term Dry Storage Condition

Currently, the interim storage pools of spent fuels in South Korea are expected to become saturated from 2024. It is required to prepare an operation plan of a domestic dry storage facility during a long-term period, with the researches on safety evaluation methods. This study modified the FRAPCON code to predict the spent fuel integrity evaluation such as the axial cladding temperature, the hoop stress and hydrogen distribution in dry storage. The cladding temperature in dry storage was calculated using the COBRA-SFS code with the burnup information which was calculated using the FRAPCON code. The hoop stress was calculated using the ideal gas equation with spent fuel information such as rod internal pressure. Numerical analysis method was used to calculate the degree of hydrogen diffusion according to the hydrogen concentration and temperature distribution during a dry storage period. Before 50 years of dry storage, the cladding temperature and hoop stress decreased rapidly. However, after 50 years, they decreased gradually and the cladding temperature was below 400 K. The initial temperature distribution and hydrogen concentration showed a parabolic line, but hydrogen was transferred by the hydrogen concentration and temperature gradient over time.

Design of a non-destructive test for validating models of hydrogen migration

High-strength steels, despite their excellent mechanical properties in normal conditions, can be susceptible to hydrogen embrittlement. Due to the service loads or residual stresses, hydrogen migrates within the component and accumulates in the regions where the highest tensile hydrostatic stress occurs. As a consequence, component brittle failure can occur even if the initial or mean hydrogen concentration is lower than the critical value. The availability of models predicting the hydrogen diffusion within the component is a crucial task for the design. Several diffusive models have been presented in the literature and some general-purpose finite element codes have already implemented some of them. However, the validation of those models is still an open issue due to the difficulty in performing accurate local measurements of the hydrogen concentration. This study deals with the design of a test potentially able to validate hydrogen migration models. In the test, a four-point bending configuration is applied to a properly shaped hourglass specimen, previously charged with hydrogen, extracted from thin high-strength steel sheets. The specimen geometry and the loading configuration were designed to obtain a central region in which the stress and strain field is uniform in plane and exhibits a quasi-uniform gradient in the thickness direction. As a consequence, it is expected a large enough central region of the specimen in which the Hydrogen can migrate only in the thickness direction during the typical duration of the test. The local hydrogen concentration is evaluated by measuring the flux leaving the tensile surface of the specimen by a solid-state hydrogen sensor.

Models of hydrogen influence on the mechanical properties of metals and alloys

The article yields a survey of the key models of mechanics that are used to describe the effects of hydrogen embrittlement, hydrogen cracking, and hydrogen-induced destruction. The main attention is paid to models which are used to calculate the stress-strain state of metal samples, parts and machine components and have the potential for specific engineering applications. From a mechanical perspective, the effect of hydrogen on the material properties is a classic problem of the influence of a small parameter, since the hydrogen concentrations critical for the strength and ductility of metals are usually small. In the vast majority of models this effect is reduced to the hydrogen redistribution within the material volume and localization of concentrations in the critical fracture zones. The authors identified four main approaches that allow one to take into account the influence of a small parameter: (i) hydrogen-enhanced decohesion (HEDE), (ii) hydrogen-enhanced localized plasticity (HELP), (iii) account for additional internal pressure due to the hydrogen dissolved in metals, and (iv) bi-continuum approach that takes into account the internal hydrogen pressure and weakening of material in the framework of a special model of a solid. The links between the main approaches are established. Systematization of publications was carried out, similarities and differences in the description of the internal transport and accumulation of hydrogen in metals are highlighted. It is indicated that the predominant number of publications is devoted to the HEDE model, but so far there is no published data on the application of this model to real problems of engineering practice; only modeling the results of mechanical tests of cylindrical and prismatic samples were considered. In fact, other less popular approaches have more practical applications. The main unresolved issue in the verification of all models is the local concentration of hydrogen, which is a source of premature destruction of metals under load. All the methods for measuring local concentrations are indirect. Even in the case of applying sophisticated physical methods, mechanical surface preparation is required, which destroys the initial natural concentration of hydrogen. The lack of reliable data on the distribution of hydrogen concentration excludes the possibility of unambiguously determination of all the model parameters. On the one hand, it allows fitting to any experimental data, and on the other hand, it reduces the predictive engineering value of all models, since a qualitative fitting is not sufficient for engineering strength analysis.

Modeling of gas porosity and microstructure formation during dendritic and eutectic solidification of ternary Al-Si-Mg alloys

A two-dimensional (2-D) multi-component and multi-phase cellular automaton (CA) model coupled with the Calphad method and finite difference method (FDM) is proposed to simulate the gas pore formation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys. In this model, the pore growth, and dendritic and eutectic solidification are simulated using a CA technique. To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification, the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method. The diffusion equations of hydrogen and two solutes are solved by FDM. The developed CA-FDM coupled model can be used for simulating the evolution of gas microporosity and microstructures, involving dendrites and irregular binary and ternary eutectics, of ternary hypoeutectic Al-Si-Mg alloys. It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics, the competitive growth among the growing gas pores of different sizes, together with the time-evolving concentration fields of hydrogen and solutes. The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation. The influences of the initial hydrogen concentration and cooling rate on the microporosity formation are investigated. It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time. The varying features of simulated porosity percentage, the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores, while the size of gas pores will significantly reduce with increasing cooling rate, leading to a lower final porosity percentage.

A step toward the numerical simulation of catalytic hydrogenation of nitrobenzene in Taylor flow at practical conditions

While computational studies on hydrodynamics of gas-liquid Taylor flow in microreactors are increasing, inclusion of mass transfer and reactions pose a challenge. Especially simulations for technically relevant reactions are lacking. This study undertakes a step in this direction by considering the hydrogenation of nitrobenzene to aniline. Physical properties are estimated from literature and a lack of data for practically relevant temperature and pressure is identified. By neglecting any feedback on hydrodynamics, the hydrogenation of nitrobenzene in a micro-channel is studied by two-dimensional numerical simulations with a volume-of-fluid method using a one-step reaction mechanism. Simulations are performed for three different bubble velocities at intermediate capillary numbers. Increase of bubble velocity enhances mass transfer and aniline production rate due to intensified recirculation in the liquid slug. Even for the channel height of 100 µm investigated here, the reaction is mass transfer limited and independent on initial concentration of hydrogen in liquid nitrobenzene.

Oxidation induced cubic-tetragonal phase transformation in titanium hydride powders

We studied the effect of oxidation temperature and initial hydrogen concentration on the occurrence of cubic (δ) to tetragonal (ε) phase transformation induced by thermal oxidation of titanium hydride powders in air. X-ray diffraction analysis indicates that δ → ε transformation occurs during oxidation at 400 °C, but not at lower (300 °C) or higher (500 °C) oxidation temperatures. The relative amounts of hydrogen loss during oxidation were determined by temperature-programmed desorption mass spectrometry (TPD-MS). The findings suggest that oxygen stabilizes the tetragonal phase (ε), and that the δ/ε phase composition following oxidation depends on both oxygen and hydrogen concentrations. For high oxidation temperatures (500 °C), some degree of hydride decomposition leads to a decrease in H concentration and to destabilization of the tetragonal phase after cooling down to room temperature.

Determination of hydrogen embrittlement stress intensity threshold by fractography

Electroplating of aerospace parts generates hydrogen embrittlement, which is controlled by 200-h sustained-load tests performed on cylindrical notched samples. Those tests are costly and offer only a pass-or-fail assessment, with limited interpretability. In this article, we propose a new fractographic method to obtain a quantitative indication of the embrittlement severity of electroplated samples that failed the sustained-load test. On the fracture surfaces of failed samples, intergranular (IG) zones were observable at fracture initiation sites, surrounded by ductile zones, with intermediate regions containing mixed (ductile+IG) features. Within the framework of the recently proposed Hydrogen-Enhanced Plasticity-Mediated DEcohesion mechanism (HELP-mediated HEDE), we argue that there is a correlation between the intergranular appearance of the fracture surfaces and the subcritical nature of the crack propagation. Using this argument, we computed the stress intensity factor K for each crack and loading condition, with the input of intergranular zone sizes and finite element analysis. Minimum and maximum K values were identified and statistics of the extremes was used to calculate the minimum expected K values for various embrittlement conditions. It is proposed that this value is an estimate of the threshold for embrittlement, Kth. Values of Kth between 9 MPa m0.5 and 30 MPa m0.5 were measured for various plating conditions. Those estimations are more conservative than the thresholds reported in the literature, which were measured using more complex experimental procedures. Moreover, the fracture toughness estimated with the maximum K values was possibly affected by high initial hydrogen concentrations as per the HELP+HEDE embrittlement mechanism.

Modeling of microporosity formation and hydrogen concentration evolution during solidification of an Al–Si alloy**Project supported by the National Natural Science Foundation of China (Grant No. 51901148), the Fund of the State Key Laboratory of Solidification Processing (Northwestern Polytechnical University), China (Grant No. SKLSP202006), and the State Key Lab of Advanced Metals and Materials (University of Science and Technology Beijing), China

We simulate the evolution of hydrogen concentration and gas pore formation as equiaxed dendrites grow during solidification of a hypoeutectic aluminum–silicon (Al–Si) alloy. The applied lattice Boltzmann-cellular automaton-finite difference model incorporates the physical mechanisms of solute and hydrogen partitioning on the solid/liquid interface, as well as the transports of solute and hydrogen. After the quantitative validation by the simulation of capillary intrusion, the model is utilized to investigate the growth of the equiaxed dendrites and hydrogen porosity formation for an Al–(5 wt.%)Si alloy under different solidification conditions. The simulation data reveal that the gas pores favorably nucleate in the corners surrounded by the nearby dendrite arms. Then, the gas pores grow in a competitive mode. With the cooling rate increasing, the competition among different growing gas pores is found to be hindered, which accordingly increases the pore number density in the final solidification microstructure. In the late solidification stage, even though the solid fraction is increasing, the mean concentration of hydrogen in the residue melt tends to be constant, corresponding to a dynamic equilibrium state of hydrogen concentration in liquid. As the cooling rate increases or the initial hydrogen concentration decreases, the temperature of gas pore nucleation, the porosity fraction, and the mean porosity size decrease, whilst the mean hydrogen concentration in liquid increases in the late solidification stage. The simulated data present identical trends with the experimental results reported in literature.

Analysis of Energy Consumption of the Reduction of Fe2O3 by Hydrogen and Carbon Monoxide Mixtures

Energy consumption is directly related to the energy supply and production costs of gas-based direct reduction ironmaking, which is an effective choice to reduce the energy consumption of iron making. In this paper, the minimum Gibbs free energy principle was used to calculate the equilibrium composition under the conditions of reduction gas consisting of hydrogen and carbon monoxide (hydrogen concentration of 0–100%, reduction gas amount of 0–6.0 mol, reduction temperature of 790–1100 °C, and 0.5 mol Fe2O3). According to the enthalpy change, a simplified energy consumption model of a gas-based direct reduction ironmaking process was established, and the energy consumption per mole of metallic iron produced was calculated in detail. The following conclusions were drawn: at the stage when the reduction reaction occurred, the utilization rate of hydrogen or carbon monoxide remained unchanged with the increase in the amount of reduction gas or the increase in the hydrogen concentration of initial gas. The direct energy consumption increased with the increase in the hydrogen concentration at 790–980 °C and the opposite was true at 980–1100 °C. At 790–980 °C, the total energy consumption per ton of iron was greater than 0 and increased with the increase in initial hydrogen concentration from 40% to 100%, and it was less than 0 and increased with the increase in initial hydrogen concentration from 0% to 30%. It was possible to achieve zero total energy consumption with a hydrogen concentration of 30% and a 973 °C reduction.

Four-point-bend tests on high-burnup advanced fuel cladding tubes after exposure to simulated LOCA conditions

ABSTRACT To evaluate the fracture resistance of high-burnup advanced fuel cladding tubes during the long-term core cooling period following loss-of-coolant accidents (LOCAs), laboratory-scale four-point-bend tests were performed using the following advanced fuel cladding tubes with burnups of 73–84 GWd/t: low-tin ZIRLOTM, M5®, and Zircaloy-2 (LK3). Three four-point-bend tests were performed on the high-burnup advanced fuel cladding tube specimens subjected to the integral thermal shock tests which simulated LOCA conditions (ballooning and rupture, oxidation in high-temperature steam, and quench). During the four-point-bend tests, all the specimens that were oxidized at 1474 K to 9.9%–21.5% equivalent cladding reacted exhibited brittle fractures. The maximum bending moments were comparable to those of the conventional Zircaloy cladding tube specimens. Furthermore, the effects of oxidation and hydriding on the maximum bending moment were comparable between the high-burnup advanced fuel cladding tube specimens and the unirradiated Zircaloy-4 cladding tube specimens. Therefore, it can be concluded that the post-LOCA fracture resistance of fuel cladding tubes is not significantly reduced by extending the burnup to 84 GWd/t and using the advanced fuel cladding tubes, though it may slightly decrease with increasing initial hydrogen concentration in a relatively lower ECR range (< 15%), as observed for the unirradiated Zircaloy-4 cladding tubes.

Hydrogen detection using membrane-type surface stress sensor

This study shows a possibility of the application of a membrane-type surface stress sensor (MSS) with a Pd film to a hydrogen sensor. It was able to detect hydrogen concentrations from 5 to 40000 ppm in a nitrogen gas mixture. In the case of a conventional sensor using a hydrogen-occluding material, it is necessary to wait for a state of saturation. In contrast, the proposed method can detect hydrogen quickly by the initial rate of hydrogen absorption. The relationship between the initial absorption rate and hydrogen concentration is explained by considering the two-step reaction kinetics of hydrogen absorption into bulk Pd via Langmuir dissociative adsorption on surface.

Effect of microstructural inhomogeneity on hydrogen diffusion in welded joints

Hydrogen is a main factor in cold cracking or hydrogen-induced cracking. The most crack susceptible region of a steel welded joint is the heat affected zone (HAZ). Formulation and functional-analytical solution of the one-dimensional problem of hydrogen diffusion in an inhomogeneous butt-welded joint taking into account weld and joint dimensions as well as hydrogen diffusion coefficients and solubilities are presented. It is shown that the peak hydrogen concentration in the HAZ of inhomogeneous joints varies in direct proportion to the initial hydrogen concentration in the weld metal. Peak hydrogen concentration is inversely proportional to the ratio of hydrogen solubilities in the weld and HAZ and the square root of diffusion coefficient ratio of the HAZ and weld. The peak hydrogen concentration in the HAZ can exceed 80% of the initial concentration in the weld if martensitic steel is welded using low-carbon low-alloy welding consumables. Utilization of austenitic consumables leads to reduction in hydrogen concentration in the HAZ by 75–85% in comparison with non-austenitic consumables.

Development of a numerical model for hydrogen bubble generation, dynamics and trapping during solidification of aluminium alloys through Eulerian-Lagrangian framework

ABSTRACTIn the present study, the authors developed a new numerical model to predict the bulk fluid flow, heat transfer, hydrogen species transfer with the hydrogen bubble generation, dynamics and trapping during solidification of aluminium through the continuum theory with Lagrangian bubble dynamics. The generated bubble size is assumed to be dependent on the microstructure. The bubble drag force is newly proposed in the Euler-Lagrange framework. In a model casting system, almost all generated hydrogen bubbles are trapped into a solidified aluminium. However, the hydrogen bubbles are capable of generating in the melt phase and detaching from the mushy zone when the initial hydrogen concentration is high. In this case, the hydrogen bubbles move upward due to buoyancy and this movement is slightly affected by the surrounding melt flow.

Evaluation of material susceptibility to hydrogen embrittlement (HE): An approach based on experimental and finite element (FE) analyses

A stress coupled hydrogen diffusion finite element model has been developed to simulate incremental step load (ISL) test. It is further integrated with a cohesive zone model (CZM) based on a stiffness-separation law. The influence of hydrogen on the cohesive law has been implemented through a newly proposed approach rooted in the hydrogen enhanced decohesion (HEDE) mechanism. Finally, this coupled model has been utilized to assess hydrogen embrittlement (HE) susceptibility of materials through the prediction of crack initiation time. The crack initiation time decreases with the increase of initial hydrogen concentration in a quench and tempered 36NiCrMo4 (4340) steel showing a ductile-brittle transition behavior. Similar observations were done from ISL experiments. In addition to initial hydrogen concentration, the influence of yield strength, fracture toughness and hydrogen diffusivity on material susceptibility was also studied. Three different materials-iron, niobium, 304 stainless steel in addition to 36NiCrMo4 (4340) steel were selected for the study. Iron shows better resistance to HE failure owing to its low yield strength and high fracture toughness while 304 austenitic stainless steel reveals least susceptibility to hydrogen induced cracking. This is largely attributed to the lower diffusivity of hydrogen in 304 stainless steel.

Self-Gathering Effect of the Hydrogen Diffusion in Welding Induced by the Solid-State Phase Transformation.

The hydrogen diffusion in welding was investigated by using thermal-mechanical-hydrogen diffusion sequential coupled procedures based on finite element method. A self-gathering effect induced by the solid-state phase transformation was discovered. Because of the self-gathering effect, the hydrogen concentration in weld metal was accumulated to a peak value which can be larger than the initial hydrogen concentration in molten pool, and subsequently the hydrogen concentration in heat affect zone was redistributed. In multi-pass welding, the gathered effect not only happened inside a weld pass, but also in the inter-pass, which further increased the sensitivity of the hydrogen-assisted cold cracking. Controlling should be adopted to restrain the hydrogen accumulation. Welding stress evolution during the solid-state phase transformation process had limited effect on the hydrogen diffusion.

A Hydrogen Atom above a Plane with the “Not Going Through” Boundary Condition

This article studies the behavior of the electronic levels of a hydrogen atom in a space bounded by a flat surface on which a condition of the spatial confinement of electronic wave functions is specified. It is shown that a significant amount of energy can be released during adsorption of hydrogen atoms by a sample with a clean smooth surface, high affinity, and low initial internal hydrogen concentration. The results are obtained using both direct numerical calculation using the finite element method and variational estimates based on the selection of special test functions.

Fracture limit of high-burnup advanced fuel cladding tubes under loss-of-coolant accident conditions

ABSTRACTTo evaluate the fracture limit of high-burnup advanced fuel cladding tubes under loss-of-coolant accident (LOCA) conditions, laboratory-scale integral thermal shock tests were performed using the following advanced fuel cladding tubes with burnups of 73–85 GWd/t: M-MDATM, low-tin ZIRLOTM, M5®, and Zircaloy-2 (LK3). In total eight integral thermal shock tests were performed for these fuel cladding tube specimens, simulating LOCA conditions including ballooning and rupture, oxidation, hydriding, and quenching. During the tests, the specimens were oxidized to 10%–30% equivalent cladding reacted (ECR) at approximately 1473 K and were quenched under axial restraint load of approximately 520–530 N. The effects of burnup extension and use of the advanced fuel cladding tubes on the ballooning and rupture, oxidation, and hydriding under LOCA conditions were inconsiderable. Further, the high-burnup advanced fuel cladding tube specimens did not fracture in the ECR values equal to or lower than the fracture limits of the unirradiated Zircaloy-4 cladding tube reported in previous studies. Therefore, it can be concluded that the fracture limit of fuel cladding tubes is not significantly reduced by extending the burnup to approximately 85 GWd/t and using the advanced fuel cladding tubes, though it slightly decreases with increasing initial hydrogen concentration.

Effect of energy parameters of pulsed laser welding of Zr-1%Nb alloy on metal contamination with gases and properties of welds

Effect of pulsed laser welding parameters (pulse energy and duration, frequency, welding speed and, as a result, overlapping factor) on properties of E110 (Zr-1%Nb) alloy welds is presented. Butt welded joints of two identical plates with a thickness of 0.5 mm were investigated. EDS analysis of a surface layer of the welds was done, initial hydrogen concentration in the weld metal was measured and influence of absorbed gases (oxygen, nitrogen, carbon and hydrogen) on tensile strength, microhardness, hydrogen absorption rate and corrosion resistance of the welds was evaluated. The weld metal contained initial hydrogen concentration within the limits set by standards. However, mixing argon flows with atmospheric gases caused the weld metal contamination with oxygen, nitrogen and carbon. As a result, inclusions of complex chemical composition and surface branching microcracks were formed. They were stress raisers where weld failures occurred. Furthermore, it was the reason of increased hydrogen absorption rate and decreased corrosion resistance of the weld metal. The obtained results showed that absence of discoloration on surfaces of the welded joints did not guarantee metal purity and, as a consequence, required performance properties. Future research is required to determine the optimal overlapping factor; shielding gas compositions (Ar, He or their mixture) and flow rates; configuration and inclination angle of nozzles as well as distance from the nozzles to the weld zone.

Prediction Model for Weld Hydrogen Cracking in High Strength Steel Weld

Prediction model for weld hydrogen cracking (so called cold cracking) in high strength steel weld was developed by a coupled thermo-elastic-plastic and hydrogen diffusion analysis in the y-grooved weld joint. Critical conditions of cracking was given as the function of principal stress and accumulated hydrogen concentration in the root region where the cracking occurs. In order to clarify the critical conditions of cold cracking, y-grooved cold cracking tests were first conducted using the steel plate with tensile strength level of 780MPa. Plate thickness of the plates were 25 mm and 50 mm. Hydrogen concentration in the weld metal was changed by applying hydrogen into the shield gas in the gas-metal arc welding (GMAW), and different pre-heating conditions were applied to evaluate the critical pre-heating temperature to prevent cold cracking. It was found that the critical pre-heating temperature was increased with thicker plate thickness and higher hydrogen concentration in the weld metal. The cracks were initiated at the weld metal in the root region which exhibits significant tensile residual stress by the welding. Hydrogen accumulation into the root region may have enhanced cracking. Based on the above y-grooved weld cold cracking tests, a simulation model for estimating the critical pre-heating temperature was developed. The y-grooved weld joints with different plate thickness were modeled by the FEM with the actual weld configuration and material properties in the base metal, heat affected zone (HAZ) and weld metal. Hydrogen was put in the weld metal as an initial condition, then welding simulation was conducted by coupled thermo-elastic-plastic and hydrogen diffusion analysis. In the hydrogen diffusion analysis, the α-multiplication method, which stress gradient term in the diffusion law was multiplied, was applied to express realistic hydrogen diffusion enhanced by hydrostatic stress field. Hydrogen accumulation occurred in the root region where showed highest hydrostatic stress. The point where showed the hydrogen accumulation was well corresponded to the crack initiation site. It was found that cracking was enhanced with thicker plate and higher initial hydrogen concentration because of higher tensile residual stress and higher hydrogen accumulation in the root region, respectively. These models were implemented into the Materials Integration (MI) System to estimate the critical pre-heating temperature in the y-grooved weld tests with different welding conditions. Although, further validation and optimization are needed for the models, this hydrogen cracking prediction system has a lot of potential as a material development support tool for reducing experimentation and speeding up the development.

Cohesive zone modeling of hydrogen-induced delayed intergranular fracture in high strength steels

A sequentially coupled hydrogen diffusion-cohesive zone modeling approach was applied to the simulation of hydrogen-induced delayed intergranular (IG) fracture in high-strength low-alloy steels. The effects of multiple hydrogen trap sites and mechanical deformation on the diffusion and cohesive strength of grain boundaries (GB) were taken account, in order to reveal that the hydrogen trapped at GB play a dominant role in the degradation processes of hydrogen of high-strength low-alloy steels, which leads to the IG fracture. The approach was implemented by Abaqus software in the form of a two-steps procedure including the coupled elastoplastic-transient hydrogen diffusion analysis and cohesive stress analysis. To validate the approach, the constant load tests of hydrogen pre-charged AISI 4135 high-strength low-alloy steel notched bars in literature were analyzed. Good agreement is observed between the simulation and experimental data of time to failure. The results confirm that hydrogen-induced IG fracture of high strength low-alloy steels can be related to the hydrogen concentration trapped at GB. The critical hydrogen concentration at GB for crack initiation is independent of the initial hydrogen concentration but depends strongly on the local stress level and stress triaxiality. The critical hydrogen concentration linearly decreases with increasing normalized peak maximal principal stress normalized by the critical cohesive strength in absence of hydrogen.