Vacuum Hot Extraction Research Articles

Kinetics of hydrogen absorption/desorption in the Zr-2.5Nb alloy.

The Zr-2.5Nb alloy is a typical pressure tube material in heavy water nuclear reactors, and an increase of hydrogen isotope content in the alloy during service can pose major safety risks; hot vacuum extraction-mass spectrometry is an efficient method for evaluating hydrogen isotope concentrations in the Zr-2.5Nb alloy. This work investigates the kinetics and thermodynamic properties of deuterium (D) absorption and desorption of the Zr-2.5Nb alloy using the constant volume adsorption method and the hot vacuum extraction method. In addition to the previously reported volume contraction model, it was observed that at 600 °C and above, the reaction between D2 and Zr-2.5Nb is dominated by diffusion, while the reaction is predominantly influenced by surface adsorption and dissociation below 600 °C. Phase transition sequence of Zr-2.5Nb deuterides during non-isothermal desorption was established using quantitatively calibrated thermal desorption spectra combined with the phase transition process of deuteride decomposition. These results can provide important references for optimizing the process parameters of the hot vacuum extraction-mass spectrometry method.

Study of oxidation behavior and associated microstructure evolution of Zircaloy-4 exposed to high temperature pure steam

Oxidation reaction is inevitably consuming the thickness of load bearing matrix of Zircaloy-4 and releases hydrogen that can be ingress into the matrix, thereby resulting in the degradation of its structural integrity. The present work aims to simultaneously investigate the oxidation behavior, hydrogen pickup, and the associated microstructure evolution of Zircaloy-4 exposed to 800 °C–1000 °C pure steam. Gain weight, growth of the oxide layer and α-Zr(O) layer in Zircaloy-4 follow the parabolic law. The intensification of oxide corrosion versus temperature and duration time is attributed to the severe Sn segregation that leads to the oxide layer delamination. α-Zr(O) layer just appeared when the oxidation temperature does not exceed the phase transition temperature. Radial cracks are prone to initiating at the concave position of the oxide/metal undulation interface. The hydrogen pickup measured by vacuum hot extraction was no more than 40 ppm, and no precipitated hydrides were identified. A mild Sn segregation in the outer oxide layer has a small influence in deteriorating the outer oxide layer protectiveness, the hinder effect of an unbroken outer oxide layer to the diffusion of hydrogen into the α-Zr(O) layer and matrix metal, and then resulting in a smaller influence of hydrogen embrittlement in this study.

Sintering and foaming of bioactive glasses

AbstractSintering, crystallization, and foaming of 44.8SiO2–2.5P2O3–36.5CaO–6.6Na2O–6.6K2O–3.0CaF2 (F3) and 54.6SiO2–1.7P2O3–22.1CaO–6.0Na2O–7.9K2O–7.7MgO (13–93) bioactive glass powders milled in isopropanol and CO2 were studied via heating microscopy, differential thermal analysis, vacuum hot extraction (VHE), Infrared spectroscopy, and time‐of‐flight secondary ion mass spectrometry. Full densification was reached in any case and followed by significant foaming. VHE studies show that foaming is driven by carbon gases and carbonates were detected by Infrared spectroscopy to provide the major foaming source. Carbonates could be detected even after heating to 750°C, which hints on a thermally very stable species or mechanical trapping. Otherwise, dark gray compact colors for milling in isopropanol indicate the presence of residual carbon as well. Its significant contribution to foaming, however, could not be proved and might be limited by the diffusivity of oxygen needed for carbon oxidation to carbon gas.

Foaming Species and Trapping Mechanisms in Barium Silicate Glass Sealants

Barium silicate glass powders 4 h milled in CO2 and Ar and sintered in air are studied with microscopy, total carbon analysis, differential thermal analysis (DTA), vacuum hot extraction mass spectroscopy (VHE‐MS), Fourier‐transformed infrared (FTIR) spectroscopy, X‐ray photoelectron spectroscopy (XPS), and time‐of‐flight secondary‐ion mass spectrometry (TOF−SIMS). Intensive foaming of powder compacts is evident, and VHE studies prove that foaming is predominantly caused by carbonaceous species for both milling gases. DTA shows that the decomposition of BaCO3 particles mix‐milled with glass powders occurs at similar temperatures as foaming of compacts. However, no carbonate at the glass surface could be detected by FTIR spectroscopy, XPS, and TOF−SIMS after heating to the temperature of sintering. Instead, CO2 molecules unable to rotate identified by FTIR spectroscopy after milling, probably trapped by mechanical dissolution into the glass bulk. Such a mechanism or microencapsulation in cracks and particle aggregates can explain the contribution of Ar to foaming after intense milling in Ar atmosphere. The amount of CO2 molecules and Ar, however, cannot fully explain the extent of foaming. Carbonates mechanically dissolved beneath the surface or encapsulated in cracks and micropores of particle aggregates are therefore probably the major foaming source.

Studies on diffusion of hydrogen in PWR clad material

Zr-1%Nb alloy is used as cladding material in pressurised water reactors (PWR). Diffusion of hydrogen in this material is one of the important input parameters required for simulating the kinetics of hydrogen embrittlement. For this purpose, one end of cut piece of Zr-1%Nb alloy clad tube of PWR was polished, electrochemically charged with hydrogen and then annealed at required temperatures and time. Hydrogen content was determined in cut pieces of clad tube in axial direction employing hot vacuum extraction quadrupole mass spectrometry (HVE-QMS) technique. From the hydrogen profile in axial direction, diffusion coefficient of hydrogen was estimated employing the solution of Fick’s second law of diffusion with semi-infinite system boundary conditions. Activation energy (Ea) for diffusion of hydrogen in Zr-1%Nb was calculated from the slope of the Arrhenius plot between Log DH and 1/T. The obtained D0 and Ea are well in the range of expectations based on the literature data in various Zr based materials by considering micro-constituents in these materials.

Nondestructive hydrogen analysis of steam-oxidized Zircaloy-4 by wide-angle neutron scattering

A nondestructive neutron scattering method to precisely measure the hydrogen content in high-temperature steam-oxidized Zircaloy-4 cladding was developed. Zircaloy-4 cladding was used to produce hydrided specimens with hydrogen content up to ≈500 wppm. Following hydrogen charging, the hydrogen content of the hydrided specimens was measured using the vacuum hot extraction method, by which the samples with desired hydrogen concentrations were selected for the neutron study. The hydrided samples were then oxidized in steam up to ≈6.0 wt. % at 1100 °C. Optical microscopy shows that our hydriding procedure results in uniform distribution of circumferential hydrides across the wall thickness, and uniform oxide layers were formed on the sample surfaces by the steam oxidation. Small- and wide-angle neutron scattering were simultaneously performed to provide a quick (less than an hour per sample) measurement of the hydrogen content in various types of hydrided and oxidized Zircaloy-4. Our study demonstrates that the hydrogen in pre-oxidized Zircaloy-4 cladding can be measured very accurately by both small- and wide-angle neutron scattering. For steam-oxidized samples, the small-angle neutron scattering is contaminated with coherent scattering from additional structural features induced by the steam oxidation. However, the scattering intensity of the wide-angle neutron scattering increases proportionally with the hydrogen charged in the samples. The hydrogen content and wide-angle neutron scattering intensity are highly linearly correlated for the oxidized cladding samples examined in this work, and can be used to precisely determine the hydrogen content in steam-oxidized Zircaloy-4 samples. Hydrogen contents determined by neutron scattering of oxidation samples were also found to be consistent with the results of chemical analysis within acceptable margins for error.

The role of hydrogen in zirconium alloy corrosion

Hydrogen enters zirconium metal as a result of the corrosion process and forms hydrides when present in quantities above the solubility limit at a given temperature. Zircaloy-4 coupons of different thicknesses (0.4 mm-2.3 mm) but identical chemistry and processing were corroded in autoclave at 360 °C for various times up to 2800 days. Coupons were periodically removed and weighed to determine weight gain, which allows follow of the corrosion kinetics. Coupon thickness differences resulted in different volumetric concentrations of hydrogen, as quantified using hot vacuum extraction. The thinnest coupons, having the highest concentration of hydrogen, demonstrated acceleration in their corrosion kinetics and shorter transition times when compared to thicker coupons. Furthermore, it was seen that the post-transition corrosion rate was increased with increasing hydrogen concentration. Corrosion rates increased only after the terminal solid solubility (TSS) was exceeded for hydrogen in Zircaloy-4 at 360 °C. Therefore, it is hypothesized that the corrosion acceleration is caused by the formation of hydrides. Scanning electron microscope (SEM) examinations of fractured oxide layers demonstrate the oxide morphology changed with hydrogen content, with more equiaxed oxide grains in the high hydrogen samples than in those with lower hydrogen content. Additionally, locations of advanced oxide growth were correlated with locations of hydrides in the metal. A hypothesis is proposed to explain the accelerated corrosion due to the presence of the hydrides, namely that the metal, locally, is less able to accommodate oxide growth stresses and this leads to earlier loss of oxide protectiveness in the form of more frequent oxide kinetic transitions.

Application of multichannel diffusion model to analysis of hydrogen measurements in solid

Measurements have been performed of the hydrogen contents in reference samples of aluminum alloy using the methods of vacuum hot extraction, vacuum melting and melting into the graphite crucible in the flux of gas carrier or argon carrier fusion method. The effect of additional “squeezing out” of hydrogen from the metal under its crystallization was not observed. It has been shown that during the sample heating time typical for the analysis by argon carrier fusion method the bound hydrogen with high activation energy is not extracted completely. This leads to errors in performed measurements. It has been proved that the mean velocity of hydrogen transport and mean coefficients of diffusion depend on the initial population of the energy levels, and they can vary for the same material.

Sintering and Foaming of Barium Silicate Glass Powder Compacts

The manufacture of sintered glasses and glass-ceramics, glass matrix composites and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, DTA, Vacuum Hot Extraction (VHE) and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere sig¬ni¬ficantly affected foaming. The strength of this effect increased in the order Ar  N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.

Multichannel diffusion vs TDS model on example of energy spectra of bound hydrogen in 34CrNiMo6 steel after a typical heat treatment

The paper is concerned with determination of the energy spectra of hydrogen in steels by the Vacuum Hot Extraction method (VHE). The investigation is performed on example of 34CrNiMo6 steel and the benchmark study of Vacuum Hot Extraction method and Thermal Desorption Spectroscopy (TDS) is provided. The spectrum of the hydrogen binding energy and the hydrogen concentration in samples are identified. Two additional energy levels are found out which are possibly related to strong covalent bonds. An advantage of VHE in comparison with TDS is that VHE approach does not require pre-saturation of samples by hydrogen. It keeps the original structure of the samples and ensures that all types of traps are not destroyed in tests. Additionally, the present approach opens new opportunity of accurate analysis on the base of the model of hydrogen multichannel diffusion in steels.

Sintering and Foaming of Barium and Calcium Silicate Glass Powders

Sintering and foaming of barium and calcium silicate glass powder compacts have been studied for different powder milling. Sintering was measured by means of heating microscopy backed up by XRD, DTA, Vacuum Hot Extraction (VHE) and electron microscopy. Foaming intensity strongly increased with decreasing glass particle size. Although powder compacts were uniaxially pressed and sintered in ambient air, foaming was affected by the milling atmosphere and most intensive for milling in CO2. Conformingly, VHE studies revealed that foaming of fully sintered samples was mainly driven by CO2, even for powders milled in technical air, Ar and N2. Prolonged storage of air milled barium silicate glass powders in ambient air before pressing and sintering caused further increase of foaming intensity. These findings indicate that carbonaceous species are preferentially trapped to or close beneath the powder surface during milling and later storage. The temperature range of CO2 degassing from fully sintered barium and calcium silicate glass powder compacts fits the temperature ranges of decomposition of BaCO3 and CaCO3 mix-milled with the respective barium and calcium silicate glass powders.

Fast, quantitative, and nondestructive evaluation of hydrided LWR fuel cladding by small angle incoherent neutron scattering of hydrogen

A nondestructive neutron scattering method to precisely measure the uptake of hydrogen and the distribution of hydride precipitates in light water reactor (LWR) fuel cladding was developed. Zircaloy-4 cladding used in commercial LWRs was used to produce hydrided specimens. The hydriding apparatus consists of a closed stainless-steel vessel that contains Zr alloy specimens and hydrogen gas. Following hydrogen charging, the hydrogen content of the hydrided specimens was measured using the vacuum hot extraction method, by which the samples with desired hydrogen concentrations were selected for the neutron study. Optical microscopy shows that our hydriding procedure results in uniform distribution of circumferential hydrides across the wall thickness. Small angle neutron incoherent scattering was performed in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Our study demonstrates that the hydrogen in commercial Zircaloy-4 cladding can be measured very accurately in minutes by this nondestructive method over a wide range of hydrogen concentrations from a very small amount (≈20ppm) to over 1000ppm. The hydrogen distribution in a tube sample was obtained by scaling the neutron scattering rate with a factor determined by a calibration process using standard, destructive direct chemical analysis methods on the specimens. This scale factor can be used in future tests with unknown hydrogen concentrations, thus providing a nondestructive method for determining absolute hydrogen concentrations.

Experimental determination of parameters of multichannel hydrogen diffusion in solid probe

The multi-channel diffusion mechanism is modeled by diffusion equations.The method for analysis of experimental dynamical curves of vacuum hot extraction (VHE) is developed, which allows to determine the binding energy and diffusion constants of hydrogen in the probe under study.The experimental data have been obtained using industrial hydrogen analyzer, which allows to carry out the absolute measurements of the dynamical curves vacuum hot extraction of the hydrogen from a solid probe. Experimental verification of mathematical models gives adequate results.The differences of the proposed approach with the method of TDS are discussed.

Hydrogen pickup measurements in zirconium alloys: Relation to oxidation kinetics

The optimization of zirconium-based alloys used for nuclear fuel cladding aims to reduce hydrogen pickup during operation, and the associated cladding degradation. The present study focuses on precisely and accurately measuring hydrogen pickup fraction for a set of alloys to specifically investigate the effects of alloying elements, microstructure and corrosion kinetics on hydrogen uptake. To measure hydrogen concentrations in zirconium alloys two techniques have been used: a destructive technique, Vacuum Hot Extraction, and a non-destructive one, Cold Neutron Prompt Gamma Activation Analysis. The results of both techniques show that hydrogen pickup fraction varies significantly with exposure time and between alloys. A possible interpretation of the results is that hydrogen pickup results from the need to balance charge. That is, the pickup of hydrogen shows an inverse relationship to oxidation kinetics, indicating that, if transport of charged species is rate limiting, oxide transport properties such as oxide electronic conductivity play a key role in the hydrogen pickup mechanism. Alloying elements (either in solid solution or in precipitates) would therefore impact the hydrogen pickup fraction by affecting charge transport.

Nondestructive Evaluation on Hydrided LWR Fuel Cladding by Small Angle Incoherent Neutron Scattering of Hydrogen

AbstractA non-destructive neutron scattering method was developed to precisely measure the uptake of total hydrogen in nuclear grade Zircaloy-4 cladding. The hydriding apparatus consists of a closed stainless steel vessel that contains Zircaloy-4 specimens and hydrogen gas. By controlling the initial hydrogen gas pressure in the vessel and the temperature profile, target hydrogen concentrations from tens of ppm to a few thousands of ppm have been successfully achieved. Following hydrogen charging, the hydrogen content of the hydrided specimens was measured using the vacuum hot extraction method (VHE), by which the samples with desired hydrogen concentration were selected for the neutron study. Small angle incoherent neutron scattering (SAINS) were performed in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Our study indicates that a very small amount (≈ 20 ppm) hydrogen in commercial Zircaloy-4 cladding can be measured very accurately in minutes for a wide range of hydrogen concentration by a nondestructive method. The hydrogen distribution in a tube sample was obtained by scaling the neutron scattering rate with a factor, which is determined by calibration process with direct chemical analysis method on the specimen. This scale factor can be used for future test with unknown hydrogen concentration, thus provide a nondestructive method for absolute hydrogen concentration determination.

Cold neutron prompt gamma activation analysis, a non-destructive technique for hydrogen level assessment in zirconium alloys

We propose a novel use of a non-destructive technique to quantitatively assess hydrogen concentration in zirconium alloys. The technique, called Cold Neutron Prompt Gamma Activation Analysis (CNPGAA), is based on measuring prompt gamma rays following the absorption of cold neutrons, and comparing the rate of detection of characteristic hydrogen gamma rays to that of gamma rays from matrix atoms. Because the emission is prompt, this method has to be performed in close proximity to a neutron source such as the one at the National Institute of Technology (NIST) Center for Neutron Research. Determination shown here to be simple and accurate, matching the results given by usual destructive techniques such as Vacuum Hot Extraction (VHE), with a precision of ±2mgkg−1 (or wtppm). Very low levels of hydrogen (as low as 5mgkg−1 (wtppm)) can be detected. Also, it is demonstrated that CNPGAA can be applied sequentially on an individual corrosion coupon during autoclave testing, to measure a gradually increasing hydrogen concentration. Thus, this technique can replace destructive techniques performed on “sister” samples thereby reducing experimental uncertainties.

Thermo Analytic Investigation of Hydrogen Effusion Behavior – Sensor Evaluation and Calibration

AbstractThe well established carrier gas analysis (CGA) method was used to test different hydrogen detectors comprising a thermal conductivity detector (TCD) and a metal oxide semiconducting (MOx) sensor. The MOx sensor provides high hydrogen sensitivity and selectivity, whereas the TCD exhibits a much shorter response time and a linear hydrogen concentration dependency. Therefore, the TCD was used for quantitative hydrogen concentration measurements above 50 µmol/mol. The respective calibration was made using N2/H2 gas mixtures. Furthermore, the hydrogen content and degassing behaviour of titanium hydride (TiH2‐x) was studied. This material turned out to be a potential candidate for a solid sample calibration. Vacuum hot extraction (VHE) coupled with a mass spectrometer (MS) was then calibrated with TiH2‐x as transfer standard. The calibration was applied for the evaluation of the hydrogen content of austenitic steel samples (1.4301) and the comparison of CGA‐TCD and VHE‐MS.

Release of gases from uranium metal at high temperatures

Depending on the ambient environmental conditions, different gaseous species could get entrapped in uranium metal ingots or pellets. On heating, melting or vapourising uranium metal, these get released and depending on the composition, may cause detrimental effects either within the metal matrix itself or on the surrounding materials/environment. For instance, these gases may affect the performance of the uranium metal, which is used as fuel in the heavy water moderated research reactors, CIRUS and DHRUVA. Hence, detailed investigations have been carried out on the release of gases over a temperature range 875–1500 K employing hot vacuum extraction technique, in specimen uranium pellets made from uranium rods/ingots. Employing an on-line quadrupole mass spectrometer, the analysis of released gases was carried out. The isobaric interference between carbon monoxide and nitrogen at m/ e = 28 in the mass spectrometric analysis has been resolved by considering their fragmentation patterns. Since no standards are available to evaluate the results, only the reproducibility is tested. The precision (relative standard deviation at 3 σ level) of the method is ±5%. The minimum detectable gas content employing the method is 5.00 × 10 −09 m 3. About 4 × 10 −04 m 3/kg of gas is released from uranium pellets, with hydrogen as the main constituent. The gas content increases with storage in air.

Occluded Gas Content in Fast Breeder Reactor Fuels

A comprehensive analytical methodology based on hot vacuum extraction principle was developed for the determination of total/occluded gas content in ceramic fast reactor nuclear fuels such as (U, Pu)C and (U, Pu)O2 of different bulk compositions. In view of the stringent specifications imposed on these fuels with respect to total gas, detailed investigations were carried out to understand and interpret the implications of the definition of the occluded gas content. These investigations include selecting the temperature and duration of heating, method of gas collection (with and without gas refrigeration) as well as measuring the gas composition employing quadrupole mass spectrometer. Hydrogen was found to be the major constituent of the occluded gas (>95%) followed by minor quantities of N2, CO and CO2. These observations showed the necessity of introducing a vacuum degassing step after sintering and also helped in redefining the specification of total gas content. The total gas content is less than 0.1 cm3.g−1 excluding hydrogen. The measurement error is ±5%.

Effect of β-Zr decomposition on the solubility limits for H in Zr–2.5Nb

The zirconium alloy Zr–2.5Nb (Zr–2.5 wt.% Nb) is used to fabricate pressure tubes for CANDU®11CANDU® (CANada Deuterium Uranium) is a registered trademark of AECL (Atomic Energy of Canada Limited). nuclear reactors. The microstructure of the as-extruded tubes consists of elongated h.c.p. α-Zr grains surrounded by a network of b.c.c. β-Zr containing at least 20 wt.% Nb. The β-Zr phase is meta-stable at temperatures below 900 K and gradually dissociates into its constituent elements α-Zr and β-Nb. The aim of this study was to determine the effect of β-Zr decomposition on the terminal solid solubility for dissolution (TSSD) for H in Zr–2.5Nb. Specimens of Zr–2.5Nb were charged to different hydrogen concentrations and aged at temperatures ranging from 673 to 773 K for 30 to 6000 min, to decompose their β-Zr component by different amounts. Hydride dissolution temperature and hydrogen concentration were determined using differential scanning calorimetry and hot vacuum extraction mass spectrometry, respectively. The results show that the as-extruded pressure tube structure, with the β-Zr almost intact, has the highest TSSD and that the TSSD for structures with fully decomposed β-phase is the lowest and approaches that of unalloyed Zr.