Stress-reorientation of hydrides and hydride embrittlement of Zr-2.5 wt% Nb pressure tube alloy

Hydrogen in excess of the terminal solid solubility precipitates out as a brittle hydride phase in zirconium alloys. The hydrides acquire platelet shaped morphology due to their accommodation in the matrix and can cause severe embrittlement, especially when these are oriented normal to the tensile stress axis. The precipitation of hydride platelets normal to the tensile stress when cooled under stress from a solution-annealing temperature is commonly referred to as ‘stress-reorientation’. The stress-reorientation is associated with a threshold stress below which no reorientation is observed. In this work, stress-reorientation of hydrides was investigated for unirradiated, cold worked and stress-relieved Zr-2.5 wt% Nb pressure tube material for a reorientation temperature of 423-723 K. The effect of the reoriented hydrides on the tensile properties of the Zr-2.5 wt% Nb pressure tube alloy was evaluated in the temperature range of 298-573 K.     

Correlation between microstructure and corrosion behavior of Zr-Nb binary alloy

To investigate the correlation between microstructure and corrosion characteristics of Zr-Nb alloy, the microstructural observation and corrosion test with the change of cooling rate from beta temperature and the variation of Nb content were performed. The oxide characterization was also carried out by synchrotron XRD and TEM. When the Nb is contained less than solid solution limit (0.6 wt%) in Zr matrix, the difference of corrosion rate was not observed in spite of showing the significant changes of microstructures with cooling rate. While, when the Nb content in the alloy is more than 0.6 wt%, the corrosion properties were deteriorated with increasing the supersaturated Nb concentration in matrix and increasing the area fraction of β_Zr. Also it was observed that the supersaturated Nb in matrix was more effective to decrease the corrosion resistance than the β_Zr phase in the same Nb containing alloy, while the equilibrium Nb concentration below solubility limit in the matrix played an important role to enhance the corrosion resistance. During the corrosion testing in steam at 400 °C, the formation of β_Nb phase in water-quenched specimen would result in the reduction of Nb concentration in matrix. Thus, the corrosion resistance is enhanced with the formation of β_Nb phase. It is suggested from this study that the equilibrium Nb concentration below solubility limit in α matrix would be a more dominant factor in the enhancement of corrosion resistance than β phase (β_Nb or β_Zr), supersaturated Nb, precipitate, and internal microstructure such as twin, dislocation and plate.     

Application of hydrogen analysis by neutron imaging plate method to Zircaloy cladding tubes

Effectiveness of neutron imaging plate (NIP) method for hydrogen analysis is investigated by using standard samples with known hydrogen concentrations. A relationship between hydrogen concentration in Zircaloy tubes and numerical data in the NIP images was obtained by image analysis process. By using the relationship, local hydrogen concentrations in segregated tubes with heterogeneous hydrogen distribution were estimated in a small area; 0.1 × 0.1 mm². Contribution of an oxide film in the tubes to the images is also investigated by using oxidized samples with and without hydrides. In the NIP images of the oxidized samples, oxide film was not recognized in the images of the sample. Results of numerical analysis also show no effect of the oxide film. These results show that the effect of oxygen in the image can be neglected when hydrogen analysis is performed on the Zircaloy tube with oxide film and hydrides by NIP method.     

Micro-texture of extruded Zr-2.5Nb tubes

We report the micro-texture of two extruded Zr-2.5Nb tubes determined using scanning electron microscopy combined with electron back-scattering diffraction (SEM/EBSD) and transmission electron microscopy and selected area diffraction (TEM/SAD). The pole figures determined by SEM/EBSD correspond well with bulk pole figures previously determined by X-ray diffraction (XRD). Three components of texture are seen to correlate with the shape and morphology of the α-grains and their contained dislocation substructures. The first component corresponds to elongated alpha grains containing a high density of a and c + a dislocations in which the c-axis is oriented at a relatively high angle to the long dimension of the α-grains as viewed in transverse section; these grains comprise a texture component with the c-axes in the radial transverse plane, tilted towards the radial direction. The second component corresponds to elongated α-grains containing a low dislocation density in which the c-axis is oriented parallel to the long dimension of the alpha grains: these grains also comprise a texture component with the c-axis in the radial/transverse plane, but predominantly in the transverse direction. The final component corresponds to colonies of Widmanstätten-like α-grains that are transformed from the β-phase: the majority of these grains have their c-axes in the axial direction. These grain have very low dislocation densities and are probably developed during cooling, after extrusion.     

Delayed hydride cracking in Zr-2.5Nb tube with the cooling rate and the notch tip shape

The objective of this study is to demonstrate the feasibility of the Kim’s delayed hydride cracking (DHC) model. To this end, this study has investigated the velocity and incubation time of delayed hydride cracking (DHC) for the water-quenched and furnace-cooled Zr-2.5Nb tubes with a different radius of notch tip. DHC tests were carried out at constant K_I of 20 MPa √m on cantilever beam (CB) specimens subjected to furnace cooling or water quenching after electrolytic charging with hydrogen. An acoustic emission sensor was used to detect the incubation time taken before the start of DHC. The shape of the notch tip changed from fatigue cracks to smooth cracks with its tip radius ranging from 0.1 to 0.15 mm. The DHC incubation time increased remarkably with the increased radius of the notch tip, which appeared more strikingly on the furnace-cooled CB specimens than on the water-quenched ones. However, both furnace-cooled and water-quenched CB specimens indicated little change in DHC velocity with the radius of the notch tip unless their notch tip exceeded 0.125 mm. These results demonstrate that the nucleation rate of hydrides at the notch tip determines the incubation time and the DHC velocity becomes constant after the concentration of hydrogen at the notch tip reaches terminal solid solubility for dissolution (TSSD), which agrees well with the Kim’s DHC model. A difference in the incubation time and the DHC velocity between the furnace-cooled and water-quenched specimens is attributed to the nucleation rate of reoriented hydrides at the notch tip and the resulting concentration gradient of hydrogen between the notch tip and the bulk region.     

Characterization of hydrogen concentration in Zircaloy claddings using a low-frequency acoustic microscope with a PVDF/LFB transducer

Hydrogen embrittlement is one of the major mechanisms responsible for the degradation of ductility of Zircaloy cladding materials. Currently the characterization of hydrogen concentration (HC) very often relies on destructive methods that are time-consuming and costly. In this research, an ultrasound-based nondestructive evaluation (NDE) technique is reported for the determination of HC in Zircaloy claddings. This ultrasound-based NDE technique employs a low frequency acoustic microscope (AM) with a PVDF/LFB transducer and a Fourier-based signal processing technique. With this AM technique, a relation between the ultrasound wavespeed and the HC of Zircaloy is established. A resolution of HC measurements with the current technique is demonstrated to be better than 200 ppm. This NDE technique has been developed with an aim to have a better resolution and also to be potentially applied to poolside inspection.     

Detection of intergranular cracking susceptibility due to hydrogen in irradiated austenitic stainless steel with a superconducting quantum interference device (SQUID) sensor

To investigate the detection method of intergranular (IG) cracking susceptibility by hydrogen in irradiated austenitic stainless steel (SS), magnetic and mechanical properties were examined after two repeats of hydrogen charging and discharging (hydrogen treatment) in Type 304 SS which had been irradiated during use in different reactor cores. The residual magnetic flux density (Br) was measured with a superconducting quantum interference device sensor and Br increased with increased neutron fluence and repeated hydrogen treatments. Elongation decreased with an increase of Br and IG cracking appeared above Br of 2×10⁻⁵ T for this measuring method after repeated hydrogen treatments. These phenomena would be caused by hydrogen-induced martensite phase being formed on grain boundaries. It was thought the appearance of IG cracking susceptibility due to hydrogen in irradiated SS could be predicted by measuring the Br of the steel.     

The influence of hydride blisters on the fracture of Zircaloy-4

The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.     

Effect of radial hydrides on the axial and hoop mechanical properties of Zircaloy-4 cladding

The effect of radial hydrides on the mechanical properties of stress-relief annealed Zircaloy-4 cladding was studied. Specimens were firstly hydrided to different target hydrogen levels between 100 and 600 wt ppm and then thermally cycled in an autoclave under a constant hoop stress to form radial hydrides by a hydride reorientation process. The effect of radial hydrides on the axial properties of the cladding was insignificant. On the other hand, the cladding ductility measurements decreased as its radial hydride content increased when the specimen was tested in plane strain tension. A reference hydrogen concentration for radial hydrides in the cladding was defined for assessing the fuel cladding integrity based on a criterion of the tensile strength 600 MPa. The reference hydrogen concentration increased with the specimen (bulk) hydrogen concentration to a maximum of ∼90 wt ppm at the bulk concentration ∼300 wt ppm H and then decreased towards higher concentrations.     

Steam oxidation of Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr and Zircaloy-4 at 900-1200 °C

The steam oxidation characteristics for the Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr (HANA-4) and Zircaloy-4 claddings were elucidated at LOCA temperatures of 900-1200 °C by using a modified thermo-gravimetric analyzer. After the oxidation tests, the oxidation behaviors, oxidation rates, surface appearances, and microstructures of the as-received, as-oxidized, and burn-up simulated claddings were evaluated in this study. The high-temperature oxidation resistance of the as-received HANA-4 cladding was superior to that of the Zircaloy-4. The superior oxidation resistance of the HANA-4 cladding could be attributed to the higher Nb and the lower Sn within its cladding. The pre-oxidized layer formed at the low temperatures below 500 °C could retard the oxidation rate at the high temperatures above 900 °C. And the soundness of the pre-oxidized layer formed at a lower temperature could influence the oxidation kinetics and the rate constants during a steam oxidation at LOCA temperatures from 900 to 1200 °C.     

High-pressure hydriding of Zircaloy cladding by the thermogravimetry and tube-burst techniques

Hydriding kinetics of modified Zircaloy claddings was studied by the thermogravimetric method at 400 °C and the tube-burst technique at 315 °C. Some specimens were prefilmed with a thin oxide layer by air oxidation on both the inner and outer surfaces which were either pickled or blasted. In the thermogravimetric test, the hydriding rates of bare cladding specimens (no oxide prefilm) were in the range 0.9-1.6 mg/cm² h with little effect of the surface treatment. Incubation times were less than 1 h or even zero. In the tube-burst test, immediate and extensive hydrogen uptake was observed for these non-coated specimens. On the other hand, the cladding specimens with oxide prefilm exhibited lower hydriding rates ranging from 0.01 to 0.05 mg/cm² h and incubation times increased to 42 h. In addition, no hydrogen uptake was observed for all oxide-coated specimens for 100-750 h.     

Hydrogen absorption by zirconium alloys at high temperatures

At high temperatures, e.g. during a hypothetical severe accident, zirconium and its alloys are not stable to other materials and to oxidising atmospheres. Exothermic reactions with steam cause the production of hydrogen which will be released to the atmosphere and, thus, endanger the containment or may be absorbed by the remaining metal. The hydrogen solubility in Zircaloy-4 and Zr-1Nb was measured in the temperature range of 1230-1730 K and at hydrogen partial pressures between 10 and 100 kPa. The parameters of the Sieverts constant were determined. No significant differences between the two alloys were observed. The hydrogen solubility of oxygen containing Zircaloy-4 decreases with increasing oxygen content.     

Influence of annealing in air on transport properties of long-time oxidized layers of zirconium alloys

The change of resistivity and permittivity of oxide films grown 1554 days in water at 360 °C on tubes of Zr1Nb, ZrSnNb(Fe) and IMP Zry-4 was investigated by I-V measurements after long-time annealing in air at 123 °C. Thereby the room temperature resistivity, highest in Zry-4W and lowest in Zr1Nb, increased by more than two orders of magnitude due to a strong decrease of mobility, with nearly constant carrier concentration. The polarity of the zero current changed from negative to positive values in the pA range. The changes are thought to be due to continued oxidation.     

Characterization of hydrogen concentration in Zircaloy-4 using ultrasonic techniques

The relationship between hydrogen concentration precipitated as hydride particles and ultrasonic parameters, such as velocity and attenuation, was examined in Zircaloy-4 samples for potential applications in the Non-Destructive Test Field. Different amounts of hydrogen (up to 517 ppm) were introduced in the samples by gaseous charging. Ultrasonic attenuation measurements were performed with compressive waves at frequencies of 10 and 30 MHz, and propagation velocity measurements were performed at 10 MHz. Ultrasonic velocity showed an approximately linear increase with hydrogen concentration and it could be used as an assessment parameter when the hydrogen level is high enough. Attenuation versus hydrogen concentration has been fitted by a logarithmic equation at 10 MHz. At 30 MHz a fluctuating behavior of the attenuation prevented measurement of the hydrogen concentration.     

Effect of β phase, precipitate and Nb-concentration in matrix on corrosion and oxide characteristics of Zr-xNb alloys

The corrosion test and oxide characterization were performed on the specimens having different Nb-content in the range of 0-5 wt%. The specimens were heat-treated at 570 °C for 500 h to get the α+β_Nb phase and at 640 °C for 10 h to get the α+β_Zr phase after β-quenching. The corrosion tests were carried out at 360 °C. In the low Nb-contents of 0.1-0.2 wt% where Nb was soluble in the matrix without the formation of Nb-containing precipitates or β phase, the samples showed the excellent corrosion resistance and their corrosion resistance was not affected by heat-treatment. The corrosion resistance was improved by the stabilization of tetragonal ZrO₂ and columnar oxide structure when all added Nb was soluble in the matrix to equilibrium concentration. In the high Nb-contents of 1.0-5.0 wt%, the corrosion rate was very sensitive to the annealing condition. The transformation of oxide crystal structure from tetragonal ZrO₂ to monoclinic ZrO₂ and oxide microstructure from columnar to equiaxed structure was accelerated in the samples having β_Zr phase, while retarded in the sample having β_Nb phase. This means that the formation of β_Nb phase resulted in the reduction of Nb concentration in the α matrix, thus the corrosion resistance was enhanced with the formation of β_Nb phase. From the corrosion test and oxide characterization, it is suggested that the equilibrium concentration of Nb in the α matrix would be a more dominant factor to enhance the corrosion resistance than the Nb-containing precipitates, supersaturated Nb, and β phase (β_Nb or β_Zr).     

Experimental investigation of the Zr corner of the ternary Zr-Nb-Fe phase diagram

Intermediate phases in the Zr-rich region of the Zr-Nb-Fe system have been investigated by X-ray diffraction, optical and electron microscopy and electron microprobe analysis. The chemical composition ranges covered by the alloys studied here are: (41-97) at.% Zr, (32-0.9) at.% Nb and (0.6-38) at.% Fe. The phases found in this region were: the solid solutions α-Zr and β-Zr, the intermetallic Zr₃Fe with less than 0.2 at.% Nb in solution, two new ternary intermetallic compounds (Zr+Nb)₂Fe `λ<sub>1</sub>' with a cubic Ti<sub>2</sub>Ni-type structure in the composition range (2.4-13) at.% Nb and (31-33) at.% Fe, and (Fe+Nb)<sub>2</sub>Zr `λ₂' indexed as hexagonal Laves phase MgZn₂ type (C14) with a wide range of compositions close to (35-37) at.% Zr, (12-31) at.% Nb and (32-53) at.% Fe.     

Effects of natural aging on the tensile properties of water-quenched U-6%Nb alloy

Uranium-6 wt% niobium (U-6%Nb) alloy has been in use for many years in the water-quenched (WQ) condition. The purpose of this work was to determine the effect of natural aging on tensile properties of the WQ U-6%Nb alloy. The materials studied were hemispherical shells after 15 and 20 years in storage. The alloy was successfully tested in the original curved configuration, using the specially designed tensile test apparatus. Finite element analysis confirmed the validity of the test method. The results of the tensile tests clearly indicated that in the WQ condition, the material is changing and after 15 and 20 years, the yield strength exceeds the original maximum allowable specification. The fracture mode transitions from highly ductile, microvoid coalescence in new material to a mixed mode of shallow dimples and inclusion-induced voids in the naturally aged material.     

Laser annealing in combination with mass spectroscopy, a technique to study deuterium on tokamak carbon samples, a tool for detritiation

In this study, a method is presented based on mass spectroscopy to measure the areal density of deuterium on a graphite surface exposed to tokamak discharges. The studied sample was cut from a bumper limiter exposed in the TEXTOR tokamak and annealed by a 1 J Excimer laser (KrF). The energy used was 400 mJ cm⁻², which is below the threshold for ablation, 1 J cm⁻². The release of HD and D₂ was measured by a mass spectroscopy set-up and no other species released from the sample were detected in this experiment. The amount of D released from the sample after 20 laser pulses was measured to 7 × 10¹⁶ D atoms per cm⁻² (for this particular sample) and most of the hydrogen at the surface was released in the first pulse, as checked by nuclear reaction analysis (NRA) techniques, which gave changes of the amount of deuterium before and after laser annealing. The sensitivity in this experiment was 5 × 10¹⁴ atoms per cm⁻² for HD and 5 × 10¹³ atoms per cm⁻² for D₂.     

Evaluation of effect of hydrogen on toughness of Zircaloy-2 by instrumented drop weight impact testing

Hydride-assisted degradation in fracture toughness of Zircaloy-2 was evaluated by carrying out instrumented drop-weight tests on curved Charpy specimens fabricated from virgin pressure tube. Samples were charged to 60 ppm and 225 ppm hydrogen. Ductile-to-brittle-transition behaviour was exhibited by as-received and hydrided samples. The onset of ductile-to-brittle-transition was at about 130 °C for hydrided samples, irrespective of their hydrogen content. Dynamic fracture toughness (K_ID) was estimated based on linear elastic fracture mechanics (LEFM) approach. For fractures occurring after general yielding, the fracture toughness was derived based on equivalent energy criterion. Results are supplemented with fractography. This simple procedure of impact testing appears to be promising for monitoring service-induced degradation in fracture toughness of pressure tubes.     

Dissolution and precipitation behavior of hydrides in Zircaloy-2 and high Fe Zircaloy

In order to elucidate the terminal solid solubility during the dissolution of hydrides at heatup (TSSD) and during the precipitation of hydrides at cooldown (TSSP) for hydrogenated Zircaloy-2 and high Fe Zircaloy, differential scanning calorimetry (DSC) measurements have been carried out in the temperature range of 50-600 °C. The hydrogen concentrations in the two kinds of alloys ranged from 40 to 542 ppm. There was no difference in either TSSD or TSSP solvi between Zircaloy-2 and high Fe Zircaloy, and best-fit equations were derived for the two curves. In the present TSSP data, two different activation energies, separating into high and low temperature ranges at 260 °C, were obtained. Based on the widths of the DSC peak obtained during cooldown, the average precipitation (nucleation plus growth) rates of zirconium hydrides from super-saturated state were assessed. The activation energy of the precipitation rate was approximately equivalent to reported values of hydrogen diffusion coefficients of Zr and Zircaloys.