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.     

Volume conservation during irradiation growth of Zr-2.5Nb

Dimensional changes are reported in three dimensions for cold-worked Zr-2.5 Nb pressure tube material irradiated to a fast fluence of 174 × 10²⁴ nm⁻², E > 1 MeV at a nominal temperature of 250 °C. The observed dimensional changes in the longitudinal and transverse directions (up to ∼1.2% and ∼−0.5%, respectively) are consistent with earlier data at 280 °C and 310 °C, and the previously reported negative temperature dependence. The observed growth in the radial direction is negative (up to ∼0.7%). Initially, there is a small volume increase (0.05-0.1%) but this gradually decays to < 0.05% and the long term rate of volume change is negligible, within the accuracy of the measurement, demonstrating that the phenomenon observed is, indeed, irradiation growth.     

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.     

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.     

Influence of temperature on threshold stress for reorientation of hydrides and residual stress variation across thickness of Zr-2.5Nb alloy pressure tube

Threshold stress, σ_th, for reorientation of hydrides in cold worked and stress-relieved (CWSR) Zr-2.5Nb pressure tube material was determined in the temperature range of 523-673 K. Using tapered gage tensile specimen, mean value of σ_th was experimentally determined by two methods, half thickness method and area compensation method. The difference between local values of σ_th measured across the thickness of the tube and the mean σ_th values yielded the residual stress variation across the tube thickness. It was observed that both the mean threshold stress and residual stress decrease with increase in reorientation temperature. Also, the maximum value of residual stresses was observed near the midsection of the tube.     

Fracture toughness of Zr-2.5 wt% Nb pressure tubes

Measurements of fracture toughness of HT Zr-2.5 wt% Nb pressure tubes have been made by studying internally pressurizing (burst) test specimens and small bending test specimens. These tests were conducted from a viewpoint of the effects of hydrogen content, hydride orientation, temperature and crack configuration on the fracture thoughness K_c. Results of the experiments showed that K_c decreased with increasing hydrogen content, but is little affecting by hydrogen content at reactor operating temperature. The value of K_c can be quantitatively evaluated by RHC defined by radial hydride content (RHC) perpendicular to the tensile stress.     

Metallurgical properties of irradiated cold-worked Zr-2.5 wt% Nb pressure tubes

Three cold-worked pressure tubes of Zr-2.5 wt% Nb were irradiated as components of Chalk River reactor loops, under operating conditions similar to those in the Canadian design of heavy water cooled, heavy water moderated power reactor. A fourth tube, part of which was artificially prehydrided to about 200 ppm H₂, was irradiated unstressed, in air.     

A high-temperature creep model for Zr-2.5 wt% Nb pressure tubes

For a postulated loss-of-coolant accident in a CANDU reactor, in which the primary cooling circuit fails to remove the heat generated in the core, the temperature of the pressure tubes could rise very quickly. Since any deformation of the pressure tubes would control how the core heat is transferred to the surrounding moderator, which is a large heat sink, the accurate prediction of this transient deformation is essential. The majority of the pressure tubes in CANDU reactors are cold-worked Zr-2.5 wt% Nb and creep equations for this material have been developed from uniaxial creep tests. These creep equations were successful in predicting the creep strain in constant-stress uniaxial tests in which the temperature was ramped at rates ranging from 1° C/s to 50° C/s. They also successfully predicted the ballooning of internally pressurized sections of pressure tube that were heated at about 5° C/s.     

The determination of hydrogen and deuterium in Zr-2.5Nb material by hot vacuum extraction mass spectrometry

An analysis method for the determination of H and D concentrations in Zr-2.5Nb material has been established based on hot vacuum extraction and isotope dilution mass spectrometry. Hot vacuum extraction enables complete removal of the hydrogen isotopes from the sample. The isotope dilution technique is used to determine quantitatively the amount of hydrogen isotopes in the extracted gas. Methods for preparing standards of H, D or H and D in Zr-2.5Nb have also been established. These ‘in-house’ standards are used to assess the performance of the analysis method. The analysis uncertainty, based on the determined content relative to the dosed H or D isotope content of each standard, is 1% (at a level of confidence of 95%) for samples containing greater than ∼5 μmoles H or D. The uncertainty increases to 5% as the sample content decreases to 0.5 μmoles H or D. The uncertainty of this analysis method is well within the requirements for surveillance examinations of CANDU^® reactors and post-irradiation examinations of reactor components.     

Textural nonuniformity in sheets of Zr-2.5% Nb alloy

Moscow Engineering Physics Institute. Translated from Atomnaya Énergiya, Vol. 72, No. 2, pp. 181–184, February, 1992.     

A high-temperature longitudinal strain rate equation for Zr-2.5 wt% Nb pressure tubes

The pressure tubes in CANDU reactors are horizontal. Thus, if, during a postulated loss-of-coolant accident, the pressure tube temperature should rise sufficiently, the self-weight of the pressure tubes together with the weight of the fuel could cause the pressure tubes to sag. Since any pressure tube deformation would control how the core heat is transferred to the surrounding moderator, which is a large heat sink, the accurate prediction of this sag is essential. Most CANDU reactors have pressure tubes of cold-worked Zr-2.5 wt% Nb. A longitudinal strain rate equation was developed for this material using four-point bend tests. This strain rate equation was successful in predicting the longitudinal strain, due to bending, in specimens for which the temperature was ramped at 1°C/s and 5°C/s.     

Annealing of cold worked two-phase Zr-2.5 Nb—Associated microstructural developments

Pilgered two-phase Zr-2.5 Nb was subjected to recovery (400 °C) and recrystallization (700 °C) annealing - broad objective was to bring out associated microstructural developments. Effects of recovery and recrystallization were indexed in terms of changes in hardness, lattice strain, stored energy of cold work and residual stress. Though recovery did not cause significant changes in grain/phase structure; visible coarsening of 2nd phase was associated with recrystallization. Such coarsening continued even after the completion of primary recrystallization, with concurrent and noticeable changes in microstructure and in crystallographic texture.     

Effect of unloading on crack growth rate of Zr-2.5Nb tubes

Crack growth rates (CGRs) of a heat-treated Zr-2.5Nb tube were determined using compact tension specimens with 60 ppm H at 250 °C under the constant and cyclic loads where the load ratio R was changed from 0.13 to 0.68. CGR was the highest under the constant load and decreased under the cyclic load with decreasing R despite a decrease of the critical hydride length indicating the enhanced rate of hydride cracking. Hence, the decreased CGR under the cyclic load is due to unloading during the cyclic load inducing the compressive stress at the crack tip. This compressive stress suppresses hydride nucleation rate, leading it to govern the CGR, according to Kim’s new model. Evidence is provided by citing Simpson’s experiment demonstrating that unloading from 15 MPa √m decreased the CGR of a cold-worked Zr-2.5Nb tube but annealing did the reverse. This study demonstrates for the first time that the retarded CGR due to an overload during the DHC tests is understood in view of crack growth kinetics using Kim’s model.     

Creep ductility of cold-worked Zr-2.5 wt% Nb and Zircaloy-2 tubes in-reactor

Deformation of internally pressurized 15 mm diameter tubes of cold-worked Zr-2.5 wt% Nb and Zircaloy-2 has been measured in axial and hoop directions for irradiations up to 24 000 h at 571 K. Specimens have deformed up to 18.1% in the hoop direction, with hoop stresses up to 495 MPa, without rupture. Since the stress sensitivity of creep rate is high (>5) under the test conditions, and is low (≈1) under operating conditions for pressure tubes, creep ductility of pressure tubes should be very high.     

Burnup effect on Nb/Zr ratio-cooling time correlation

The axial ⁹⁵Nb/⁹⁵Zr ratio distribution in a fuel assembly of the typical research reactor (IRT) was determined experimentally by gamma scanning. The results showed that this ratio is stable along the fuel assembly axis regardless of the position of the scanned section. This allows to limit gamma scanning of the whole assembly on the measurement of the central section only. This will save time, efforts and experimentalist’s exposure to radiation. In addition, the effect of burnup on the ⁹⁵Nb/⁹⁵Zr-cooling time correlation was investigated. The results showed that, using this correlation to determine cooling time, will include a systematic error of about 12%.     

Influence of metallurgical variables on delayed hydride cracking in Zr-Nb pressure tubes

During service, Zr-2.5Nb pressure tubes of nuclear power reactors may be prone to suffer from crack growth by delayed hydride cracking (DHC). For a given hydrogen plus deuterium concentration there is a critical temperature (T_C) below which DHC may occur. In this work, T_C was measured for specimens cut from pressure tubes made in Canada (CANDU) and in Russia (RBMK). Hydrogen was added to the specimens to get concentrations ranging from 24 to 60 wt ppm. It was found that T_C was higher than the corresponding precipitation temperature. The crack propagation velocity (V_P), measured in axial direction, increases from a minimum at T_C to a maximum at a temperature close but higher than the precipitation temperature. At lower temperatures, when hydride precipitates are present in the bulk, V_P follows an Arrhenius law: V_P = A exp(−Q/RT), with an activation energy Q of 66-68 kJ/mol for both tubes. The RBMK material presented lower velocities than CANDU one.     

Towards a more comprehensive microstructural analysis of Zr-2.5Nb pressure tubing using image analysis and electron backscattered diffraction (EBSD)

Zr-2.5Nb pressure tubes used in CANDU (CANada Deuterium Uranium) reactors have a very complex microstructure, with two major crystallographic phases, α and β. These phases include a fair amount of deformation from the extrusion process and the cold working (∼25%) performed at the end of the manufacturing process. This microstructure (texture, grain aspect ratio, etc.) changes along the tube’s length and differs from tube to tube. In order to better understand the deformation mechanisms, these microstructural differences must be statistically characterized. Scanning electron microscopy combined with direct image analysis or with electron backscattered diffraction (EBSD) are good techniques for carrying out such a measurement. However it is not possible, using specimen preparation methods specific for each of these techniques, to reveal all of the grain and phase boundaries. We have thus developed post-treatment algorithms to be able to partially analyze the revealed Zr-2.5Nb microstructure. The first algorithm was used for image analysis treatments of micrographs taken at 5 kV on the radial-tangential plane of etched samples using a reactive ion etch (RIE, CF₄ + O₂). The second was developed for EBSD grain mapping and can be used to characterize α-Zr grain shape and orientation. The two techniques are complementary: EBSD gives information about the micro-texture and the relationship between the microstructure and micro-texture while image analyses of SEM micrographs reveal the direction and distribution of the α-Zr lamellae more easily and over a greater sample area than EBSD. However, the SEM micrographs that were used did not reveal any grain boundary (only phase boundary). An analysis of EBSD grain maps reveals that the average α-Zr grain size, mainly in the elongated direction (tangential), is smaller than what is normally obtained from an image analysis of SEM micrographs. The grain size distribution of type I α-Zr grains (deformed original (prior) α-Zr) and type II (stress-induced β-Zr → α-Zr phase transformation) is also shown to be different for sizes greater than 0.4 μm².     

Anisotropy of crack resistance of the channel tubes produced using Zr-2.5% Nb alloy

Conclusions Using experimentally determined values of the critical opening of the differently oriented cracks (_c), we evaluated the anisotropy of crack resistance of the material of nine channel tubes (Zr-2.5% Nb alloy) under different structural conditions. Determination of the strucutral (_o) and texture (R) parameters of crack resistance according to the bend tests carried out using specimens having a cross section of 4×4 mm makes it possible to evaluate the individual contribution of the texture and the structure to the crack resistance of the material of the channel tubes. The parameter _c is a constant that depends on the type of the alloy and its structural state: the higher the crack resistance of a material, the higher is the magnitude of ₀. The parameter R depends on the crack orientation in the tube. It affects the coefficient of rigidity of the stress state (K_ij) at the crack tip and, thereby, characterizes the embrittling effect of the given crack.The experimental results obtained when determining _c of the material of the channel tubes having cracks of different orientations can be expressed by the following equation _c=0.741₀(2.35-K _ij), where K_ij is calculated using the parameters of anisotropy of plastic deformation of the material of the tubes.Translated from Atomnaya Énergiya, Vol. 69, No. 4, pp. 230–233, October, 1990.     

Influence of prior dislocation structure on anisotropy of thermal creep of cold-worked Zr-2.5Nb tubes

Normally, creep anisotropy of hcp metals is thought to be controlled by the crystallographic texture. Here, we show that the creep anisotropy of cold-worked Zr-2.5Nb tubes is also very dependent on the anisotropic dislocation structures introduced by cold-work. The contribution of each slip system to the creep deformation of an individual grain orientation depends upon the activity of that slip system during prior cold-work. This conclusion is reached by comparing the self-consistant visco-plastic polycrystalline models with thermal creep tests performed on internally pressurized thin-wall capsules with different textures under a transverse stress of 300 MPa at 350 °C, where dislocation creep is the dominant operating mechanism. The non-uniform dislocation distributions prior to creep were derived by simulating the cold-work process of Zr-2.5Nb tubes from an Elasto-Plastic Self-Consistent (EPSC) model.