Nondestructive nuclear-physical methods of determining the boron content in zirconium

A. A. Bochvar All-Union Scientific Research Institute of Standardization and Mechanical Engineering. Translated from Atomnaya Énergiya, Vol. 73, No. 6, pp. 497-498, December, 1992.     

Radiation-induced swelling of zirconium hydride

Physics and Energy Institute (FÉI), Obninsk. Translated from Atomnaya Énergiya, Vol. 71, No. 2, 178–180, August, 1991.     

Dislocations generated by zirconium hydride precipitates in zirconium and some of its alloys

The character of dislocations emitted during the precipitation of γ-zirconium hydride in zirconium, Zircaloy-2, Zr-1% Al and Zr-1% Cr has been determined. Contrast experiments in the transmission electron microscope have shown that the dislocations possess Burgers vectors ($\text{b}$) of the type $\text{1}\text{3}\text{a <11}\text{2}\text{?}\text{0>}$. Of the three possible $\text{b}$'s only one or two are observed associated with a given hydride needle, those giving a large component of $\text{b}$ along a direction perpendicular to the needle. Dislocation generation is thought to result from the dilatational misfit associated with the hydride needles. The creation of dislocations with $\text{b}$'s along the $\text{c-}\text{axis}$ is more difficult and the misfit strain along this direction is not relieved by plastic deformation.     

Critical length and thickness of hydride plates with delayed hydride cracking in zirconium alloys

A method for calculating the critical length and thickness of hydrides with delayed cracking in commercial zirconium alloys is developed. The critical characteristics of hydride precipitates are estimated for pipes made of the alloy Zr–2.5%Nb. It is shown that the theoretical results agree with the experimental data.Translated from Atomnaya Ènergiya, Vol. 97, No. 4, pp. 280–286, October, 2004.     

Mitigation of hydride embrittlement of zirconium by yttrium

Brittle hydrides of plate-shaped morphology are known to embrittle the host zirconium matrix. The embrittlement effect is a strong function of the aspect ratio of hydride plates and their major dimension, as thinner plates behave akin to cracks resulting in stress-concentration around their edges, especially when tensile load is acting normal to the broad face of the plates. The embrittlement of the host matrix is due to loss in load bearing area as a result of cracking of the hydride plates under load and severe localized deformation of the ligaments joining the hydride plates. In this work, mitigation of hydride embrittlement was attempted by exploiting the synergistic effect of yttrium addition to zirconium and microstructural modification of the Zr-Y alloy by quenching. This was expected to enable creation of a very high density of nucleation sites much stronger than those available otherwise and, thus, facilitate precipitation of much smaller hydrides that do not embrittle the host matrix. The results obtained in the present work on a dilute Zr-Y alloy do support this idea.     

The effect of metallurgical factors on hydride phases in zirconium

The effects of grain size, solution heat-treating temperature, specimen purity, and hydrogenation technique on the formation of gamma-phase and delta-phase hydrides in zirconium have been investigated. Only specimen purity, in particular oxygen content, was found to influence which hydride phases were formed. Low oxygen content resulted in formation of the gamma phase, whereas the delta phase formed in the presence of high oxygen content. Transformation strain energy calculations suggest that this behaviour may be due to the strengthening effect of oxygen on the zirconium matrix.     

Isotope effect and hydrogen content dependence on the heat capacity and thermal conductivity of zirconium hydride and deuteride

Zirconium (Zr) hydride and deuteride are considered as candidates of neutron reflector materials in fast reactors (FRs). Thus, it is important to evaluate their fundamental physical and chemical properties. In this study, we prepared ?-phase Zr deuteride (?-ZrD_1.90), whose physical properties have been investigated and also the heat capacity (C_P) and thermal conductivity (κ) have been measured from room temperature to 673 K. The obtained data were compared with the literature data of δ-phase Zr hydride (δ-ZrH_1.66), ?-phase Zr hydride (?-ZrH_1.94), and deuteride (?-ZrD_1.95). Both C_P and κ of ?-ZrD_1.90 were higher than those of δ-ZrH_1.66. The higher C_P of ?-ZrD_1.90 than that of δ-ZrH_1.66 is mainly due to the smaller vibration frequency of deuterium atoms than that of hydrogen atoms. On the other hand, the higher κ of ?-ZrD_1.90 than that of δ-ZrH_1.66 is due to the larger hydrogen isotope content of ?-ZrD_1.90 than that of δ-ZrH_1.66. The data reported here would be useful when Zr hydride and deuteride are installed into FRs.     

Nature and thermal stability of radiation-induced defects in zirconium hydride

Atomic Energy - The authors wish to thank V. I. Shcherbak for great help in the electron-microscope investigations.     

The first step for delayed hydride cracking in zirconium alloys

Two models for delayed hydride cracking (DHC) in zirconium alloys are distinguished by their first step:

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The loading of a crack induces hydride precipitation. The hydride is postulated to create a hydrogen concentration gradient, where the bulk concentration is greater than that at the crack tip. This concentration gradient is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the precipitate first model (PFM).

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The tensile stress at the crack tip induces a gradient in chemical potential that promotes the diffusion of hydrogen to the crack tip. Hydrides form if the hydrogen concentration reaches the solubility limit for hydride precipitation. The mechanism is postulated to create a hydrogen concentration gradient, where the bulk concentration is lower than that at the crack tip. The gradient in chemical potential is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the diffusion first model (DFM).

The second model, DFM, is developed. This model is shown to describe the main features of the experimental observations of DHC, without invoking new phenomena, such as reduction in the solubility limit for precipitation of hydride, as required by the PFM.     

An in-situ study of the dissolution of γ-zirconium hydride in zirconium

The dissolution of γ-zirconium hydride has been examined in single crystal specimens of zirconium using a hot stage in a high voltage electron microscope. It was found that a significant proportion of the dislocations generated by the hydride needles during growth were not annihilated when dissolution occurred on heating to well above the solvus temperature. This is contrary to earlier work where similar experiments were carried out with thinner specimens at conventional accelerating voltages. Electron irradiation completely prevented annihilation of the hydride dislocations during dissolution. The results are discussed in relation to (a) repeated nucleation at the same sites during thermal cycling, (b) external shape changes (growth), (c) strain steps observed during thermal cycling under creep conditions, (d) positron annihilation experiments, and (e) the terminal solid solubility of hydrogen in zirconium.     

Metallographic studies of fatigue in 20% cold-worked zircaloy-2 containing zirconium hydride

The fatigue behaviour of unhydrided and hydrided 20% cold-worked zircaloy-2 reactor pressure-vessel tubing has been studied for fluctuating tension at room-temperature and 300 °C and for reversed torsion at room-temperature. At room-temperature, high concentrations of zirconium hydride markedly reduce the critical crack length. This effect is attributed to a lowering of the fracture toughness, since hydrides fracture in the path of crack propagation and promote matrix cleavage. At 300 °C, hydride particles constrained by the matrix are plastically deformed without fracturing; the toughness is probably unaffected by the presence of hydride.     

Mechanism of the reduction of zirconium and titanium oxides by calcium hydride

The theoretical principles of the reduction of stable metallic oxides by calcium hydride are considered, on the basis of a study of the reduction reactions of zirconium and titanium oxides, Thermodynamical calculations and special experiments intended to clarify the mechanism of reactions between oxides and CaH₂ are used to compare the reducing activity of CaH₂ with those of the calcium and atomic hydrogen formed in its thermal dissociation. The view which exists in the literature, according to which the high reducing power of CaH₂ is explained by the action of the atomic hydrogen formed, is shown to be erroneous. It is shown that the principal reducing action in this process is effected by metallic calcium, formed by dissociation of CaH₂. Metallic calcium at temperatures of 1000° and higher reacts with chemically stable oxides not only in the liquid but also in the vapor state, which creates favorable conditions for rapid and complete reduction reactions. It is also shown that the active hydrogen liberated when metals saturated with hydrogen are heated is capable, to some extent, of additionally reducing the oxides which contaminate these metals.