Hydride blister formation in Zr-2.5wt%Nb pressure tube alloy

Hydrogen-enhanced degradation and oxide effects in zirconium alloys for nuclear applications

Abstract: The zirconium alloys used in nuclear industry include mainly Zr–Sn and Zr–Nb alloys of different chemical composition, microstructure and susceptibility to both hydrogen degradation and oxidation. The hypothetic nuclear accidents can create a real danger to the Zr alloys and stability of parts made of these alloys, and especially such as loss of coolant accident (LOCA) and reactivity initiated accidents (RIA). The hydrogen degradation can manifest itself in an appearance of hydride phases resulting in a substantial loss of plasticity, an increase in ductile–brittle transition, sometimes in a decrease in mechanical strength. The oxidation can prevent the hydrogen entry but at high temperatures the cracking of the oxide layer can form the easy hydrogen diffusion channels. Based on a substantial number of tests made so far and well-known thermodynamic and kinetic parameters, the general microstructure-dependent and temperature-dependent degradation model considering both hydrogen and oxidation could be elaborated.

Terminal solid solubility of hydrogen in Zr-alloy pressure tube materials

Abstract: In this work, terminal solid solubility (TSS) of hydrogen for Zr-alloy pressure tube materials was determined corresponding to end of hydride dissolution (TSSD) and start of hydride precipitation (TSSP) using the dilatometry technique. For this, Zircaloy-2 and Zr–2.5Nb pressure tube alloy coupons were gaseously charged with controlled amount of hydrogen in the range 10–100 μg/g. Change in length of cylindrical specimens of length ∼10 mm and diameter ∼3.5 mm machined from the coupons were measured as a function of temperature using a dilatometer. The samples were heated at 2 °C/min to 430 °C, held for 30 min at 430 °C and cooled back to the ambient temperature at 2 °C/min. The transition temperatures corresponding to the end of dissolution of hydrides during heating and beginning of precipitation of hydrides during cooling in these alloys were determined from thermal strain (e) versus temperature (T), average slope (of e versus T plot) versus T and differential thermal strain versus T plots. The enthalpies of hydride dissolution and precipitation for Zircaloy-2 pressure tube material were found to be 30–34.5 and 25.9–26.3 kJ/mol, respectively, whereas the corresponding enthalpies for Zr–2.5Nb pressure tube material were found to be 35.44 and 17.2–22.8 kJ/mol, respectively. This difference in the enthalpies between TSSD and TSSP is explained in terms of the different roles played by the components of strain energy associated with the elastic and plastic deformation in the matrix and precipitate, as a result of hydride accommodation in this alloy during heating and cooling process.

Delayed hydride cracking in Zr–2.5Nb pressure tube material☆

Abstract: Delayed hydride cracking (DHC) is one of the localized forms of hydride embrittlement caused by hydrogen migration up the tensile stress gradient. In this work, DHC velocity was measured along the axial direction of the double melted, cold worked and stress-relieved Zirconium–2.5Niobium pressure tube material in the temperature range of 162–283 °C. The DHC crack growth was monitored using the direct current potential drop (DCPD) technique. The calibration curves between the normalized DCPD output and the normalized crack length at different test temperatures were also used to determine the DHC velocity. A simple model capable of explaining the observed features of DHC is proposed. The model explains the basis for the occurrence of incubation period associated with DHC crack initiation. Activation energy associated with the DHC in this alloy was found to be 56 kJ/mole.

Phase and texture analysis of a hydride blister in a Zr―2.5%Nb tube by synchrotron X-ray diffraction

Abstract: This paper presents a detailed phase and texture study within and around a hydride blister grown on the surface of a Zr-2.5%Nb pressure tube. The analysis is based on synchrotron X-ray diffraction experiments using an 80 keV photon beam and a high-speed area detector placed in transmission geometry. It was found that the blister is composed of two main phases, {alpha}-Zr and {delta}-ZrH, with a composition which changes locally across the blister. No location within the blister presents pure {delta} zirconium hydride, with a maximum of 80% for the volume fraction of {delta} hydride at the center of the blister. The texture observed for both phases in the original pressure tube remains essentially unaltered across the hydride blister. A detailed analysis of this texture using well-known parent-precipitate relationships shows that some selective precipitation occurs at {alpha}-Zr grains with their c-axis under a tensile stress, and on grains with grain boundaries favorably aligned for hydride nucleation.

Radiation Safety Information Computational Center

Abstract: The September 2003 edition, of the International Handbook of Evaluated Criticality Safety Benchmark Experiments is scheduled to be ready for distribution on CD-ROM near the end of September. Twenty newly approved evaluations are included in this version in addition to all previously approved evaluations. Editorial and technical corrections have been made to some of the previously approved evaluations. Revision status of each individual evaluation is noted at the bottom of each page. In addition, a revision status table, noting specific technical revisions made to each evaluation, is included on the CD-ROM in a directory designated “Revision”. For questions or comments contact J. Blair Briggs by telephone (208) 526-7628, fax (208) 526-2930, or by email at bbb@inel.gov.

The terminal solid solubility of hydrogen and deuterium in Zr-2.5Nb alloys

Abstract: The terminal solid solubility (TSS) of hydrogen or deuterium is an important parameter in Zr alloys that are used in the nuclear industry. If this solubility is exceeded, it can make the alloys susceptible to delayed hydride cracking. Accurate expressions for the TSS are necessary for modelling delayed hydride cracking as well as deuterium ingress into pressure tubes and blister formation in pressure tubes in contact with their calandria tubes. Measurements of the changes in the dynamic elastic modulus have been used to establish expressions for the TSS as a function of temperature and to study the hysteresis between hydride dissolution and precipitation. It is shown that the hysteresis is particularly sensitive to the thermal history of the sample (such as the prior maximum temperature, hold time at maximum temperature and cooling rate). As a result, the precipitation TSS (TSSP) has a range of values bounded by the solubility equations designated TSSP1 and TSSP2. These are obtained, respectively, by cooling from an upper- and a lower-bound maximum temperature. The TSSD equation obtained in this study is very close to previously determined expressions, but the TSSP equations differ significantly from earlier results.

Hydride embrittlement in ZIRCALOY-4 plate: Part I. Influence of microstructure on the hydride embrittlement in ZIRCALOY-4 at 20 °C and 350 °C

Abstract: The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350 °C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, andβ-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably.In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20 °C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350 °C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect.

Environmentally-induced cracking of zirconium alloys — A review

Abstract: The general field of environmentally-induced cracking of zirconium alloys has been reviewed and the phenomena that are observed and the progress in understanding the mechanisms are summarized. The details of the industrially important pellet-clad interaction failures of nuclear reactor fuel have been left for a companion review and only observations on the mechanism are summarized briefly here. It is concluded that in the zirconium alloy system, by virtue of the physical peculiarities of the System, it is easier to reach unambiguous conclusions about the environmental cracking mechanisms that are operating than with other systems. Thus, chemical dissolution in either liquid or vapour phase is thought to be the principal mechanism for intergranular cracking, while adsorption-induced embrittlement is thought to be the most common transgranular quasi-cleavage process. Hydrogen embrittlement in this system can be identified because it requires precipitated hydride that gives characteristic fractography when cracked. Only in a few instances does stress-corrosion cracking appear to proceed by a hydride cracking mechanism.

The habit planes of zirconium hydride in zirconium and zircaloy

Abstract: The habit planes for precipitation of zirconium hydride in Zircaloy-2 and Zircaloy-4 have been determined to be {1017}. The standardization of experimental techniques allows one to make a valid comparison of this result with the {1010} habit plane determined earlier for pure zirconium. Discrepancies between the results of this study and earlier investigations are discussed and possible explanations are proposed.

Stress reorientation of hydrides in zirconium--2.5% niobium

Abstract: By tests on both plate material and specimens cut from tubing, an extensive assessment of the factors governing the realignment of hydride platelets in Zr−2.5% Nb under stress has been carried out. Stress reorientation readily occurs providing that the fabrication texture of the material results in a high concentration of basal poles in the direction of applied stress. The results clearly demonstrate that only hydride taken into solution can become reoriented, because the stress affects precipitation during cooling under stress, and the maximum degree of reorientation is therefore dependent upon temperature and hydrogen content. The function of stress is to compensate for the lattice strain that occurs when hydride is precipitated and the degree of reorientation is readily described by an equation derived from a consideration of nucleation frequencies in different directions.