Structure and Thermodynamical Properties of Zirconium Hydrides from First‐Principle

Abstract

Zirconium alloys are used as nuclear fuel cladding material due to their mechanical and corrosion resistant properties together with their favorable cross-section for neutron scattering. At running conditions, however, there will be an increase of hydrogen in the vicinity of the cladding surface at the water side of the fuel. The hydrogen will diffuse into the cladding material and at certain conditions, such as lower temperatures and external load, hydrides will precipitate out in the material and cause well known embrittlement, blistering and other unwanted effects. Using phase-field methods it is now possible to model precipitation build-up in metals, for example as a function of hydrogen concentration, temperature and external load, but the technique relies on input of parameters, such as the formation energy of the hydrides and matrix. To that end, we have computed, using the density functional theory (DFT) code GPAW, the latent heat of fusion as well as solved the crystal structure for three zirconium hydride polymorphs: δ-ZrH 1.6 , γ-ZrH, and -ZrH 2 .

Modeling of Some Physical Properties of Zirconium Alloys for Nuclear Applications in Support of UFD Campaign

“…Both diagrams agree well in establishing the phase fields of the δ-and ε-hydrides with homogeneity ranges on the basis of the stoichiometric compounds ZrH 1.5 (experimentally established value is ZrH 1.66 , [35] . Partial molar entropy of hydrogen: comparison of the results of modeling (left) and experimental data [16]; black solid line -the results of thermodynamic assessment in [55] and experimental work [54] Figure 11.…”

“…Calculation using ThermoCalc; (b) experimental phase diagram [48]. Both diagrams agree well in establishing the phase fields of the δ-and ε-hydrides with homogeneity ranges on the basis of the stoichiometric compounds ZrH 1.5 (experimentally established value is ZrH 1.66 , [35]). …”

“…It means that although H would energetically prefer to be located at an interstitial position, metastable Zr-H-Zr configurations are also possible if the process is driven kinetically. There are three distinctly different types of zirconium hydride structures described in the literature: γ−ZrH; δ−ZrH 1.66 (which was modeled in this work as ZrH 1.5 ) and ε−ZrH 2 [35]. In Figure 3 we present the results of our first-principles calculations yielding the three energy-relaxed crystal lattice structures.…”

“…First principles calculations of the equilibrium crystalline lattice parameters, free energy of formation at different temperatures, and the phonon density of states were discussed by Blomquist and by Zhu et al, [35,36]. A detailed overview of the problem of hydrogen solubility in Zr and its alloys was provided by Anghel [13].…”

The thermodynamics of hydride precipitation: The importance of entropy, enthalpy and disorder

“…Some works have focused on the octahedral site as the main location for H atom solution, however such works appear to be in the minority 23,24 . It has been theoretically predicted that the FCT structure of ZrH 2 can have another stable phase for c/a ¿ 1 25,36 . Zhu et al 16 studied the ordered hydride phases using DFT; they concluded that δ-ZrH 1.5 is thermodynamically less stable than the other phases at high temperature.…”