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[en] The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal ε phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with ¹⁸¹Ta as the nuclear probe. Two EFGs were found in the ε phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the ε-phase, are due to the changes in the lattice parameters a₀, c₀ as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies. (orig.)

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[en] The paper presents the results of high-temperature ampoule tests for advanced absorbing materials based on hafnium hydride (HfHx). An installation and a methodology have been developed for the ampoule testing. The ampoule test installation consists of two vertical furnaces coaxially connected to each other. A low-temperature furnace is used for the ampoule preheating and exposure between cycles, and a high-temperature furnace is used for isothermal exposure to the temperature specified by the test program. Samples of δ- and ε-phase hafnium hydride were prepared, and an up to 5μm thick oxide coating was applied to some of the δ-HfHx samples. Experimental data were obtained on the thermal stability of hafnium hydride in an inert gas environment (argon, helium) at temperatures of up to 900°C. A comparison was made between the resistance of hafnium hydride obtained by direct (through) saturation and sintering of powder in conditions of cyclic thermal loading. Based on the results of the HfHx testing in a closed inert environment, it was noted that high-temperature exposure of the δ-HfH1,6 pellets (to temperatures of up to 900°C) does not lead to the hydride dissociation, while ε-HfH1,9 in an inert environment remains stable at up to 650°C, and there is hydrogen release taking place as a temperature of 800°C is reached, which leads to an increase in the ampoule pressure and to the ampoule depressurization. The experiment was numerically simulated by the finite element method using the ANSYS Mechanical 16.0 code, and the hafnium hydride heating rate and time to the specified test temperature were determined.

[en] The enthalpy for the direct reaction of H₂ (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H₂ (g) + 1(b-a)HfH_a(α) → 1/(b-a)HfH_b(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases and a and b are the corresponding (H/Hf) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of ΔH degree _f (HfH_1.5) = -107.5 kJ/mol and ΔH degree _f (HfH_2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |ΔH_H| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3

[en] Published in summary form only

[en] The macroscopic diffusivity D and the spin-lattice relaxation rate Γ₁ of hydrogen in hafnium dihydrides, HfH_x (1.71≤x≤1.94), were measured at temperatures up to 770 K. The activation enthalpy for hydrogen diffusion, obtained by fitting an Arrhenius expression to the diffusivities measured by PFG-NMR, increases slightly with increasing hydrogen concentration from H_a=0.63 eV (x=1.71) to H_a=0.71 eV (x=1.94). These H_a values agree satisfactorily with those deduced from the dipolar spin-lattice relaxation Γ_1,dip. A combined analysis of both Γ₁ and PFG measurements shows that hydrogen atoms jump predominantly from occupied tetrahedral sites (T sites) to nearest-neighbour vacant T sites. The results for HfH_x are compared with those for TiH_x and ZrH_x, the dihydrides of the other Group IVb transition metals

[en] Compatibility between the hafnium hydride control rod pin - hafnium hydride pellets, cladding material with hydrogen permeation barrier and liquid sodium for thermal bonding- was investigated. The compatibility between the hafnium hydride control rod pin was relatively good. (authors)

No abstract available