[en] The development of an oxide on zirconium alloys is governed by the diffusion of the oxygen ions through the oxide scale and the appearance and growth of compressive stress at the oxide - metal interface. It is admitted that the diffusional driving force is the compressive strain gradient in addition to a chemical potential gradient across the oxide scale. Each of these two contributions to the kinetic curve can be treated separately, allowing the evaluation of their dependency on the oxide scale thickness and the evolution in time of both flows given by the two potential gradients. In the first part of our work we present the theoretical treatment of the kinetical curves in both pre- and post- transition regions. The equations obtained where used in the treatment of kinetic data obtained experimentally on isothermal oxidation of Zy-4 cladding in 873 - 1073 K temperature range. The contribution of chemical potential gradient to the kinetic curve is linear for whole range with different kinetic coefficients for the pre-transition and post-transition regions. The average compressive stress exponentially increases in time, in the pre-transition region which means a linear increase with the oxide scale thickness. For the post-transition region the shape of stress evolution curves became more complicated due to the cracks and pores formation. The periodical stress relaxation can be related to the kinetical behaviour over the transition and explains the multilayered structure of the oxide scale. (author).

[en] The severe accident scenarios show the evolution of reactor core damage finalized with the corium and debris bed formation. Generally located above the corium, the debris bed has its temperature range evaluated between 1300°C (bottom) and 300°C (top). At the air ingress, in the debris bed the main chemical phenomena contributing to the subsequent degradation and fission products release are: oxidation of the Zircaloy 4 sheaths of the still intact rods, oxidation of the mixtures composed of Zr and UO₂ in the configuration of solid debris (either as relocated drops due to metallic melting or in the form of rubble debris particles) and oxidation of pure UO₂ in the fuel pellets remnants. When air penetrates into the debris bed, the remaining zircaloy4 claddings are oxidized, the oxidation rate decreasing from bottom to top. In the lower part of the debris bed (high temperature) the pins are completely oxidized and may undergo rapid destruction under their own weight, while the pins claddings in the upper part are oxidized with a smaller rate. By the destruction of pins, new sintered pellets with free surface are exposed; part of them remaining in debris bed alongside the material resulted from reactor core relocation and the other part falling down on corium. The oxidation of Zircaloy 4 sheaths is a dynamic process, dependent on the atmosphere, the temperature distribution into the debris bed and the cooling rate of the debris bed. The main objective of FIPRED (Fission Product Release from Debris Bed) Romanian Project is to evaluate the post severe accident fission products release from debris bed in air ingress conditions, tacking in account of UO₂ sintered pellets selfdisintegration by oxidation. The paper presents the scientific objectives and main steps of the project. The equipment (FIPRED EQ), the experimental test matrix and results obtained the mechanism of selfdisintegration of UO₂ sintered pellets by oxidation are presented, also. (author)

[en] Tritium occurs in CANDU reactors predominantly following neutron capture in the deuterium of heavy water. It is a source of irradiation for operators, consequently, considerable efforts are done worldwide for its extraction and storing. The tritium storage should require a chemical form as stable as possible. Currently, the gaseous tritium is stored in steel containers, a means implying leaks. Hence, the solid form of tritium immobilization is highly advisable. To attain the objective of secure tritium storage experimental methods were worked out to study the thermodynamical properties of the methods able to be used in tritium storage. First, the pressure-temperature-composition diagrams (P-T-C) were determined and then a method for determining the diffusion of hydrogen isotope in metals was achieved for the study of absorption/desorption kinetics of the systems. In the first part of this work the experimental setup is described as well as the results obtained in determining the P-T-C diagrams on metallic Ti and U samples as well as on hydrated ZrNi and ZrCo samples. A plot of the characteristic logP vs. 1/T for the system ZrNi-H is shown. The results obtained for both metallic U and Ti sponge are in good agreement with the data from literature. In the second part of the work the experimental method for determining the diffusion coefficient of the hydrogen isotopes in metals as well as the results obtained are presented. A thin superficial and uniform hydrogen layer was implanted upon one of the sample's faces by luminescent discharge in hydrogen atmosphere. The implanted samples were then submitted to various diffusion regimes in isothermal conditions, inside a furnace with controlled atmosphere. Following this treatment the samples were examined by metallographic analysis to determine the hydrogen concentration profile over the entire sample's width. The concentration profile equation is given. The dependence of the diffusion coefficient on the implanted hydrogen concentration was determined. The measurements were done on Zr-2.5%Nb alloy and results obtained are in good agreement with data from the literature

[en] The zirconium alloys oxidation and the oxidation kinetics is governed by the diffusion of oxygen in the oxide scale formed at the surface of the metal, and the oxide scale is formed normally under compression. With the increase of the oxide scale thickness the compressive stress at the oxide-metal interface increase until plastic deformation of the oxide and metal ensues, meanwhile the compressive stress at oxide-environment interface decreases and may become tensile. The kinetic curve reflects the changes of the compressive stress by changes in the rate of oxygen uptake. The volume expansion associated with this transformation is the cause of the oxide cracking. The crack initiation is the primary cause of the cyclicity of the post-transition kinetic curves at temperatures up to 700°C. The purpose of the present paper is to correlate the oxide scale thickness at cracking obtained from the kinetic curves and those obtained by SEM. (authors)

[en] Hydrogen storage is an important topic chiefly because of its relevance to the energy economy of the future. Since hydrogen is a versatile fuel and can be readily generated from and converted to other forms of energy, to store hydrogen is to store energy. The hydrides of titanium, zirconium, and hafnium are stable to very high temperatures. The enthalpy of the hydride forms was determined by using the DTA method. (Author)

[en] The method of first order stress determination by diffraction implies interplanar separation (d) measurements and applying the well-known Bragg relation, nλ = 2dsinθ, where λ is the wave length of the monochromatic beam while 2θ is the scattering angle corresponding to the given diffraction peak. Deformation is measured relative to the d₀ value corresponding to the unstressed state. The lattice deformation ε is determined in diffraction measurement from the relation: ε Δd/d₀ = - cotθΔθ. The direction along which one determines deformation is the direction of scattering vector. The optimal resolution in determining the deformation is obtained for θ = 90 angle, hence for a scattering of 180 angle (back reflection) while the optimal spatial mapping resolution is obtained for a scattering angle around 90 angle (θ = 45 angle). For focusing configuration such as that of TRIGA Pitesti reactor the sample should be positioned to satisfy the focusing condition. The requirements to be fulfilled by the experimental device for deformation determination are indicated. Measurements of deformation were carried out on six samples of HZr extracted from pressure tubes of sizes 45 x 20 mm, 3 mm thickness, 20 x 10, 4 mm thickness, 15 x 12 mm, 3 mm thickness. The working parameters of the experimental setup are indicated. The experimental data were processed by means of ORIGIN Microsoft fitting code. The accuracy in the determination of the diffraction line position as resulting from fitting procedure was 10^-2, what corresponds to an accuracy in determining the scattering values of Δε cotθΔx_c = 6 x 10^-4. The deformation measurements were performed for two sample orientations: 14 angle 40 min (corresponding to symmetric reflection) and 25 angle 40 min. The first one gives directly ε_N, while the second one allows determining ε_P (<ε₁₁> = <ε₂₂> ε_P and <ε₃₃> ε_N, where ε_ii are the deformation tensor normal components. A table is given showing that for three samples contractions across the thickness occur, while for the other samples a dilation on the same direction occurs

[en] A generalization of the intensity formula by Dorner, in terms of flux spectral distribution, elementary cell density, differential cross section, irradiated sample's volume and transmission function of the spectrometer is presented. For a given sample's volume and beam intensity, the integral intensity is proportional with the density of each element included in elementary cell structure. Hence, it allows obtaining a nondestructive method of evaluation of elements' concentration in sample composition, as for instance, the hydrogen concentration in metallic hydrides as HZr. HZr samples were prepared using controlled hydration of several Zr 2.5% Nb samples as plates of 40 x 25 x 1.62 mm size. Details concerning hydration procedures and chemical-mechanical treatment of the samples are presented. A table gives for 9 samples the values of the parameters characterizing the hydration process. The conclusion is drawn that although not accurate, this method is convenient for nondestructive estimation of hydrogen concentration in metallic hydrides. For a number of 6 samples, extracted from pressure tubes, the hydrogen concentration as obtained by this method are given

[en] This paper investigated Incoloy 800 HT alloy after following thermal transient tests: fast heating rates (50° and 90°C/minute) up to 1,000°C, maintaining this temperature level (0 and 60 minutes), furnace-cooling until 220°C, and then air-cooling. This alloy is one of the candidate materials for construction of the steam generators of the future NPP reactors. The analysis consisted in metallographic examination and traction tests. The samples were investigated using the Olympus GX 71 optical microscope, the OPL microdurometer with automatic cycle and WALTER BAI traction device. The average grain size was determined by linear interception method. The micro hardness was calculated by the relationship from the device technical book. On the traction diagrams were obtained: strength resistance (Rm), elongation at rupture (A) and elastic modulus (E). The tested alloy was compared with the ''as received'' material, and the results showed a good behavior of this alloy in the presented conditions. (authors)

[en] This paper investigated Incoloy 800HT (UNS N08811) alloy after some heat-cycling tests. The study continues prior tests realized in INR Pitesti concerning utilization of some nickel-based alloys in the heat exchangers and steam generators construction. The thermal-cycling consist in a successive series of heating and cooling with some rates in a range temperature. Technical parameters of thermal cycling: 50 & 200 cycles, 25 °C/minute heating-cooling rate, temperature range 450-1000°C, and argon working medium. The analysis consisted in metallographic examination (microstructure), Vickers microhardness, and traction tests. The average grain size was determined by linear interception method (ASTM E-112). The micro hardness was calculated by the relationship of the device technical book. On the Strength-Deformation diagrams were obtained: tensile strength and elongation. The tested samples were compared with the ''as received'' material. The results showed a good metallographic and mechanical behaviour of Incoloy 800HT at these thermal-cycling tests. (authors)

[en] The purpose of this work is the measurement of the heat capacity, thermal expansion and thermal conductivity of UZrH fuel, having 45%U, at temperatures between 300 and 1400 K. The methods used for the specific heat capacity were the differential scanning calorimetry, dilatometric measurements for linear expansion, and differential thermal analysis for thermal diffusivity. For all of these methods, were used dynamic argon atmosphere measurements in thermal transient conditions, at heating and cooling rates ranging between 0.0833 and 0.833 Ks^-1. The results on the cool-down regions of the measurements show two distinct behaviours, at temperatures corresponding to the δ (350÷700K) and the β structures of the zirconium hydride (700÷1355 K). From the equations of heat capacity, density and those of the thermal diffusivity, the equation of thermal conductivity of UZrH was found. For a large range of temperature (400÷1100 K) the thermal conductivity was found to be independent of temperature. (author).