Recovery of cold-work in extruded Zr-2.5 wt% Nb

X-ray line broadening measurements and electron microscopy have been used to characterize the dislocation substructures in extruded, cold-worked and stress relieved Zr-2.5 wt% Nb pressure tube materials. Variation in dislocation substructure deduced from the X-ray line broadening measurements give good agreement with thin film observations. Recovery of cold-work occurs in three “stages” in Zr-2.5 wt% Nb. Between 575 and 725 K the dislocation density decreases from $\text{≈1.5–1.8 × 10}{}^{15}\text{m}{}^{\mathrm{?2}}\text{to ≈3–4 × 10}{}^{14}\text{m}{}^{\mathrm{?2}}$ with little change in sub-grain size or dislocation arrangement below 725 K. From 725 K to 875 K the sub-grain size increases from 〈 100 nm to ≈ 400 nm while the dislocation density decreases slowly to $\text{≈ 1.4–1.7 × 10}{}^{14}\text{m}{}^{\mathrm{?2}}$. Above 875 K the sub-grain size increases to ?800 nm, some grain growth occurs and only a few well defined dislocation networks remain. As-extruded Zr-2.5 wt% Nb has a sub-grain size of ≈600 nm and a dislocation density of ≈8 × 10¹³ m ^?2. The implications of the measurements are discussed.     

Evolution of defects in micropellet fuel during the fabrication of HTGR compacts

The stress distribution arising in micropellets and cylindrical fuel compacts during fabrication, the stress concentration in micropellets located near the surface of a compact, and the evolution of defects in micropellets as a function of the type of stress state are investigated.It has been found that an ensemble of micropellets with a large number of particles contains a continuous spectrum of defects in the range 10^–4–10² µm. Mechanical stresses engender evolution of the defects according to the scheme accumulation of microdefects microcracks cracks through defects.Recommendations are formulated for lowering the number of defects in micropellets during deposition of coatings on the micropellets and compaction.Research Institute of the Luch Scientific and Industrial Association.__________Translated from Atomnaya Énergiya, Vol. 98, No. 1, pp. 44–50, January, 2005.     

Micro-texture of extruded Zr-2.5Nb tubes

We report the micro-texture of two extruded Zr-2.5Nb tubes determined using scanning electron microscopy combined with electron back-scattering diffraction (SEM/EBSD) and transmission electron microscopy and selected area diffraction (TEM/SAD). The pole figures determined by SEM/EBSD correspond well with bulk pole figures previously determined by X-ray diffraction (XRD). Three components of texture are seen to correlate with the shape and morphology of the α-grains and their contained dislocation substructures. The first component corresponds to elongated alpha grains containing a high density of a and c + a dislocations in which the c-axis is oriented at a relatively high angle to the long dimension of the α-grains as viewed in transverse section; these grains comprise a texture component with the c-axes in the radial transverse plane, tilted towards the radial direction. The second component corresponds to elongated α-grains containing a low dislocation density in which the c-axis is oriented parallel to the long dimension of the alpha grains: these grains also comprise a texture component with the c-axis in the radial/transverse plane, but predominantly in the transverse direction. The final component corresponds to colonies of Widmanstätten-like α-grains that are transformed from the β-phase: the majority of these grains have their c-axes in the axial direction. These grain have very low dislocation densities and are probably developed during cooling, after extrusion.     

Thermal stability of hydrostatically extruded EUROFER 97 steel

EUROFER 97 steel is a candidate structural material for the future fusion power reactors, as well as for the European Test Blanket Modules (TBMs) to be tested in ITER. In the reported study, the microstructure of EUROFER 97 was modified by hydrostatic extrusion (HE) which reduced the grain size from 400 to 86 nm and that of the carbide particles from 111 to 75 nm. The changes in the microstructure significantly improved the strength of the extruded samples. However, it is important that the enhanced properties of nanostructured materials are stable over the required range of intended service temperature. The thermal stability of the nanostructured EUROFER steel was evaluated by subjecting the hydrostatically extruded samples to annealing at temperatures ranging from 473 to 1073 K (200–800 °C) for 1 h. Tensile tests and microhardness measurements with a 200 g load were carried out on the annealed samples to determine the effect of the heat treatment. The results show that the highest microhardness (403 HV_0.2) was achieved for samples annealed at 673 K. However, the tensile and yield strength decreased at the higher temperature of 873 K and the total elongation increased to 15%, compared to only 3% for as-extruded samples. The changes in the mechanical properties were rationalized by the examination of the microstructural changes. During heating the initial grain size remains virtually unchanged below a temperature of 873 K. However, above 873 K the grain size increased and it is very likely that growth will be very rapid at higher temperatures.     

Factors affecting the swelling during degassing of compacts containing uranium-aluminum intermetallics dispersed in aluminum

Above 500 °C the swelling of dispersions of UAl₃ in aluminum occurs in two stages. In the first, the swelling is gradual and believed to be due to adsorbed gases that are released from the surfaces of the aluminum and UAl₃ powder particles. After a varying “incubation time”, the second stage of swelling at a greatly increased rate begins. This incubation period increases with increasing UAl₃ particle size and decreasing temperature. The second stage is due to the transformation of the UAl₃ to UAl₄ through reaction with the aluminum matrix.     

Fracture mechanical properties and neutron irradiation effects of fuel compacts for the HTTR

To simulate the nuclear fuel for High Temperature Engineering Testing Reactor (HTTR), fuel compact models using SiC-kernel coated particles instead of UO₂-kernel coated particles were prepared under the same conditions as those for the real fuel compact. The mechanical and fracture mechanics properties were studied at room temperature. The thermal shock resistance and fracture toughness for thermal stresses of the fuel compact were experimentally assessed by means of arc discharge heating applied at a central area of the disk specimens. These model specimens were then neutron irradiated in the Japan Material Testing Reactor (JMTR) for fluences up to 1.7 × 10²¹n/cm² (E ·> 29 fJ) at 900°C ± 50°C. The effects of irradiation on a series of fracture mechanical properties were evaluated and compared with the cases of graphite IG-110 used as the core materials in the HTTR.     

Irradiation effects in magnesium and aluminum alloys

Effects of neutron irradiation on microstructure, mechanical properties and swelling of several magnesium and aluminum alloys were studied. The neutron fluences of $\text{2?3 × 10}{}^{22}\text{n/cm}{}^2$, $\text{> 0.2}\text{MeV}$ produced displacement doses of 20 to 45 displacements per atom (dpa). Ductility of the magnesium alloys was severely reduced by irradiation induced recrystallization and precipitation of various forms. Precipitation of transmuted silicon occurred in the aluminum alloys. However, the effect on ductility was much less than for the magnesium alloys. The magnesium and aluminum alloys had excellent resistance to swelling: The best magnesium alloy was Mg/3.0 wt% Al/0.19 wt% Ca; its density decreased by only 0.13%. The best aluminum alloy was 6063, with a density decrease of 0.22%.     

Sintering studies of urania powder compacts — Analysis of a flow model

An attempt has been made to analyse the sintering data of urania powder compacts prepared form its precursor ammonium diuranate calcined and sintered in hydrogen atmosphere for various soaking times and temperatures. Better sintering behaviour was observed in compacts prepared from powders calcined below 800 ° C, which were non-crystalline under X-ray diffraction study. Frenkel's sintering mechanism as applicable for the non-crystalline powders was applied to understand the densification behaviour in which the viscosity term was linearly related to shear stress and strain rate as also its relationship with time was assumed to be either linear or exponential in nature. A plot between shear stress (to induce densification) and time indicated that (i) a critical time (T_crit) is necessary to induce any observable sintering stress in these powders and T_crit, values increased with decrease in calcination temperature and (ii) the amount of shear stress increased with higher sintering temperatures and was related to time, exponentially.     

Homogenization in powder compacts of U02-Pu02

The homogenization kinetics in mixed UO₂-PuO₂ compacts have been studied by adopting a concentric core-shell model of diffusion. An equation relating the extent of homogenization expressed in terms of the fraction of UO₂ remaining undissolved and the time of annealing has been derived. From the equation, the periods required at different annealing temperatures to attain a specified level of homogenization have been calculated. These calculated homogenization times have been found to be in fair agreement with the experimentally observed homogenization times. The derived relationship has also been shown to satisfactorily predict homogenization in Cu-Ni powder compacts.     

Thermal neutron cross-sections of magnesium

Basic data necessary for the calculation of the thermal neutron scattering cross-section of magnesium were defined. The calculation scheme is the standard one used in reactor physics. For this purpose, the NJOY code is the main piece of software. Small modifications were needed in the NJOY's module LEAPR to take into account the specific lattice structure of magnesium. As a result we obtain a ENDF-6 standard conforming thermal data evaluation containing both coherent elastic scattering and incoherent inelastic scattering. The calculated values were checked against experimental values. Finally, a comparison of transmission coefficients through magnesium annular structure was done using the free gas and crystal models.     

EDXRF方法测量菱镁矿中镁元素含量

采用间接测量的方法,对菱镁矿的化学分析过程进行处理,建立目标元素Mg与中间观测元素Ba之间的对应关系,以Ba的能量色散X射线荧光(Energy Dispersive X-ray Fluorescence,EDXRF)测量结果,计算出菱镁矿测试样品中镁的含量为29.43%,相对标准偏差为0.58%。借助于具有含量梯度的验证样品对方法的准确性进行了验证,得到的定标曲线具有良好的线性关系,测量结果显示准确度和可靠性高、稳定性和重复性好。该方法克服了EDXRF对轻元素直接探测效率低、结果精度差的缺点,为镁的测量提供了一种快速、准确、经济的检测思路和方法。     

Aggregation of ion-implanted silver in magnesium oxide single crystals

Ag precipitates are obtained by thermal annealing between 25°C and 1500°C of MgO single crystals implanted at room temperature with Ag⁺ ions (180 keV, 5 × 10¹⁶−10¹⁷ ions cm⁻²). These results are analysed in the light of MIE theory which has bee justified by TEM observations.The TEM enabled us to clarify the nature of the precipitates (metallic silver), their shape, their size and their orientation in relation to the matrix.This relationship was defined as: [100]_Ag∥[100]_MgO and (001)_Ag∥(001)_MgO.     

Radiation-induced luminescence in magnesium aluminate spinel crystals and ceramics

Radioluminescence (RL) and thermoluminescence (TL) in spinel crystals and ceramics were investigated to elucidate the radiation-induced electronic processes in single crystals grown by Verneuil and Czochralski methods as well as transparent and translucent ceramics. Both RL and TL spectra demonstrate a UV-band related to electron–hole recombination luminescence at intrinsic defects; green and red luminescence are identified with emission of Mn²⁺- and Cr³⁺-ions, respectively. The kinetics of growth of different RL luminescence bands depending on dose at the prolonged X-irradiation shows the competitive character of charge and energy transfer between defects and impurity ions. The dependence of RL intensity on the temperature of the sample was measured in the range of 300–750 K and compared with TL for different emission bands. The variety of maxima in the temperature dependence of RL and in the glow curves of TL measured for different luminescence bands in spinels of different origins and crystalline forms is used to show that charge carrier traps and luminescence centers are not isolated defects but are complexes of defects and impurities. The formation, structure and properties of these complexes depend on the processing conditions.     

PIGE analysis of magnesium and beryllium

In this work, we present an alternative method for PIGE analysis of magnesium and beryllium in thick samples. This method is based on the ERYA – Emitted Radiation Yield Analysis – code, which integrates the nuclear reaction excitation function along the depth of the sample. For this purpose, the excitations functions of the ²⁵Mg(p,p′γ)²⁵Mg (E_γ = 585 keV) and ⁹Be(p,γ)¹⁰B (E_γ = 718 keV) reactions were employed. Calculated gamma-ray yields were compared, at several proton energy values, with experimental yields for thick samples made of inorganic compounds containing magnesium or beryllium. The agreement is better than 5%. Taking into consideration the experimental uncertainty of the measured yields and the errors related to the stopping power values, this agreement shows that effects as the beam energy straggling, ignored in the calculation, seem to play a minor role.     

Study of homogenisation and cation interdiffusion in mixed UO2-PuO2 compacts by X-ray diffraction

Homogenisation in mixed UO₂-PuO₂ compacts has been studied by X-ray diffraction. It is observed that the homogenisation proceeds, mainly, by the assimilation of UO₂ into PuO₂. This near one-way flow of material, from UO₂ to PuO₂, is shown to be due to high activity (large BET surface area) of the PuO₂ powder as compared with that of the UO₂ powder.An X-ray line profile analysis method of determining various mixed composition fractions in sintered mixed compacts has been used to evaluate homogeneity in terms of the fraction of UO₂ that has gone into PuO₂. A concentric core-shell diffusion model, in which UO₂ forms a solute core and PuO₂ forms a solvent shell, was used to determine cation interdiffusion coefficients from the homogenisation data. The temperature dependence of cation diffusivity in the range 1573–1873 K is obtained as $\text{D = 2.55 × 10}{}^{\mathrm{?11}}\text{exp(?2.22 × 10}{}^5\text{/8.31 T)}$ m²/s. The low value (222 kJ/mole) of activation energy for cation interdiffusion is attributed to the hypostoichiometry of the mixed compacts studied.The diffusivity values at 1573 and 1673 K separately give an activation energy of 126 kJ/mole, which suggests grain-boundary diffusion as the primary mechanism of homogenisation in this temperature range.