Hadean isotopic fractionation of xenon retained in deep silicates.

Our understanding of atmosphere formation essentially relies on noble gases and their isotopes, with xenon (Xe) being a key tracer of the early planetary stages. A long-standing issue, however, is the origin of atmospheric depletion in Xe1 and its light isotopes for the Earth2 and Mars3. Here we report that feldspar and olivine samples confined at high pressures and high temperature with diluted Xe and krypton (Kr) in air or nitrogen are enriched in heavy Xe isotopes by +0.8 to +2.3‰ per AMU, and strongly enriched in Xe over Kr. The upper +2.3‰ per AMU value is a minimum because quantitative trapping of unreacted Xe, either in bubbles or adsorbed on the samples, is likely. In light of these results, we propose a scenario solving the missing Xe problem that involves multiple magma ocean stage events at the proto-planetary stage, combined with atmospheric loss. Each of these events results in trapping of Xe at depth and preferential retention of its heavy isotopes. In the case of the Earth, the heavy Xe fraction was later added to the secondary CI chondritic atmosphere through continental erosion and/or recycling of a Hadean felsic crust.

A new thermo-desorption laser-heating setup for studying noble gas diffusion and release from materials at high temperatures.

A new heating and gas treatment line for Thermo-Desorption Spectrometry (TDS) of noble gases (He, Ne, Ar, Kr, and Xe) is presented. It was built with the primary objective to offer advanced temperature controls and capabilities while working in a cold environment. By choosing a high-power continuous wave laser as the heating source and using a proportional-integral-derivative controller system, TDS of noble gases can now be performed with fast and highly steady heating ramps (e.g., less than 1 °C deviation from the set point for ≤1 °C s-1 ramps). Sample temperature over 2000 °C can also routinely be reached, with limited heating of the sample support and the sample chamber, offering the possibility to have several samples awaiting in the ultra-high vacuum chamber. We also present the development efforts made to increase temperature homogeneity of the heated sample while limiting the contact with the sample holder. Recent results acquired with this TDS setup on krypton thermal diffusion in uranium dioxide (UO2) as a function of O2 additions are also presented as an application example.

On the stoichiometry of zirconium carbide.

The dependencies of the enhanced thermomechanical properties of zirconium carbide (ZrCx) with sample purity and stoichiometry are still not understood due to discrepancies in the literature. Multiple researchers have recently reported a linear relation between the carbon to zirconium atomic ratio (C/Zr) and the lattice parameter, in contrast with a more established relationship that suggests that the lattice parameter value attains a maximum value at a C/Zr ~ 0.83. In this study, the relationship between C/Zr atomic ratio and the lattice parameter is critically assessed: it is found that recent studies reporting the thermophysical properties of ZrCx have unintentionally produced and characterised samples containing zirconium oxycarbide. To avoid such erroneous characterization of ZrCx thermophysical properties in the future, we propose a method for the accurate measurement of the stoichiometry of ZrCx using three independent experimental techniques, namely: elemental analysis, thermogravimetric analysis and nuclear magnetic resonance spectroscopy. Although a large scatter in the results (ΔC/Zr = 0.07) from these different techniques was found when used independently, when combining the techniques together consistent values of x in ZrCx were obtained.

Development toward a double focusing isotopic separator for noble gas isotope enrichment.

A double focusing sector field mass filter used in Nier-Johnson geometry has been built in order to perform Kr isotope enrichment for 81 Kr and 85 Kr isotopes. The principle consists in implanting Kr+ ions accelerated at 7 keV in Al foils after separation using the magnetic sector. A specific ion source has been designed capable of generating high Kr+ ion beams (>0.5 µA) to transfer into the collecting Al foils in 3 to 5 h significant fractions of large Kr samples (1015 to 1016 atoms) initially introduced in the instrument. Implanted Kr isotopes can be further selectively released from the Al foil by surface ablation using an infrared laser beam. Implantation yields and enrichment factors are measured using a conventional mass spectrometer. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis and DFT investigation of new bismuth-containing MAX phases.

The Mn + 1AXn phases (M = early transition metal; A = group A element and X = C and N) are materials exhibiting many important metallic and ceramic properties. In the present study powder processing experiments and density functional theory calculations are employed in parallel to examine formation of Zr2(Al1-xBix)C (0 ≤ x ≤ 1). Here we show that Zr2(Al1-xBix)C, and particularly with x ≈ 0.58, can be formed from powders even though the end members Zr2BiC and Zr2AlC seemingly cannot. This represents a significant extension of the MAX phase family, as this is the first report of a bismuth-based MAX phase.

Peculiar Behavior of (U,Am)O(2-δ) Compounds for High Americium Contents Evidenced by XRD, XAS, and Raman Spectroscopy.

In U(1-x)Am(x)O(2±Î´) compounds with low americium content (x ≤ 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, Am(III+) cations are charge-balanced by an equivalent amount of U(V+) cations while the fluorite structure of pure U(IV+)O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U(0.5)Am(0.5)O(2±Î´) compound. Our results indicate that americium is again only present as Am(III+), while U(V+) remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (Am(III+)) and oxidized (U(V+)) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxygen sublattice, with the observation of both vacancies and interstitials, the latter being apparently consistent with the insertion of U6O12 cuboctahedral-type clusters (as observed in the U4O9 or U3O7 phases). These results thus highlight the specificities of uranium-americium mixed oxides, which behave more like trivalent lanthanide-doped UO2 than U(1-x)Pu(x)O(2±Î´) MOX fuels.

New insight into self-irradiation effects on local and long-range structure of uranium-americium mixed oxides (through XAS and XRD).

Uranium-americium mixed oxides could be used as transmutation targets to lower Am inventory in spent nuclear fuels. Due to (241)Am activity, these materials are subjected to α-self-irradiation which provokes crystallographic disorder. Previous studies on U-Am mixed oxides gave first insight into α-radiation tolerance of these compounds, but have never been carried out for more than a year, whereas these compounds might be stored up to a few years between fabrication and irradiation. In this work, we study effects of self-irradiation on the structure of U(1-x)Am(x)O(2±Î´) solid solutions (x = 0.15 and 0.20) aged 3 to 4 years. Especially, X-ray diffraction and X-ray absorption spectroscopy are combined to observe these effects from both long-range and local perspectives. Results show that the fluorite-type structure of U-Am mixed oxides withstands (241)Am α-irradiation without major damage. Despite the increase of interatomic distances and crystallographic disorder observed during the first months of storage, the present results show that a steady state is then reached. Thus, no detrimental factors have been identified in this study in terms of structural damage for several-year storage of U(1-x)Am(x)O(2±Î´) pellets before irradiation. Furthermore, comparison between long-range and local evolution suggests that α-self-irradiation-induced defects are mainly located in low-ordered domains. Based on literature data and present results, the steady state appears related to the equilibrium between radioinduced defect formation and material self-healing.

XRD monitoring of α self-irradiation in uranium-americium mixed oxides.

The structural evolution under (241)Am self-irradiation of U(1-x)Am(x)O(2±Î´) transmutation fuels (with x ≤ 0.5) was studied by X-ray diffraction (XRD). Samples first underwent a preliminary heat treatment performed under a reducing atmosphere (Ar/H2(4%)) aiming to recover the previously accumulated structural defects. Over all measurements (carried out over up to a full year and for integrated doses up to 1.5 × 10(18) α-decay events·g(-1)), only fluorite U(1-x)Am(x)O(2±Î´) solid solutions were observed. Within a few days after the end of the heat treatment, each of the five studied samples was slowly oxidized as a consequence of their move to air atmosphere, which is evidenced by XRD by an initial sharp decrease of the unit cell parameter. For the compounds with x ≤ 0.15, this oxidation occurred without any phase transitions, but for U0.6Am0.4O(2±Î´) and U0.5Am0.5O(2±Î´), this process is accompanied by a transition from a first fluorite solid solution to a second oxidized one, as the latter is thermodynamically stable in ambient conditions. In the meantime and after the oxidation process, (241)Am α self-irradiation caused a structural swelling up to ∼0.8 vol %, independently of the sample composition. The kinetic constants of swelling were also determined by regression of experimental data and are, as expected, dependent on x and thus on the dose rate. The normalization of these kinetic constants by sample α-activity, however, leads to very close swelling rates among the samples. Finally, evolutions of microstrain and crystallite size were also monitored, but for the considered dose rates and cumulated doses, α self-irradiation was found, within the limits of the diffractometer used, to have almost no impact on these characteristics. Microstrain was found to be influenced instead by the americium content in the materials (i.e., by the impurities associated with americium starting material and the increase of cationic charge heterogeneity with increasing americium content).