XRD Monitoring of α Self-Irradiation in Uranium–Americium Mixed Oxides

Abstract

The structural evolution under (241)Am self-irradiation of U(1-x)Am(x)O(2±δ) 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±δ) 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±δ) and U0.5Am0.5O(2±δ), 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).

In situ characterization of uranium and americium oxide solid solution formation for CRMP process: first combination of in situ XRD and XANES measurements

“…The coexistence of these two phases can be explained by a low oxidation kinetic at RT, as was previously reported. 32 Oxidation process starts from the surface of the particles to the bulk, as illustrated by the minor contribution of the smaller-lattice-parameter phase corresponding to the oxidized phase. A new diagram was recorded after 3 days of reoxidation, on another pellet fabricated with the same process.…”

Melting behaviour of americium-doped uranium dioxide

“…However, because a delay of few weeks (for XRD) to a couple of months (for XAS) was inevitable between the laser heating/cooling treatments and the post-melting material characterisation, the samples underwent some self-irradiation damage, notably those with the highest Am content. This could be viewed as a partial explanation for the complex phase splitting and higher O/M ratio observed in the U 0.8 Am 0.2 O 2+d sample as several studies have shown that a-selfirradiation-induced structural effects on (U, Am)O 2 compounds are limited to lattice swelling reaching up to 0.8 vol.% and a small structural disorder increase [45,46,[52][53][54].…”

Comparative XRPD and XAS study of the impact of the synthesis process on the electronic and structural environments of uranium–americium mixed oxides

“…1. All three sintered samples exhibit a single fluorite-type phase whose lattice parameters are summarized in Table 2 with values corrected for selfirradiation effects (based on data reported by Horlait et al [26]). Close initial (i.e.…”