Search

The high stability of zirconium dioxide in aqueous environments is known and demonstrated, and this property is strongly used in nuclear industry to ensure the long term storage of wastes. However, only upper limits of its aqueous solubility are known reliably and lower limits linked to very well crystallized ZrO2 are much less assessed. Indeed, the low dissolution rate of zirconia makes the solubility measurements a challenging task. To overcome, high S/V ratios of nanoparticles zirconia were used. This work also improved the sensitivity of analytical techniques (HR ICP-MS) and methodologies, and a reliable experimental procedure was developed to measure zirconium (quantification limit ≈10−11 mol∙L−1). New Zr(IV) dioxide solubility data at pH between 0 and 2 were obtained approaching solubility from under-saturated conditions in (Na,H)Cl and (Na,H)ClO4 medium. Two crystalline nanoparticle structures were compared: monoclinic and yttrium stabilized cubic zirconia. Very low solubility was measured for monoclinic phase between pH 1.5 and 2: between (1.8±1.2) × 10−10 mol∙L−1 at pH 2 and (2.3±1.0) × 10−10 mol∙L−1 at pH 1.5. The cubic zirconia showed higher solubility. Integrating the effect of ionic strength, particle size and aqueous speciation, solubility constants of log K s 0 = (-8.43±0.69) for the monoclinic nanoparticles and log K s 0 = (-7.12±0.35) for the yttrium stabilized cubic nanoparticles were obtained. High-resolution techniques (HR-TEM, SAXS and STEM-HAADF) were also used to assess the evolution of morphology and surface before, during and at equilibrium. Analysis of these results shows that the morphology and surface of nanoparticles in the raw state and after reaching equilibrium in (Na,H)Cl and (Na,H)ClO4 medium are similar.

Radiation effects in four synthetic ferrate garnets (A3B2(XO4)3, Ia3d, Z = 8) were examined by ion beam irradiations with in situ observation (T = 298–873 K) using transmission electron microscopy (TEM) at the IVEM Tandem Facility at Argonne National Laboratory. The garnet compositions include: A = Ca, Gd, Th, and Ce; B = Zr, Fe. The critical amorphization temperatures (Tc), the temperature above which the target material cannot be amorphized due to dynamic annealing, were between 820 and 870 K. The amorphization doses at room temperature are between 0.17 and 0.19 dpa (displacement per atom), which is similar to that of silicate- and aluminate-garnets. The small variations in the amorphization dose and Tc of the different compositions suggest that radiation effects in ferrate garnets are structurally constrained. Qualitative analyses of the valence states of Ce and Fe in garnet before and after irradiation were completed using electron energy-loss spectroscopy (EELS). The Ce and Fe in the unirradiated garnet were dominantly trivalent and divalent, respectively. The characteristic peaks of Ce 4+ ,a t5 eV higher energy for the M-edges, were present in unirradiated garnet as a minor peak, and the peaks did not disappear after complete amorphization, suggesting that the valence state did not change significantly. EELS analysis was conducted on a nearly pure andradite, Ca3Fe2Si3O12, which ideally contains only ferric iron. The andradite was amorphized at 0.18 dpa. EELS analysis revealed that some of ferric iron was converted to ferrous iron during the irradiation. � 2004 Elsevier B.V. All rights reserved.

Britholite, ideally Ca4−xREE6+x(SiO4)6O2 (REE=rare earth elements), has the hexagonal structure of apatite, a candidate waste form for actinides. Two synthetic britholites: Ca3.05Ce2.38Fe0.25Gd5.37Si4.88O26 (N56) and Ca3.78La0.95Ce1.45Zr0.78Fe0.14Nd2.15Eu0.50Si6.02O26 (N88) (P63; Z=1) were irradiated with 1.0 MeV Kr2+ and 1.5 MeV Xe+ over the temperature range of 50–973 K. The process of ion irradiation-induced amorphization, including the effects of the target mass and the ion mass, and the recrystallization of amorphous domains due to ionizing irradiation were investigated. The critical amorphization temperature, Tc was determined to be 910 K for N56 (1.0 MeV Kr2+), 880 K for N88 (1.0 MeV Kr2+) and 1010 K for N88 (1.5 MeV Xe+). The sequence of increasing Tc correlates with the mass of the incident ion; whereas, the ratio of electronic to nuclear stopping power (ENSP) is inversely correlated with Tc. Electron irradiations were conducted on previously amorphized britholite (N56) with an electron flux of 1.07 × 1025 e−/m2/s. The ionizing radiation resulted in recrystallization at the absorbed dose of 6.2 × 1013 Gy. This result suggests that the ionizing radiation can induce recrystallization in silicate apatites, similar to that observed for phosphate apatite.

The radiation susceptibility of garnet structure types (A3B2(XO4)3, Ia3d, Z=8) has been examined by 1.0 MeV Kr2+ irradiation with in situ transmission electron microscopy over the temperature range of 50–1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate–aluminates (G-series). The critical amorphization temperature (Tc), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. Tc of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the Tc, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.