From X-ray diffraction, scanning electron microscopy, X-ray absorption near edge structure (XANES) and diffuse reflectance (DR) spectroscopic studies, it was found that approximately 0.03 formula units (f.u.) of mixed hexavalent and pentavalent U, but <0.001 f.u. of NI or Np, can be dilutely incorporated into the Ti site of rutile, TiO2, sintered at 1400 degrees C in air. The valence of 0.01 f.u. of U in Zr(l-x)YxO2-x/2 (x = 0.23-0.5) fired in air is also between +5 and +6 based on XANES. The presence of U5+ is confirmed in both rutile and doped zirconia samples by DR spectroscopy. The valence of 0.01 f.u. of Np in Zr(1-x)YxO2-x/2 (x = 0.23-0.5) fired in air is found to be +6 from DR study. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.

Diffuse reflectance spectroscopy (DRS) measurements were made over the wavenumber range of 4000–20,000cm−1 at room temperature on monoclinic and stabilized zirconia and pyrochlore-structured Y2Ti2O7, which were doped with Np or Pu and sintered at ∼1400°C in various atmospheres. Np4+ was present in uncompensated monoclinic zirconia samples sintered in air, Ar or H2/N2. Np6+ was obtained in air/Ar-sintered monoclinic zirconia that also contained Y3+ as charge compensators, and Np4+ was formed on sintering in H2/N2. Np4+ was present in Y-stabilized zirconia after firing in air, Ar or H2/N2. Pu4+ was observed in air-fired Pu-doped monoclinic zirconia, and sintering in Ar or H2/N2 produced Pu3+ while only Pu4+ was identified in Y-stabilized zirconia sintered in air, Ar or H2/N2. Only Pu4+ and Np4+ were observed in Y2Ti2O7 after sintering in air or argon, even when Ca was substituted for Y to try to encourage the formation of higher valence states of Pu and Np.

By using Na+ as a charge compensator and varying sintering conditions, Pu3+ and Pu4+ were incorporated on the Ca site of fluorapatite through sol–gel synthesis, as evidenced by X-ray diffraction, scanning electron microscopy and diffuse reflectance spectroscopy. It appears that Pu incorporation would be unlikely if prepared by a dry powder method, which has implications for previous radiation damage studies of fluorapatite doped with 238Pu.

Tetravalent U, Np and Pu can be substituted by ceramic methods into the rare earth site of xenotime and monazite in air atmospheres using Ca ions as charge compensators, while no evidence of penta- or hexavalent actinide ions was found. Some Pu3+ and Np3+ can be incorporated in xenotime samples fired in a reducing atmosphere.

The dissolution of the thorium analogue of brannerite (ThTi2O6-I) and U(IV)/U(V) doped Th-brannerite (Th0.97U0.03Ti2O6-II and Th0.955U0.03Ca0.015Ti2O6-III) in aqueous media under atmospheric conditions has been studied to elucidate the effects of pH and uranium valence state on the dissolution rate. The dissolution of I is nearly stoichiometric but slightly preferential release of U occurs for II and preferential release of Ca and U occur for III. The V-shape pH dependence previously observed for U-brannerite only occurs for U (not other matrix elements) for II, indicating that the pH dependence is related to the U oxidation state upon dissolution. The normalised U dissolution rates of III are nearly an order of magnitude higher than those of II for pH values over 3, suggesting brannerite is less durable with U(V) doping. TEM examination of specimens after leaching revealed few surface alteration products, which is consistent with the nearly stoichiometric dissolution of thorium brannerite.