Preparation of UO2, ThO2 and (Th,U)O2 pellets from photochemically-prepared nano-powders

Photochemically-induced preparation of nano-powders of crystalline uranium and/or thorium oxides and their subsequent pelletizing has been investigated. The preparative method was based on the photochemically induced formation of amorphous solid precursors in aqueous solution containing uranyl and/or thorium nitrate and ammonium formate. The EXAFS analyses of the precursors shown that photon irradiation of thorium containing solutions yields a compound with little long-range order but likely "ThO2 like" and the irradiation of uranium containing solutions yields the mixture of U(IV) and U(VI) compounds. The U-containing precursors were carbon free, thus allowing direct heat treatment in reducing atmosphere without pre-treatment in the air. Subsequent heat treatment of amorphous solid precursors at 300-550 degrees C yielded nano-crystalline UO2, ThO2 or solid (Th,U)O-2 solutions with high purity, well-developed crystals with linear crystallite size <15 nm. The prepared nano-powders of crystalline oxides were pelletized without any binder (pressure 500 MPa), the green pellets were subsequently sintered at 1300 degrees C under an Ar:H-2 (20:1) mixture (UO2 and (Th,U)O-2 pellets) or at 1600 degrees C in ambient air (ThO2 pellets). The theoretical density of the sintered pellets varied from 91 to 97%. 2015 Elsevier B.V. All rights reserved.