Alteration of dehydrated schoepite and soddyite to studtite, [(UO2)(O2)(H2O)2](H2O)2

Abstract

The oxidation of used nuclear fuel in a geologic repository has important implications for the mobility of radionuclides and fission products in the environment. Hexavalent uranium (uranyl) minerals, including oxyhydroxides and silicates, form as alteration phases on the surface of fuel pellets in laboratory simulations. However, alpha-radiolysis of water forms hydrogen peroxide in solution, which may favor the alteration of these secondary phases to the uranyl peroxide mineral studtite. This study investigates the alteration of dehydrated schoepite, UO 3 (H 2 O), and soddyite, [(UO 2 ) 2 (SiO 4 )] (H 2 O) 2 , in the presence of aqueous solutions containing hydrogen peroxide. Crystalline samples were reacted with various concentrations of hydrogen peroxide and the resulting material was analyzed by powder X-ray diffraction. Both dehydrated schoepite and soddyite readily convert to studtite in the presence of hydrogen peroxide following the reaction stoichiometry. These results indicate that the possible impact of peroxide buildup on the stability of alteration phases in a repository setting should not be overlooked.

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

“…This means that the stabilization of becquerelite under these conditions is not as large as that of studtite phase. As it was observed experimentally by Kubatko et al 93 and Forbes et al, 148 becquerelite, dehydrated schoepite and soddyite readily transform into studtite in the presence of high hydrogen peroxide concentrations. The transformation of uranium trioxide, rutherfordine, metastudtite, schoepite and metaschoepite into studtite was also predicted in our previous works.…”

Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite

“…Although vandenbrandeite, as it occurred with schoepite, metaschoepite and becquerelite uranyl oxide hydrate phases, 115,116 is largely stabilized under high concentrations of hydrogen peroxide, its stabilization under these conditions is not as large as that of studtite phase. 124 Kubatko et al 205 and Forbes et al 206 observed experimentally that becquerelite, dehydrated schoepite and soddyite transform readily into studtite in the presence of high concentrations of hydrogen peroxide. The transformation of uranium trioxide, rutherfordine, metastudtite, schoepite and metaschoepite and becquerelite into studtite was also predicted in previous works.…”

The Application of Periodic Density Functional Theory to the Study of Uranyl-Containing Materials: Thermodynamic Properties and Stability

“…The first eight materials have been identified to be basic components of the paragenetic sequence of secondary phases arising from the alteration of uraninite ore deposits and corrosion of SNF under the final DGR conditions [42][43][44][45][46][47][48][49][50][51][52][53][54], and gamma uranium trioxide is the main oxide of hexavalent uranium [100,106,107]. Uranyl peroxides appear in the earlier stages of this paragenetic sequence [50][51][52][53][54][108][109][110][111] due to the production of hydrogen peroxide and other oxidants resulting from the radiolysis of water due to the ionizing radiation of the SNF. The uranyl oxyhydroxides also begin to appear soon from the alteration of uranium dioxide [42][43][44][45][46][47][48].…”