Microstructural Characterization of U Coprecipitated Phases Formed in Bentonic-Granitic Groundwater and under Anoxic Conditions

Abstract

For improving the accuracy of the performance assessment studies related to the spent fuel safety under storage conditions, it is necessary to develop a new matrix alteration model. These models must be based on laboratory experiences and they should be capable of extrapolating storage environmental conditions. The most recent models developed include the oxidation and dissolution process of the spent fuel matrix, but the influence of a possible process of secondary phase formation over the spent fuel surface was not yet taken into account. This is a key process that could produce a reduction of the matrix dissolution rate/radiation shielding behaviour; however, the surface precipitation of the secondary phase could induce a localized corrosion process, in which case the dissolution rate of the spent fuel would be increased.This paper is focussed on microstructural characterization of secondary phases formed in co-precipitation experiments performed under anoxic conditions in granitic-bentonitic simulated groundwater. In order to simulate the influence of the container material, the co-precipitation experiments were performed in the absence and presence of iron powder. The solid phases formed were characterized using the following techniques: XRD; SEM-EDX, and TEM-EDX.The XRD diffraction pattern showed that under anoxic conditions, a mixture of phases were obtained (sodium and potassium uranate and schoepite), whereas uranate phases were detected when only iron was present. The characterization study indicates that the U secondary phase formed (under reducing conditions and in the presence of iron powder) growth from the iron surface. The crystal size of the secondary phase is independent of the presence of iron powder (and it is always less than 3 µm). Furthermore, the microstructural study showed the growing of U phases over iron powder.Mater. Res. Soc. Symp. Proc. Vol. 985