NEPTUNIUM INTERACTION WITH CRYSTALLINE ROCK SURFACES
An area in a gneiss rock massive has been chosen as one of the prospective sites for a deep geological repository. The rock massive on this site at the repository depth consists of biotite gneisses altered to different grades, with intrusions of dolerite.
Neptunium is one of the long-lived radionuclides that is potentially mobile in the environment. It is a redox sensitive radionuclide that can be present in the environment in different oxidation states. The most stable and probable oxidation states of Np in the range of repository conditions are IV and V in anoxic and oxic conditions, respectively. Since the most probable oxidation state of neptunium at the beginning of repository operation is (V), the initial form of neptunium in all experiments was NpO2+.
The kinetics of neptunium uptake by gneiss and dolerite from ground water were evaluated in oxic and anoxic environments at 20 оС and 90 оС. All experiments were done for crushed or solid rock samples collected from the studied site and model underground water.
Neptunium (V) was sorbed from groundwater on polished surfaces of gneiss and dolerite samples collected from the above-mentioned site in oxic and anoxic environments at 20 оС and 90 оС. The distribution of sorbed neptunium was studied by autoradiography. Single mineral grains with high concentrations of sorbed neptunium were detected, and the neptunium oxidation state was studied by XPS.