Se(IV) uptake by Äspö diorite: Micro-scale distribution

Abstract

Risk assessment of deep geological repositories (DGR) for high level radioactive waste demands radionuclide (RN) sorption data, with minimised uncertainties, on repository barriers. Sorption data in crystalline rocks, are principally obtained on crushed material and reported as distribution coefficients, relative to the mass of solid (Kd in m3/kg). Main sources of uncertainty on available sorption data are due to differences in experimental approach (comparison of crushed and intact material), groundwater and rock composition, redox conditions or the high heterogeneity of crystalline rock. In this study, the effect of rock mineral heterogeneity on selenium(IV) surface distribution on diorite crystalline rock was analysed under oxic and anoxic conditions. The micro-Particle Induced X-ray Emission (lPIXE) ion beam technique was selected because it allows quantifying tracer distribution directly on intact rock samples, at mineral micro-scale. Diorite samples were extracted from the Äspö underground research laboratory (Sweden), handled and transported under anoxic conditions. Maintaining controlled redox conditions during the whole experiment is considered particularly relevant to preserve real repository conditions. Selenite distribution on Äspö diorite surface was heterogeneous, its retention being higher under anoxic conditions. By microPIXE analyses the main Se retentive regions were identified, equivalent in both oxic/anoxic conditions, and surface distribution coefficients (Ka) were determined on main diorite minerals. Sorption values ranged from near zero, on quartz or K-feldspars, to higher values on Fe-bearing minerals like biotite (Ka 7 · 10-5 m) under anoxic conditions. Experimental Ka values determined here are compared to reported distribution coefficients (Kd).