Sorption and Diffusion of Radionuclides in Rock Matrix and Natural Fracture Surfaces Studied by Autoradiography

Abstract

A method based on autoradiography was developed to determine the sorption and diffusion of cesium, strontium, cobalt, nickel, iodine and americium into rock matrix. Samples chosen for this study were filled and unfilled natural fracture surfaces and drill cores having a central drilled hole (“drill core cups”). Rock types were mica gneiss, tonalite and rapakivi granite, which were selected to represent the common rocks and minerals in Finnish bedrock. Distribution coefficients (K_a-values) of cesium and strontium determined for fissure surfaces and drill core cups were of the same order of magnitude. After three months contact time the greatest penetration depth for cesium was 2.5 mm, for a natural fissure surface sample of rapakivi granite. For strontium the penetration depths of 11 mm in three months and 35 mm in twelve months were found for filled natural fissure surface samples of rapakivi granite. The range of D_a-values was 1.5 × 10⁻¹⁵–3.2 × 10⁻¹⁴ m²/s and 1.4 × 10⁻¹⁴–2.1 × 10⁻¹³ m²/s for cesium and strontium, resp. For cobalt the D_a-values of 5 × 10¹⁶ m²/s in tonalite was obtained. In six months the penetration depths of nickel, iodine and americium were too low (< 0.5 mm) to allow calculation of D_a.