The Effect of Ground Water - Rock Interactions on the Migration of Redox Sensitive Radionuclides

Abstract

ABSTRACTInteractions of redox sensitive elements with common Finnish rocks were studied in the laboratory under oxic and anoxic conditions. Sorption and diffusion of technetium, uranium, plutonium and neptunium were studied in drill cores of granites and gneiss. Cylindrical drill core cup samples and plane surfaces sawed from drill cores were exposed for periods from six to eighteen months to synthetic ground water to which Tc-99, U-233, Pu-236 or Np-237 had been added. Identification of the sorption and the diffusion pathways were discovered by qualitative autora-diography. The concentrations of elements versus depth in the drill core were measured radiometrically, by quantitative autoradio-graphy or by both methods. The migration of plutonium was slow in all samples. Technetium, uranium and neptunium were transported effectively under both oxic and anoxic conditions. Transported species of these elements were found throughout the rock matrices investigated - in microfissures, along mineral grain boundaries and in the minerals themselves. The ranges of the measured apparent diffusivities for neptunium were (2.7 – 30) × 10−14 m2/s under oxic conditions and (1.4 – 4.3) × 10−14 m2/s under anoxic conditions. For uranium the range of Da-values under oxic conditions was (3.6 – 5.7) × 10−14 m2/s under anoxic conditions it was (1.1 × 4.3) × 10−14 m2/s in granitic ground water and (1.8 – 5.3) × 10−14 m2/s in bentonite water.