A Laboratory Study on Radionuclide Migration in Single Natural Granitic Fissures

Migration of ^I52Eu(III), ²³⁵Np(V), ²³⁷Pu(IV), ²⁴¹Am(III), ⁹⁹Tc(VII), and ⁹⁹Tc(IV) was studied in natural fissures oriented parallel to the axis of granitic drill cores. A pulse of radionuclide solution was injected at one end of the fissure and the temporal change in radionuclide concentration of the effluent measured. At the end of each experiment the fissure was opened and the radionuclide distribution on the fissure surfaces measured. The radionuclide distribution ratios R_a, calculated from the measured retardation, correlate well to published distribution coefficients K_d. The increase in retardation of ²³⁵Np with increasing pH in the pH range 6 to 9 is in accordance with the hydrolytic equilibrium NpO₂⁺ + OH‾ ⇄ NpO₂(OH). Reduction of Tc(VII) to Tc(IV) resulted in a marked increase in retardation.