A Modelling Study of the Effect of Rock Alteration on the Redistribution of Uranium

Abstract

A modelling study was carried out to understand the effect of rock alteration on uranium radionuclide concentrations in rocks, in the vicinity of the Koongarra ore deposit, Australia. The one-dimensional, advection-dispersion-sorption model considers two important factors resulting from the process, mechanism and rate of chlorite weathering, one type of rock alteration that has occurred over a million-year period; (a) the changes in the distribution coefficients of uranium and thorium over time, and (b) that in rock porosity. The distribution coefficient and rock porosity at a given time, are expressed as average values of those of the coexisting minerals. By assuming a Darcy velocity of 0.9 m/y and an initial uranium concentration in the groundwater of 0.5 ppm, similar to present-day values, we were able to derive, from the finite-element modelling, uranium concentrations in the rock which are in good agreement with observed values. The calculated values of 230Th/234U activity ratios show a similar trend to those observed, although more experimental data are necessary to confirm the similarity. However, in the absence of the rock alteration, the calculated results did not agree with those observed. The present study suggests that rock alteration should be included in models used to predict uranium migration over long, geologic timescales.

Weathering of Chlorite in a Quartz-Chlorite Schist: I. Mineralogical and Chemical Changes

“…Actinides, a major concern because of their long-lasting radioactivity, may flow from the source and react with then-coexisting minerals of the surrounding rock, which vary with time as a function of the weathering mechanisms and kinetics. Radionuclide migration is affected by mineral weathering (Murakami et al 1991a;Murakami et al 1992) and the effects have been quantitatively estimated (Ohnuki et al 1991;Murakami and Kimura 1993;Ohnuki et al 1993).…”