A Modelling Study of the Effect of Rock Alteration on the Redistribution of Uranium
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A MODELLING STUDY OF THE EFFECT OF ROCK ALTERATION ON THE REDISTRIBUTION OF URANIUM TAKASHI MURAKAMI AND HIDEO KIMURA Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
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 23 0Th/ 2 34 U 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. INTRODUCTION Groundwater reacts with rock-forming minerals near the earth's surface. The water-rock interactions over long geologic timescales result in chemical and structural changes to the minerals. Because of the different physical and chemical properties of minerals, the mineral changes occurring for millions of years may have a great influence on radionuclide retardation, and thus, radionuclide migration after the disposal of radioactive waste. However, the effect of such rock alteration on the radionuclide migration has not been estimated before. By their mineralogical study of the relationships between altered minerals and uranium distribution, Murakami et al. have pointed out that chlorite weathering has influenced the uranium redistribution at Koongarra, Australia [1]. On the basis of their observations, a modelling study was carried out to quantitatively understand the effect of rock alteration on uranium radionuclide concentrations in rocks.
CHANGES OF POROSITY AND DISTRIBUTION COEFFICIENT RESULTING FROM ROCK ALTERATION At Koongarra, quartz-chlorite schist, the host rock, has been subjected to weathering for more than one million years. Quartz is resistant to weathering, but Mat. Res. Soc. Symp. Proc. Vol. 294. ý'1993 Materials Research Society
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2.0
Time (My) Before Present 0.5 1.8
0
/
Arbitrary Uranium Concentration
Surface 1Om
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SKaot nile
Weathered Zone
Groundwater -Verutltee =Transition
Zone
/ / /Chlorlte
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