U AND TH SERIES DISEQUILIBRIUM IN UNALTERED AND HYDROTHERMALLY-ALTERED GRANITES FROM THE EL BERROCAL SITE (SPAIN) - WEATHERING EFFECTS

Within the framework of analysis of the safety assessment of high leve l radioactive waste repositories, the migration of natural radionuclid es in a fissured granitic environment, the El Berrocal site, is being studied in Spain. In order to determine the weathering effects on the natural radionuclides of the site, the top parts of the U-238 and Th-2 32 decay series have been measured in bulk samples of unaltered and di fferently altered granites by cc-particle spectrometry. The results ha ve established the recent geochemical processes involving U-238, U-234 , Th-230, Th-232 and Th-228 such as: U-234 recoil loss, bulk U assimil ation or U-234 recoil gain, equilibrium states, bulk U leaching withou t fractionation, bulk uranium assimilation with activity ratio near un ity, and loss or gain of Ra-228 (Th-228) as a function of the hydroche mical characteristics of the percolating water. These results have bee n interpreted in terms of the different degrees of weathering accordin g to literature data. Copyright (C) 1996 Elsevier Science Ltd