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 level radioactive waste repositories, the migration of natural radionuclides 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 238U and 232Th decay series have been measured in bulk samples of unaltered and differently altered granites by α-particle spectrometry. The results have established the recent geochemical processed involving 238U, 234U, 230Th, 232Th and 228Th such as: 234U recoil loss, bulk U assimilation or 234U recoil gain, equilibrium states, bulk U leaching without fractionation, bulk uranium assimilation with activity ratio near unity, and loss or gain of 228Ra (228Th) as a function of the hydrochemical characteristics of the percolating water. These results have been interpreted in terms of the different degrees of weathering according to literature data.