Migration of some elements and radionuclides across a granite-granite contact zone: A natural analogue for safe disposal of high-level radwastes

Elements and natural radionuclides in the contact zone of two granites with different ages would migrate from one to the other because of the differen ce in their chemical contents and later water-rock interactions. This migra tion could serve as an analogue for the near-field process of radwastes in a high-level radwaste deep geological disposal repository. In the contact between the Indosinian granite (whole-rock Rb-Sr isochron ag e at 214+/-3 Ma) and Hercynian granite (zircon U-Pb isochron age at 296+/-1 31 Ma) located in Ziyuan County, Guangxi, the O and Ph isotope characterist ics and the activity ratios of U-234/U-238, Th-230/U-238, Th-230/U-234 and Ra-226/Th-230 show that, based on the whale-rock chemical contents, both of the two granites have maintained a relatively open chemical system in thei r evolution processes. However, as there is no obvious open fault, the migr ation of major elements, trace elements and natural U-series nuclides takes place within only 1-2 m in the contact zone, and water-rock interactions i n the weak contact plane only affect the redistribution of chemical constit uents at a distance of 30 cm. Fractures are the pathways for fluid flow. In term of uranium-series disequ ilibrium and the radionuclide data (the abnormally high value of U-234/U-23 8=4.60 in the fracture sample), significant water-rock interactions could c ause the bordering rocks to be altered and leach some radionuclides out. U and Ra migrated selectively at the time scale of 10(6)a and 10(4)a, respect ively, but the redistribution of granite is only within 10 cm from the cont act, and Th seems to he immobile. On the other hand, clay-enriched fillings in the fractures can adsorb most of the leached radionuclides. The adsorpt ion could significantly retard radionuclide migration in the fractures, but its influence in distance is less than 10 cm from the fracture plane. The present data could be viewed as evidence for safety confidence for the high -level radwaste disposal repositories hasted by granitic rocks in China.