Uranium and Thorium Disequilibrium in Zeolitically Altered Rock

Extensive studies conducted during the past two decades regarding the behavior of ²³⁸U and its decay products in the geologic environment have shown that radioactive disequilibrium in the ²³⁸U decay series is a common phenomenon. The daughter products ²³⁴U and ²³⁰Th are especially useful as indicators of processes affecting uranium migration because of their relatively long half-lives and contrasting chemical behavior. A striking variability in ²³⁴U/²³⁸U ratios occurs in zeolitically altered volcanic tuff investigated in this study. Results indicate that initial preferential emplacement of ²³⁴U by adsorption-alpha recoil mechanisms gradually was overwhelmed by preferential displacement of ²³⁴U by leaching-alpha recoil mechanisms. The data provide guidelines for predicting the behavior of chemically analogous members of the ²³⁷Np decay series (²³³U, ²²⁹Th) in the natural environment following geologic disposal of radioactive wastes.