Clay minerals suitable for overpack in waste repositories: evidence from uranium deposits

Various clay minerals have been proposed for overpack around radioactive waste canisters. The laboratory studies of the migration of elements from the nuclear waste through proposed overpack have resulted in some disagreement. As an alternative approach to the laboratory studies, a review of data for the uranium deposits of the Grants, New Mexico, mineral belt has been undertaken. This paper reviews the evidence from the clay mineralogy, the Rb-Sr systematics and the trace element analyses of the clay size (<2μ) fraction of rocks from the uranium ore zones and the barren host rocks. The <2μ fraction is dominated by clay minerals, but small amounts of other minerals and organic matter are present as well. The information from uranium deposits suggests that chlorite will form during the mineralization process from pre-existing montmorillonite or illite or mixed-layered illite-montmorillonite accompanied by fixation of V, Mo, Se, As, REE, Th. Based on this fact coupled with the observation that the barren rocks are magnetite-rich under low Eh conditions, then an argument for the use of the very abundant sand-poor shales of the Morrison Formation as material suitable for overpack can be made. In the event of canister failure, any escaping actinides, actinide daughters or fission products should react efficiently with the intermediate cation exchange capacity (CEA) montmorillonite-illite-rich material and promote low CEC chlorite formation and sorption of actinides and lanthanides. Formation of such low CEC chlorite would not only scavenge the earliest radioactive elements released but would also retard fluid flow through the surrounding overpack by fissure filling which in turn would further impede radionuclide escape. The distribution of trace elements around uranium deposits of the Grants mineral belt is entirely compatible with such a scenario