Chapter 8 - An Evaluation of Solubility Limits on Maximum Uranium Concentrations in Groundwater

Abstract

This chapter presents a summary of an ongoing work by the Japan Atomic Energy Agency (JAEA) that addresses solubility constraints on the maximum uranium (U) concentrations in natural waters. JAEA has been evaluating such solubility limits specifically for U using field data that characterize the Tono Uranium Deposit in central Japan. The Tono Uranium Deposit provides evidence that U, which is an important component of high-level radioactive wastes (HLW), is effectively immobilized within the deposit in a chemically reducing environment over long periods of time at depths within a few hundred meters of the oxidizing conditions at the earth's surface. A comprehensive and critically evaluated database characterizing the geology, hydrogeology, hydrochemistry, and geomicrobiology of the deposit and its environs has been developed as a result of numerous field studies and drilling campaigns carried out in the Tono area over the past two decades. These data provide an opportunity to test whether maximum U concentrations in the groundwater circulating through the deposit are controlled by the solubility of various minerals that have been assumed to be “solubility controlling” in an HLW repository. The study demonstrates that maximum U concentrations in the groundwater of the Tono Uranium Deposit appear to be limited by the solubility of the amorphous, hydrous oxide UO2 (am). This conclusion supports the assumption that the solubility of UO2(am), or an analogous UO2 solid, would limit the aqueous concentrations of U released from an HLW repository over similar time scales. Equilibrium calculations suggest that UO2(am) solubilities could vary roughly between 10-6 and 10-9 mol kg-1over the range of environmental conditions currently observed in the Tono Uranium Deposit and environs.