THERMODYNAMICS OF HYDROTHERMAL URANYL MINERAL FORMATION AT NOPAL I

Oxidizing conditions at the Nopal I deposit in the Peña Blanca uranium district, Chihuahua, Mexico, promoted formation of a suite of uranyl minerals as alteration products of primary uraninite. Uranyl mineral formation at Nopal is regarded as analogous to processes, which may affect radionuclide isolation, at the potential nuclear waste repository at Yucca Mountain, Nevada. Petrographic data show that the sequence of uranyl mineral formation at Nopal (uranyl hydrate, uranyl silicate hydrate, alkali and alkaline earth uranyl silicate hydrate) is comparable to that observed in experiments designed to mimic alteration of spent nuclear fuel at Yucca Mountain. Recently published and newly estimated data on the thermodynamic properties of uranyl minerals permit evaluations of their relative stabilities in relation to variations in groundwater chemistry and temperature. Observed parageneses are consistent with increasing aqueous concentrations of environmental components, which are dominated by silica and calcium at Nopal. The uranyl mineralogy at Nopal is also consistent with a prograde hydrothermal paragenesis corresponding to the retrograde solubilities of schoepite, soddyite, and uranophane. Uranyl mineral formation by hydrothermal (rather than weathering processes) at Nopal is also consistent with a change from uranyl mineral growth to precipitation of secondary iron oxy-hydroxides, opal, and calcite, which constitute the latest forming minerals in fractures and caliche.

This abstract was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) and its contractors for the Nuclear Regulatory Commission (NRC) under Contract No. NRC–02–02–012. The activities reported here were performed on behalf of the NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste Repository Safety. This abstract is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of NRC.