ALTERATION OF URANINITE FROM THE NOPAL-I DEPOSIT, PENA-BLANCA DISTRICT, CHIHUAHUA, MEXICO, COMPARED TO DEGRADATION OF SPENT NUCLEAR-FUEL INTHE PROPOSED UNITED-STATES HIGH-LEVEL NUCLEAR WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) ha s altered almost completely to a suite of secondary uranyl minerals. T he deposit is located in a Basin and Range horst composed of welded si licic tuff; uranium mineralization presently occurs in a chemically ox idizing and hydrologically unsaturated zone of the structural block. T hese characteristics are similar to those of the proposed U.S. high-le vel nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrogra phic analyses indicate that residual Nopal I uraninite is fine grained (5-10 mum) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxida tion and formation of secondary minerals from the uraninite have occur red in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H2O) and also common to Yuc ca Mountain. In contrast, secondary phases in most other uranium depos its form from elements largely absent from spent fuel and from the Yuc ca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I u raninite and the sequence of alteration products, their intergrowths a nd morphologies are remarkably similar to those observed in reported c orrosion experiments using spent fuel and unirradiated UO2 under condi tions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory ex periments and the natural alteration of Nopal I uraninite are dominate d by uranophane [nominally Ca(UO2)2Si2O7.6H2O] with lesser amounts of soddyite [nominally (UO2)2SiO4.2H2O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the diff erences in time and physical-chemical environment between Yucca Mounta in-approximate laboratory experiments and Yucca Mountain-approximate u raninite alteration at Nopal I, suggesting that the results may reason ably represent phases likely to form during long-term alteration of sp ent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system condition s.