Publications

Coffinite (USiO{sub 4}) has been found in numerous sedimentary and hydrothermal environments including those considered as natural analogues of nuclear waste repositories. Scanning electron microscopy (SEM) and analytical electron microscopy (AEM) studies have been conducted on a uraninite sample from a U-deposit in Canada. It is observed that the uraninite (UO{sub 2+x}) is replaced by coffinite (U[SiO{sub 4}].nH{sub 2}O) and the replacing coffinite coexists with quartz. The TEM study shows {alpha}-recoil damage, lattice distortion, and low-angle boundaries among neighboring uraninite domains. Coffinitization seems more closely associated with {alpha}-recoil-damaged uraninite areas. Electron energy-loss spectroscopy (EELS) spectrum indicates that the ratio of U(+6)U(+4) in the uraninite is about 2/3, while the coffinite is dominated by U(+4). A thermodynamic calculation indicates that coffinitization can take place most likely at temperatures below 130 C if dissolved silica concentrations are limited by amorphous silica mineral phase. In a sufficiently high silica concentration environment, coffinite can form under the oxygen fugacity of 10{sup -65}-10{sup -55} atm. The equilibrium model, however, is not able to explain the coexistence of coffinite with quartz. A kinetic model that takes account of Ostwald processes is thus proposed. The kinetic model indicates that the presence of U(+6) in uraninite and the enhanced uraninite dissolution rate may be an important factor controlling uraninite coffinitization.

The Waste Isolation Pilot Plant (WIPP), located in a salt bed in southern New Mexico, is designed by US Department of Energy to demonstrate the safe and permanent disposal of design-basis transuranic waste. WIPP performance assessment requires consideration of radionuclide release in brines in the event of inadvertent human intrusion. The mobility of radionuclides depends on chemical factors such as brine pmH (-log molality of H{sup +}) and CO{sub 2} fugacity. According to current waste inventory estimates, a large quantity ({approximately} 10{sup 9} moles C) of organic materials will be emplaced in the WIPP. Those organic material will potentially be degraded by halophilic or halotolerant microorganisms in the presence of liquid water in the repository, especially if a large volume of brine is introduced into the repository by human intrusions. Organic material biodegradation will produce a large amount of CO{sub 2}, which will acidify the WIPP brine and thus significantly increase the mobility of actinides. This communication addresses (1) the rate of organic material biodegradation and the quantity of CO{sub 2} to be possibly generated, (2) the effect of microbial CO{sub 2} production on overall WIPP performance, and (3) the mechanism of using MgO to mitigate this effect.