Bret W. Leslie

Uranium transport through fractured silicic tuff and relative retention in areas with distinct fracture characteristics

Abstract The Nopal I uranium (U) deposit, in the Pefia Blanca District, Chihuahua, Mexico, has been identified as analogous in some regards to the candidate U.S. high-level waste (HLW) repository at Yucca Mountain, Nevada. Uranium transport at the Nopal I deposit has been studied to investigate mechanisms by which HLW components could be transported through silicic tuff over long time periods. This investigation focused on approximately 1400 m2 of essentially continuous bedrock outcrop spanning the Nopal I deposit and surrounding host tuff. Data collected document: (i) the distributions of U within and around the Nopal I deposit, (ii) the distribution and characteristics of the fracture network within and surrounding the deposit, and (iii) the transport of U away from the deposit mainly along fracture paths. Uranium-series isotopic measurements indicate mobilization of U along the margin of the deposit within the last 1 Ma and significant U transport at about 54 Ka. Transport of U away from the Nopal I deposit along a few relatively continuous mesofractures achieved maximum distances at least 20 times greater than transport through the general fracture network composed of thousands of less continuous microfractures within and surrounding the deposit. Uranium transport away from the deposit appears to be largely independent of variations in the general fracture network pattern. Transport of U away from individual micro- and meso-fractures into homogeneous, unfractured tuff matrix appears limited to distances less than 1 mm. At the Nopal I deposit, matrix diffusion does not appear to have been an important factor for retardation of U. This analysis suggests a ranking for U retention: (i) microfracture network retention ≫ mesofracture retention, and (ii) individual microfracture retention ≫ matrix retention.

Estimated Effects of Temperature-Relative Humidity Variations on the Composition of In-Drift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada

We define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances.

U.S. Nuclear Regulatory Commission Program on Risk-Informing the Materials and Waste Arenas: Pilot Studies

The United States Nuclear Regulatory Commission has a program to encourage the use of risk information in regulation of nuclear materials and waste. Within this program there is a project to develop methods and guidelines for risk-informed decision-making in a regulatory context. A basic framework for risk-informed decision-making has been proposed, as well as some preliminary logic algorithms and risk guidelines. To test these preliminary algorithms and guidelines, several pilot studies were conducted on specific regulatory decisions of interest. This paper will describe the overall risk-informing framework, as well as the decision algorithm applicable to these pilot studies. Although only refinements are expected in the overall process, the decision algorithms and risk guidelines are still in a developmental stage of which these pilot studies are a test phase. In this paper two of these pilot studies will be described. Insights concerning the usefulness of the decision algorithms and risk guidelines in similar situations are also stated.