URANIUM-SERIES DISEQUILIBRIUM IN TUFF AND GRANITE

Download or read book URANIUM-SERIES DISEQUILIBRIUM IN TUFF AND GRANITE written by and published by . This book was released on 2000 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Uranium occurs naturally at trace levels in the major rock-forming minerals (quartz, feldspars, micas) in volcanic and plutonic rocks and is concentrated in accessory minerals (zircon, sphene, apatite). It may attain concentrations as high as 1000 ppm in the accessory minerals. Radiometric age determinations on zircon and sphene have shown that uranium migration from these minerals is generally negligible over prolonged periods of geologic time. Zircon grains separated from highly weathered igneous rocks have been found to retain most of their uranium. In contrast, the uranium fixed onto mineral grain boundaries or present in less-resistant minerals such as biotite or hornblende can be readily leached by groundwater. The ubiquitous presence of uranium in a rock makes it an ideal ''natural analogue'' for understanding the mobility of uranium at a potential site for nuclear fuel waste disposal and one that is easily overlooked in the search for suitable analogues for a disposal site. Several of the intermediate radionuclides in the decay series of the two long-lived isotopes of uranium (238U and 235U) have half-lives greater than one year and are, therefore, of geological interest. In a sealed rock mass with no water-rock interactions, all intermediate radionuclides attain radioactive equilibrium with one another within a maximum 1-2 million years. Because rocks of the Yucca Mountain area and the Canadian Shield (both potential sites for nuclear waste disposal in the United States and Canadian programs, respectively) are considerably older, this condition (known as secular equilibrium) should exist in these rocks, and all daughter/parent radionuclide activity ratios should equal unity (1.000). If the ratios are found not to equal unity, then the rock has been disturbed, probably by groundwater transport of more soluble radionuclides into or away from the rock. How recently this migration has occurred can be determined from the half-life of the radionuclide involved. Depending on the analytical precision obtained, the observation of a 234U/238U activity ratio that is less than or greater than 1.000 clearly shows that an isotope of uranium has migrated within the rock in the last 1-2 million years. Other daughter/parent activity ratios can be used to detect radionuclide migration over shorter time-scales, such as 23°Th/234U (300,000 years) and 226Ra/23°Th (8,000 years). Uranium-series disequilibrium is, therefore, a useful technique for application to site evaluation for nuclear fuel waste disposal because it can be used to: (1) show that so-called ''intact rock'' is indeed intact (i.e. radionuclides are in secular equilibrium and are immobile), (2) determine the principal flow regimes in a rock mass by analysis of rock matrix, fracture material, etc., (3) estimate the time period of recent radionuclide migration in the rock, and (4) proxy as a natural analogue for the potential mobility of uranium at the site. Several examples of these applications have been reported. This paper describes the use of uranium-series disequilibrium in the comparison of two North American sites: the water-saturated Lac du Bonnet granite batholith on the Canadian Shield and the unsaturated tuffs from the Exploratory Studies Facility (ESF) and Cross-Drift Tunnels at Yucca Mountain, Nevada. In particular, the fact that unfractured rock should be at secular equilibrium is applied to both sites to determine if the rock matrix is a significant flow path for groundwater.