HR: 0800h
AN: V11D-0811    [Abstracts]
TI: Evaluating Consequences of Volcanism for Spent Nuclear Fuel at Yucca Mountain, Nevada
AU: * Coleman, N
EM: nmc@nrc.gov
AF: US NRC, MS T2-E26, Washington, DC 20555,
AU: Marsh, B
EM: bmarsh@jhu.edu
AF: Johns Hopkins University, 322 Olin Hall, 3400 N. Charles St., Baltimore, MD 21218,
AB: The likelihood that a volcanic dike could intersect a high-level waste (HLW) repository at Yucca Mt. is very small, 1E-9/yr to 1E-7/yr. The intersection of a cone-forming conduit is even less likely. Realistic insights about the fate of HLW in a volcanic conduit suggest that fewer waste packages may be affected and particle sizes of ejected spent fuel may be larger than previously assumed. Most HLW consists of fractured ceramic pellets of UO2 about a cm in diameter, with a melting point >2800C, much higher than magma temperatures of 1000-1200C. Spent fuel would not dissolve in magma; therefore the size range of transported fragments would largely be determined by pre-existing particle sizes in fuel rods. This range would differ from that of volcanic ejecta. The expected travel time in a conduit from repository depth to the surface would be short, allowing little time for erosion of ceramic pellets but permitting rapid quenching of magma on the relatively cold waste packages and their contents. Quench rinds would protect waste fragments during rapid transit to the surface in a column of frothy magma. Xenoliths and crush-impact studies constrain the size of spent fuel particles that may be incorporated in volcanic ash. Estimates of fuel particle size have used a log triangular distribution from 1-100 microns. For comparison, grains of table salt are 100 microns across. Talcum powder is 10 microns. One micron is the wavelength of near infrared light. However, it is unlikely that spent fuel could be reduced to this minute size range. At Lathrop Wells, tuff xenoliths eroded from conduit walls are common in the scoria cone. They vary in size from a fraction of a cm up to 30 cm and have quenched basalt rinds, providing evidence that large spent fuel fragments could survive intact over the short travel distance to the surface. Crush-impact studies at energies up to 1000 J/gram on spent fuel show less than 30% of the fuel mass reduced to <100 microns and less than 10% to <10 microns. Hypothetical doses are sensitive to assumptions about particle size because respirability decreases sharply as particles increase beyond 10 microns. In performance assessment, using a particle size range of 100-10000 microns reduces dose 200-fold compared to a range of 1-100 microns. There is a strong basis to assume that only a fraction of spent fuel entrained in a conduit would be ejected as tephra. The relative volume of ash vs. scoria cone and lava flows can be used to estimate practical limits on the fraction of ejected waste in ash that could be transported by water and wind. Compared with ash, waste in lava flows or scoria cones would be protected from erosion and transport for hundreds of thousands of years, as shown by the million-year-old cones and flows in Crater Flat near Yucca Mt. Also, it is likely that only a limited number of waste packages could be entrained because volcanic conduits would be smaller at the repository depth of 300 m than at ground surface. Lithostatic pressure keeps conduits smaller at depth, possibly <10 m in diameter. It is also possible that a conduit could form between drifts, and no waste would be entrained. Dikes are more likely to intrude pre-existing faults. Conduits form along dikes, so keeping drifts set back from larger faults would reduce the chance of a conduit intersecting the repository. In sum, only a limited number of waste packages is likely be entrained in a volcanic conduit, and there is strong evidence that a large fraction of the HLW content would not be reduced to very small particles. For more information, see the report by NRC's Advisory Committee on Nuclear Waste and Materials [http://www.nrc.gov/reading-rm/doc-collections/acnw/letters/2007/].
DE: 8400 VOLCANOLOGY
DE: 8425 Effusive volcanism
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting