Measures of Geologic Isolation
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Measures of Geologic Isolation William M. Murphy Department of Geological and Environmental Sciences California State University, Chico Chico, CA 95929-0205 USA; [email protected] ABSTRACT Isolation in a geologic setting has been the generally favored solution to the high-level radioactive waste (HLW) problem since a scientific basis for nuclear waste management began to be formulated over half a century ago. Although general features of suitable settings have been enumerated, quantitative measures of the safety of geologic isolation of HLW are challenging to devise and to implement. Some regulatory measures of isolation for the proposed repository at Yucca Mountain, Nevada, have be devised and revised involving considerations of global releases, groundwater travel time, and time and space scales for isolation. In current Yucca Mountain specific regulations, the measure of long-term safety hinges on probabilistic estimates of radiation doses to the average member of a maximally exposed group of people living about 18 km down the groundwater flow gradient within 10,000 years after permanent closure of the repository. From another perspective, hydrogeochemical studies provide quantitative measures of system openness and the ability of geologic systems to isolate HLW. Hydrogeochemical data that bear on geologic isolation of HLW at Yucca Mountain include precipitation of radionuclides in stable mineralogical products of spent fuel alteration, ages of natural secondary mineralization in the mountain, uranium decay-series isotopic data for system openness, bomb-pulse isotope occurrences, and ambient carbon-14 distributions. INTRODUCTION In 2002, based on a recommendation from the U.S. Department of Energy (DOE), the President of the United States notified Congress that it considers Yucca Mountain, Nevada, qualified for submission of an application for construction of the nation’s permanent geologic repository for disposal of high-level radioactive waste (HLW). The U.S. Congress ratified that recommendation over the objection of the Governor of Nevada later in 2002. The country and the world now await a license application from the DOE and its evaluation by the U.S. Nuclear Regulatory Commission (NRC) according to standards set by the Environmental Protection Agency (EPA). The necessity of a permanent solution to the HLW problem in the U.S. is made evident by continuing accumulation of wastes in numerous aging facilities, which are reaching temporary storage capacity and longevity, and by the hazardous lifespan of the wastes, which extends for a period of time longer than the present duration of human civilization. Although fanciful alternatives have been described and debated, isolation of HLW in a geologic setting has been the generally favored solution since a scientific basis for this problem began to be formulated over half a century ago. Scientific and social concerns regarding the safety of geologic disposal of HLW (and concomitant implications for controversial nuclear technologies in general) motivate
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