INTRODUCTION

Regardless of the current or future use of nuclear power reactors for the generation of electricity, one of the major national problems facing scientists across a range of disciplines is the safe disposal of high-level radioactive wastes. (High-level radioactive waste refers to heat-producing waste.) By current definitions, this includes two by-products of nuclear reactors—spent fuel assemblies and reprocessed spent-fuel assemblies. The current consensus of the scientific community is that the most viable means of waste isolation is through deep burial in selected rock formations. This is the present policy of the U.S. Department of Energy. The suitability of a number of sites for storage of high-level radioactive waste is now being actively investigated. Two of the sites that have undergone thorough geologic investigations are the Hanford site in southern Washington (waste burial in the Columbia River Basalt) and the Nevada Test Site (NTS) in southern Nevada (waste burial in ash-flow sheets of the Timber Mountain-Oasis Valley caldera complex).

Geologic investigations leading to selection of sites for burial of radioactive wastes pose several problems. First, detailed characterization of a block of the Earth’s crust is required on a level that is unprecedented in the science. This characterization has unique requirements. First, the repository block must be defined with a high degree of confidence, and yet penetrations of the block by exploratory drilling or tunnels must be limited so the integrity of the block is not compromised. This restriction requires an emphasis on geophysical techniques as a primary means of site exploration. Second, the potential effects of construction of a repository tunnel complex and the thermal disturbance of the rock from the emplacement of heat-producing waste must be evaluated with respect to induced changes that could alter the isolation properties of the rocks. Third, the rates of operation of natural geologic processes on the block, such as groundwater movement and tectonic uplift or erosion, must be defined to evaluate the suitability of the block for containment of radioactive waste elements. Finally, possible future changes from naturally occurring but more catastrophic tectonic processes such as seismicity or volcanism must be evaluated for the required isolation period of high-level waste. This last topic, which falls in the regime of predictive geology is the focus of this paper. More and more frequently, geologists are being asked by other scientists and the public to make specific predictions on the future activity of geologic phenomena. Although some progress has been made in prediction of earthquake and volcanic activity, most predictions are valid for periods of months or at most a few tens of years. In contrast, geologic predictions required for waste disposal must encompass a period of thousands of years. We have limited experience in the geologic methods used to make such predictions and even less experience in communicating and defending decisions based on geologic predictions in a public forum. This chapter describes recent work concerned with prediction of tectonic processes for one specific problem—volcanic hazards related to permanent isolation of high-level radioactive wastes. Two topics are emphasized: (1) the special problems posed by volcanic hazard assessment for waste disposal and (2) the use of risk-assessment techniques to evaluate volcanic hazards for the storage of high-level radioactive waste in tuff in southern Nevada (Nevada Nuclear Waste Storage Investigations).

SPECIAL PROBLEMS INHERENT IN VOLCANIC HAZARD ASSESSMENT FOR WASTE DISPOSAL SITES

Perhaps the most novel aspect of volcanic hazard assessment for radioactive waste disposal is the length of time for which hazards must be forecast. The required containment period of high-level radioactive waste is 104 yr as defined in the draft version of the Environmental Standards for Management and Disposal of Spent Nuclear Fuel, High-Level and Transuranic Radioactive Wastes [U.S. Environmental Protection Agency (1982) 40 CFR 191]. This standard, which has not yet been formally approved, is based on a radiological comparison of the projected mass of radioactive waste [105 metric tons of heavy metal (MTHM)] with an equivalent amount of unmined uranium ore. A 104-yr period has been chosen as the required interval for radioactive waste to decay to a level where the risk is about the same as the smallest estimate of the risk from an equivalent amount of uranium ore (EPA 520/1 82–025, p. 29). However, Bredehoeft et al. (1978) noted that 104 yr provides adequate containment for the short-lived radionuclides such as 90Sr or 137Cs but allows for only a limited reduction in the potential hazards of long-lived radionuclides like 129I. Gera (1982) argued that the waste should be allowed to decay until the radiotoxicity levels are equal to the levels from the amount of uranium consumed in a reactor. This would require an isolation time of about 105 yr. Whatever the required isolation period of high-level waste, this period becomes the minimum length of time for which future volcanic hazards must be forecast. Both 104 and 105 yr are long compared to the rates of operation of volcanic processes.

The long time frame of hazard assessment is a special problem of waste disposal. There is neither an estab-



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