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Locating a repository in the unsaturated zone of arid regions eliminates or simplifies many of the technological problems involved in designing a repository for operation below the water table and predicting its performance. It also offers possible accessibility and ease of monitoring throughout the operational period and possible retrieval of waste long after. The risks inherent in such a repository appear to be no greater than in one located in the saturated zone; in fact, many aspects of such a repository's performance will be much easier to predice and the uncertainties will be reduced correspondingly. A major new concern would be whether future climatic changes could produce significant consequences due to pogsible rise of the water table or increased flux of water through the repository. If spent fuel were used as a waste form, a second new concern would be the rates of escape of gaseous iodine129 and carbon-14 to the atmosphere. * * * Such a system would be above, rather than below, the regional water table, and if properly engineered. should remain dry at least much of the time. It would have the advantages of greater ease of construction, greater distance from the [Columbia] river, easier retrieval of waste canisters if this should prove 'As knowledge of the Y u m Marntain site grew. some members of a Peer Review COmmiUee. made up of representatives from universities and indushy, raised questions as to the possible utilization of potential hmt rccks in theunsakatedme. In February 1982. several scientists of the U S Geological Survey (USGS). in a letter to the Department of Energy F'mgram Manager for the Nevada Nuclear Waste Storage Investigations Program.. sd the hydrogeologic eonditions that should be present in the unsaturated m e beneath Yucca Mountain and concluded that they should provide a very favorable envirmment for waste disposaL At about the same time, the USGS sent a letter to the Nuclear Regdatay Commission (NRC) suggesting how NRCpropased rule 10 CFR Part 60. which was specificaUy limited to repe3itories in the saturated m e , might be modiKed to include t h m in the UP sahrrated m e A subsequent draft of 10 CFR Part 60 covm both m e s .

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