A t the end of the 20th century, one of for developing "generally applicable stanthe most pressing problems, political dards" for the repository, and the NRC is reand scientific, is the disposal and iso-sponsible for promulgating rules or regulalation of radioactive wastes (1). There is now tions, which if complied with, will ensure a worldwide consensus (2) that effective that EPA' s standards are met. The new rules long-term isolation for spent nuclear fuel and and standards rely on a probabilistic perforhigh-level radioactive or transuranic waste mance assessment (PPA) of the repository to can be achieved by geologic emplacement. provide a single quantitative measure of The chief elements of the geologic disposal compliance. This approach, when combined strategy are as follows: (i) it was to be deep, with a proscribed compliance period that is permanent, and long term, using to advan-short compared with the time for geologic or tage the decrease in radioactivity, thermal climate change (10,000 years), a point of output, and radiotoxicity over time; (ii) the geologic history of stable regions was taken as indicative of future stability Actlrddetr and and continued suitability; (iii) the passive hydrologic andgeochemical properties of the geologic formations were to be key to the isolation strate-8 gy. The large uncertainties in 8 10' F