A PROBABILISTIC DERIVATION OF WASTE CONCENTRATION LIMITS FOR THE RADIOACTIVE WASTE MANAGEMENT SITES AT THE NEVADA TEST SITE

摘要

A probabilistic approach to performance assessment was used to derive wasteconcentration limits for the Area 5 and Area 3 Radioactive Waste Management Sites atthe Nevada Test Site (NTS). Probabilistic techniques employed in the performanceassessments include probabilistic treatment of parameter uncertainty, the occurrence ofinadvertent human intrusion scenarios, and the longevity of institutional controls. Sitespecificand probabilistic treatment of the intruder at NTS is appropriate given thelocation of the low-level waste disposal facilities in a remote desert environment.The probability of intrusion and the length of the institutional control period wasestimated from the elicitation of a panel of subject matter experts (SMEs). The SMEsconcluded that the probability of future inadvertent human intrusion and humansettlement would be very low in the remote arid valleys of the NTS. Moreover, theSMEs believed that institutional controls were likely to be effective beyond 100 years atthe NTS.The NTS SMEs estimated the probabilities of inadvertent human intrusion for a 10,000-year compliance period and a 0.8-hectare disposal site footprint. Application ofelicitation-derived intruder probabilities to the disposal facilities was a three-step process:1) adjust probabilities to a 1,000-year performance assessment compliance period basedon new regulatory guidance; 2) adjust probabilities to the actual footprint of the wastecell; and 3) adjust for an elicitation-derived institutional control period of 250 years. Theprobability-weighted dose, or expected dose to an intruder for a disposal cell at the NTS,was determined by multiplying the dose by the probability of a well-drilling intruderscenario. When the intruder performance objectives were found to be limiting, wasteconcentration limits were derived for select radionuclides using the probability-weightedintruder dose.The paper explores the benefits of the NTS probabilistic approach on waste concentrationlimits:1 accounting for site-specific conditions at the waste disposal facility allows full creditto be taken for the advantages offered by a remote, arid site