The conceptual structure of the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) is described. This structure involves three basic entities (EN1, EN2, EN3): (i) EN1, a probabilistic characterization of the likelihood of different fimmes occurring at the WIPP site over the next 10,000 yr, (ii) EN2, a procedure for estimating the radionuclide releases to the accessible environment associated with each of the possl%le futures that could occur at the WIPP site over the next 10,000 yr, and (iii) EN3, a probabilistic characterization of the uncertainty in the parameters used in the definition of EN1 and EN2. In the formal development of the 1996 WIPP PA EN 1 is characterized by a probability space {S~fi~~pJ for stochastic (i.e., aleatory) uncertainly; EN2 is characterized by a function J that corresponds to the models and associated computer programs used to estimate radionuclide release$ and EN3 is characterized by a probability space (S~u, d SW pm) for subjective (i.e., epistemic) uncertainty. A high-level overview of the 1996 WIPP PA and references to additional sources of information are given in the context of (S~t, Sp ps:),f md (Ssu,~Su, PJ

(3 S,T IThe Waste Isolation Pilot Plant (WIPP) is under development by the US. Department of Energy (DOE) for the geologk (deep underground) disposal of transuranic (TRU) waste. This development has been supported by a sequence of performance assessments (PAS) carried out by Sandia National Laboratories (SNL) to assess what is known about the WIPP and to provide ,pidance for future DOE research and development activities. Uncertainty and sensitivity analysis procedures based on Latin hypercube sampling and regression techniques play an important role in these PAS by providing an assessment of the uncertainty in important analysis outcomes and identifying the sources of this uncertainty. Performance assessments for the WIPP are conceptually and computationally interesting due to replatory requirements to assess and display the effects of both stochastic &e., aleatory) and subjective (i.e., epistemic) uncertainty, where stochastic uncertainty arises from the possible disruptions that could occur over the 10,000 yr replatory period associated with the WIPP and subjective uncertainty arises from an inability to unambiguously characterize the many models and associated parameters required in a PA for the WIPP. The interplay between uncertainty analysis, sensitivity analysis, stochastic uncertainty and subjective uncertainty are discussed and illustrated in the context of a recent PA carried out by SNL to support an application by the DOE to the U.S. Environmental Protection Agency for the certification of the WIF' P for the disposal of TRU waste.

Abstract'The conceptual and computational structure of a performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) is described. Important parts of this structure are (i) maintenance of a separation between stochastic (i.e., aleatory) and subjective epistemic) uncertainty, with stochastic uncertainty arising from the many possible disruptions that could occur over the 10,000 yr regulatory period that applies to the WIPP and subjective uncertainty arising fiom the imprecision with which many of the quantities required in the kalysis are known, (ii)

The Waste Isolation Pilot Plant (WIPP) is under development by the U.S. Department of Energy (DOE) for the geologic disposal of transuranic waste. The construction of complementary cumulative distribution fimctions (CCDFS) for total radionuclide release from the WIPP to the accessible environment is described. The resultant CCDFS (i) combine releases due to cuttings~d cavings, spallings, direct brine release, and long-term transportin flowing groundwater, (ii) fall substantially to the left of the boundary line specified by the U.S. Environmental Protection Agency's (EPA's) standard40 CFR 191 for the geologic disposal of radioactive waste, and (iii) constitute an important component of the DOE's successfid Compliance Certification Application to the EPA for %.heWIPP.Insights and perspectives gained in the performance assessment (PA) that led to these CCDFS are described, including the importance of (i) an iterative approach to PA, (ii) uncertainty and sensitivity analysis, (iii) a clear conceptual model for the analysis, (iv) the separation of stochastic (i.e., aleatory) and subjective (i.e., epistemic) uncertainty,(v) quality assuranceprocedures, (vi) early involvement of peer reviewers, regulators, and stakeholders, (vii) avoidance of conservative assumptions, and (viii) adequate documentation.

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