Publications

The Waste Isolation Pilot Plant (WIPP) is the first operational repository designed for the safe disposal of transuranic (TRU) radioactive waste from the defense programs of the US Department of Energy (DOE). The US Environmental Protection Agency (EPA) is responsible for certifications and regulation of the WIPP facility for the radioactive components of the waste. The EPA has promulgated general radioactive waste disposal standards at 40 CFR Part 191. and WIPP-specific criteria to implement and interpret the generic disposal standards at 40 CFR Part 194. In October 1996. the DOE submitted its Compliance Certification Application (CCA) to the EPA to demonstrate compliance with the disposal standards at Subparts B and C of 40 CFR Part 191. This paper summarizes the development of the overall legal framework for radioactive waste disposal at the WIPP, the parallel development of the WIPP performance assessment (PA), and how the EPA disposal standards and implementing criteria formed the basis for the CCA WIPP PA. The CCA resulted in a certification in May 1998 by the EPA of the WIPP'S compliance with the EPA's disposal standard, thus enabling the WIPP to begin radioactive waste disposal.

The Waste Isolation Pilot Plant (WIPP) is under development by the US Department of Energy (DOE) for the geologic disposal of transuranic waste. The construction of complementary cumulative distribution functions (CCDFs) for total radionuclide release from the WIPP to the accessible environment is described. The resultant CCDFs (1) combine releases due to cuttings and cavings, spallings, direct brine release, and long-term transport in flowing groundwater, (2) fall substantially to the left of the boundary line specified by the U.S. Environmental Protection Agency's (EPA's) standard 40 CFR 191 for the geologic disposal of radioactive waste, and (3) constitute an important component of the DOE's successful Compliance Certification Application to the EPA for the WIPP. Insights and perspectives gained in the performance assessment (PA) that led to these CCDFs are described, including the importance of (1) an iterative approach to PA, (2) uncertainty and sensitivity analysis, (3) a clear conceptual model for the analysis, (4) the separation of stochastic (i.e., aleatory) and subjective (i.e., epistemic) uncertainty, (5) quality assurance procedures, (6) early involvement of peer reviewers, regulators, and stake holders, (7) avoidance of conservative assumptions, and (8) adequate documentation.

The appropriate disposal of radioactive waste is a problem of great importance, wide-spread interest, and some controversy. As part of the solution to this problem the Waste Isolation Pilot Plant (WIPP) is under development by the US Department of Energy (DOE) for the deep geologic disposal of transuranic (TRU) waste generated by defense programs in the United States. The DOE submitted a Compliance Certification Application (CCA){sup 17} for the WIPP to the US Environmental Protection Agency (EPA) in October 1996, and a positive certification decision for the WIPP was issued by the EPA in May 1998. The first disposal of TRU waste in the WIPP took place in March 1999. The 1996 CCA for the WIPP was supported by an extensive performance assessment (PA) carried out by Sandia National Laboratories (SNL), with this PA often designated the 1996 WIPP PA, the 1996 CCA PA, or simply the 1996 PA. In turn, the 1996 PA was supported by site characterization activities, experimental programs, model development programs, data development programs, uncertainty and sensitivity analyses, a dedicated computational environment, a rigorous quality assurance (QA) program and a sequence of earlier PAs. Further, this PA was carried out in a regulatory environment defined by the following EPA regulations: Environmental Radiation Protection Standards for the Management and Disposal of Spent Nuclear Fuel, High-Level and Transuranic Radioactive Wastes (40 CFR Part 191) and Criteria for the Certification and Re-Certification of the Waste Isolation Pilot Plant's Compliance with the 40 CFR Part 191 Disposal Regulations (40 CFR Part 194) The WIPP is the first licensed facility in the United States for the deep geologic disposal of radioactive waste. As a result, there is extensive interest in both the WIPP and the analyses that led to its certification by the EPA for the disposal of TRU waste. The WIPP program has produced large amounts of documentation both as part of the CCA itself and in large numbers of technical reports and supporting analysis documents. Although this information is publicly available, in practice its great quantity and availability at only specific locations (e.g., EPA Docket locations, the WIPP Records Centers in Albuquerque and Carlsbad) make obtaining a detailed understanding of the 1996 WIPP PA an arduous undertaking.

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): (1) EN1, a probabilistic characterization of the likelihood of different futures occurring at the WIPP site over the next 10,000 yr, (2) EN2, a procedure for estimating the radionuclide releases to the accessible environment associated with each of the possible futures that could occur at the WIPP site over the next 10,000 yr, and (3) 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, EN1 is characterized by a probability space (S{sub st}, P{sub st}, p{sub st}) for stochastic (i.e., aleatory) uncertainly; EN2 is characterized by a function {line_integral} that corresponds to the models and associated computer programs used to estimate radionuclide releases; and EN3 is characterized by a probability space (S{sub su}, P{sub su}, p{sub su}) 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{sub st}, P{sub st}, p{sub st}), {line_integral} and (S{sub su}, P{sub su}, p{sub su}).

Sandia National Laboratories is conducting performance assessments for the United States Department of Energy to use in evaluating compliance of the Waste Isolation Pilot Plant with EPA 40 CFR 191, Subpart B. Performance assessment is an iterative process that will lead to final compliance evaluation in 1994 or later. Monte Carlo simulations examine modeling system sensitivity to the probability of intrusion and uncertainty in the transport model for the overlying water-bearing unit. Simulations of two-phase (gas and brine) flow indicate gas generation may substantially reduce brine saturation in the waste, limiting radionuclide transport. All results are preliminary and are not suitable for evaluating compliance. Results suggest, however, that compliance can be achieved.

The United States Department of Energy (DOE) plans to use the Waste Isolation Pilot plant (WIPP) in southeastern New Mexico for disposal of transuranic wastes generated by defense programs. The DOE must first demonstrate compliance with the Environmental Protection Agency's (EPA) Environmental Standards for the Management and Disposal of Spent Nuclear Fuel, High-Level and Transuranic Radioactive Wastes (40 CFR Part 191) hereafter called the Standard. The Standard was vacated by a Federal Court of Appeals in 1987 and is undergoing revision; by agreement with the State of New Mexico the DOE will continue to evaluate repository performance with respect to the Standard as first promulgated until a new version is available. This paper reviews the results of a 1989 preliminary demonstration of the performance-assessment methodology that will be used by the WIPP project ultimately to assess compliance with the Standard. The report also describes conceptual and numerical improvements in the performance-assessment methodology made during 1990, and summarizes the progress made toward achieving the probabilistic assessment of repository performance required for regulatory compliance. 13 refs., 4 figs.

This report describes a demonstration of the performance assessment methodology for the Waste Isolation Pilot Plant (WIPP) to be used in assessing compliance with the Environmental Protection Agency. This demonstration incorporates development and screening of potentially disruptive scenarios. A preliminary analysis of the WIPP disposal system's response to human intrusion scenarios produces preliminary complementary cumulative distribution functions (CCDFs) used to assess the compliance of the WIPP with the Containment Requirements of the Standard. The conceptual model of the disposal system consists of geologic, hydrologic, and disposal system subsystems along with the physical and chemical processes associated with these subsystems. Parameter values defining the systems contain uncertainties and modeling approximations of such a disposal system contributes to those uncertainties. The WIPP compliance assessment methodology consists of a system of techniques and computer codes that estimate releases of radionuclides from the disposal system, incorporating analysis of the parameter uncertainties in the estimates. Demonstration CCDFs are presented, but are not yet credible enough to judge the probability of compliance of the WIPP with the EPA Standard. 60 refs., 75 figs., 30 tabs.