Assessments for Disposal of Radioactive Waste at the Waste Isolation Pilot Plant
Project Description and Significance
The Performance Assessment Department at Sandia is evaluating long-term performance of the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico. The evaluation is based on the Environmental Protection Agency's regulations (40 CFR 191) for disposal of radioactive wastes. These regulations require performance assessment (PA), an analysis that identifies the processes and events that might affect the disposal system, examines their effects and likelihoods, and estimates releases probabilistically. The long-term requirements in 40 CFR 191 for the WIPP specify criteria that must be met for 10,000 years after closure of the disposal facility (schematically shown below). The Performance Assessment Department uses computer simulations to model events and processes (scenarios) that may lead to radionuclide migration beyond a "controlled" boundary after wastes are emplaced and the WIPP repository is closed.
Large quantities of radioactive wastes have been generated by defense programs in the United States. Such wastes pose a potential hazard to human health and safety and must be limited (according to 40 CFR 191, for at least 10,000 years). Some defense wastes are currently stored in a manner that cannot ensure long-term (more than 100-year) isolation. The WIPP is planned as the first mined geologic repository for transuranic radioactive wastes generated by defense programs of the United States Department of Energy (DOE).
Sandia's Contribution
Although 40 CFR 191 does not require that risk be evaluated, its requirements are risk-based. Additionally, uncertainties in scientific explanation are recognized in the regulation (i.e., the correctness of hypotheses can only be judged on the basis of current knowledge). Consequently, Sandia's approach to evaluating WIPP compliance resembles a scenario-based, probabilistic risk assessment in which the performance metricradioactive release specified in probabilistic termsis a complementary cumulative distribution function (CCDF) (or probability-of-exceedance curve).
The risk-based criteria for long-term performance in 40 CFR 191 may be viewed as a triplet consisting of the answers to the following three questions:
What can happen? (scenarios)
How likely are these things to happen? (probabilities of scenarios)
What are the outcomes of these things happening? (consequences of scenarios)
The first question can be answered by a systematic scenario construction that provides a set of comprehensive and mutually exclusive scenarios for consequence analysis. The answer to the second question requires probability estimates for scenarios retained for analysis. Answering the third question requires a modeling system for estimating consequences.
The PA process for the WIPP consists of six general tasks:
Disposal-system and regional characterization through collection of data on waste properties, facility design, and regional geology and hydrology.
Scenario development that identifies combinations of events and processes whereby contaminants might be released outside a "controlled" area, and subsequent determination of which scenarios to model.
Probability modeling that provides estimates of the likelihood that the retained scenarios will occur.
Consequence analysis, including uncertainty analysis, that predicts contaminant amounts released and associated uncertainties from the calculations.
Regulatory-compliance assessment through construction of CCDFs (hypothetical CCDF illustrated below) for comparison of modeling results with long-term performance criteria in 40 CFR 191.
Sensitivity analysis that determines the parameters that most influence modeling results.
Sandia supports the DOE by providing detailed scientific and engineering understanding of the WIPP site and scientific research on issues related to disposal of transuranic radioactive wastes in bedded salt. Sandia has been involved in siting and characterization studies of the WIPP since 1977. The current program undertaken by Sandia defines and implements, subsequent to DOE approval, experiments both in laboratories and in the WIPP (see photograph) that focus on collecting and evaluating data from the following activities:
Analyses of the disposal room and drift system.
Transuranic waste experiments that study gas generation and interaction of wastes in a simulated repository environment.
Analyses of sealing systems and rock mechanics.
Studies of a variety of hydrologic parameters of the repository host rock.
Studies of a variety of hydrologic and chemical transport parameters of the formations overlying the repository host rock.
Performance assessments use input from the conceptual, mathematical, and computational models developed in the preceding five activities to demonstrate and document the overall models that assess performance of the WIPP.
Performance assessment for the WIPP is iterative, and answers to each of the three questions in the triplet will have been reexamined as the project moves toward a final evaluation of regulatory compliance planned for 1996.
Future Work
Although 40 CFR 191 is an important driver for developing methodology and analytical tools for evaluating long-term disposal of radioactive wastes, other regulations are important in assessing the impacts of disposing of both radioactive and nonradioactive hazardous wastes in the WIPP. With minor modifications, the same modeling tools used for determining compliance with 40 CFR 191 can be used to assess compliance with other long-term environmental regulations, e.g., 40 CFR 268.
For further information, contact:
D.R. (Rip) Anderson
Sandia National Laboratories, MS-1328
Albuquerque, NM 87185-1328
Phone: (505) 848-0692
e-mail: drander@sandia.gov
Submitted October 1995 Layout design by Wanda Mar.
Submitted October 1995