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In May 1998, the US Environmental Agency (EPA) certified the US Department of Energy's (DOE) Waste Isolation Pilot Plant (WIPP) as being in compliance with all of the applicable regulations governing the permanent disposal of spent nuclear fuel, high-level waste, and transuranic radioactive waste. The WIPP, a transuranic waste repository, is the first deep geologic repository in the US to have successfully demonstrated regulatory compliance with long-term radioactive waste disposal regulations and be certified to receive wastes. Many lessons were learned throughout the 25-year history of the WIPP--from site selection to the ultimate successful certification. The experiences and lessons learned from the WIPP may be of general interest to other repository programs in the world. The lessons learned include all facets of a repository program: programmatic, managerial, regulatory, technical, and social. This paper addresses critical issues that arose during the 25 years of WIPP history and how they influenced the program.

In 1975 Sandia National Laboratories (SNL) was asked by the predecessor to the Department of Energy to assume responsibility for the scientific programs necessary to assure the safe and satisfactory development of a geologic repository in the salt beds of southeast New Mexico. Sandia has continued in the role of Science Advisor to the Waste Isolation Pilot Plant (WIPP) to the present time. This paper will share the perspectives developed over the past 25 years as the project was brought to fruition with successful certification by the Environmental Protection Agency (EPA) on May 13, 1998 and commencement of operations on April 26, 1999.

The Department of Energy submitted a Compliance Certification Application for the Waste Isolation Pilot Plant to the Environmental Protection Agency (EPA) in October, 1996. A critical part of this application was a Performance Assessment which predicts the cumulative radioactive release to the accessible environment over a time period of 10,000 years. Comparison of this predicted release to the EPA standard shows a comfortable margin of compliance. The scientific understanding that was critical to developing this assessment spans a broad range of geotechnical disciplines, and required a thorough understanding of the site`s geology and hydrology. Evaluation of the geologic processes which are active in the site region establishes that there will be no natural breach of site integrity for millions of years, far longer than the 10,000 year regulatory period. Inadvertent human intrusion is, therefore, the only credible scenario to lead to potential radioactive release to the accessible environment. To substantiate this conclusion and to quantify these potential releases from human intrusion, it has been necessary to develop an understanding of the following processes: (1) salt creep and shaft seal efficacy; (2) gas generation from organic decomposition of waste materials and anoxic corrosion of metals in the waste and waste packages; (3) solubilities for actinides in brine; (4) fluid flow in Salado formation rocks, and (5) hydrologic transport of actinides in the overlying dolomite aquifers. Other issues which had to be evaluated to allow definition of breach scenarios were brine reservoir occurrences and their associated reservoir parameters, consequences of mining over the repository, and drilling for natural resources in the vicinity of the repository. Results of all these studies will be briefly summarized in this paper.

The Waste Isolation Pilot Plant (WIPP) is planned as a mined geologic repository for the disposal of transuranic (TRU) radioactive wastes generated by defense programs of the United States Department of Energy. One of the criteria for evaluating the suitability of the WIPP for disposal of TRU wastes is compliance with the United States Environmental Protection Agency`s (EPA) standards for such facilities. The Containment Requirements of those standards require calculating cumulative releases of radionuclides to the accessible environment by all combinations of events and processes (scenarios) that may affect the escape and transport of radionuclides from the repository for 10, 000 years after decommissioning of the facility. Because the release limits established by the EPA are probabilistic, scenario probabilities are also required. A panel of experts was convened to estimate the probabilities of occurrence of the events used in scenario development and to identify additional human-intrusion events for inclusion in a safety assessment of the WIPP. This report documents the background presentations that were made to the panel about the WIPP program, regulatory guidelines, and performance-assessment program, and site-specific and regional geologic and hydrologic characteristics that may affect the WIPP disposal system.