DEALING WITH UNCERTAINTIES IN GELOGICAL PREDICTION FOR RADIOACTIVE WASTE REPOSITORIES: THE EXPERIENCE OF WIPP

Before undertaking a rigorous analysis, the task of predicting the future behavior of a geological repository for radioactive waste appears extremely difficult. The difficulty arises from the need to deal with a vast array of uncertainties in the evolution of the natural environment as well as the human behavior in the long-term future. The U.S. Environmental Protection Agency (EPA) has developed a methodology for dealing with the uncertainties in this process and has codified it in the Code of Federal Regulations Title 40 Part 191 for the Waste Isolation Pilot Plant (WIPP) and in 40 CFR 197 for the proposed Yucca Mountain repository for high-level radioactive waste.

The WIPP, a repository for defense transuranic (TRU) waste in southeastern New Mexico, successfully demonstrated compliance with the EPA standards in 1998, and has submitted documentation to the EPA, demonstrating continued compliance for the first 5-year recertification in 2004. The methodology for such demonstration consisted of a rigorous process of collecting geological and hydrogeological data at the site; compiling an exhaustive list of natural and human induced features, events, and processes that may affect the integrity of the site for the EPA prescribed 10,000 years; and probabilistic analysis of what may happen and what would be the consequences to the human beings and the environment for 10,000 years. The analyses included the impact of inadvertent direct drilling in to the repository by future generations.

Scenarios that appeared potentially catastrophic prior to rigorous probabilistic analyses were not found to result in unacceptable radiation doses to the future generations when the mechanics of such exposure was carefully analyzed and put in probabilistic terms to rationally deal with uncertainties in such long-term predictions. While the worst scenarios at WIPP involved inadvertent human intrusion due to the presence of oil, gas, and potash resources at the site, the Yucca Mountain performance assessment will have to deal with the potential effect of geological disruptive events such as seismicity and volcanism. Preliminary indications from the Yucca Mountain are that there, like WIPP, the detailed analyses of potential future disruptions yield less hazardous results than what appears at the outset before analyzing the scenarios in detail.