Prioritization of technical content and impact assessment for changes to performance assessment at the Waste Isolation Pilot Plant (WIPP).

Waste Isolation Pilot Plant (WIPP) performance assessment consists of twenty-four conceptual models that are used to describe repository performance during the 10,000 year regulatory time period following closure of the facility. Following the recertification of the repository in March of 2006, Sandia National Laboratories and the U. S. Department of Energy (DOE) began a set of activities to identify which models could be improved and updated by inclusion of data from recent repository investigations. Sandia initiated this process by conducting a review of the performance assessment models to determine which models should be updated. Sandia provided the DOE with a list of recommendations, and after discussions with the DOE, Sandia proceeded with a set of scoping calculations to identify the impacts of these changes on modeling repository performance. This paper describes the methodology that Sandia used to determine which changes should be proposed and the impact of some of these changes on estimates of radionuclide releases that could occur in the event of inadvertent human intrusion into the repository. This paper is the first in a series of papers about the proposed changes. The other papers detail the specific changes and the justification for those changes to the models and parameters.