RELEASE MODES AND PROCESSES RELEVANT TO SOURCE-TERM CALCULATIONS AT YUCCA MOUNTAIN

The feasibility of permanent disposal of radioactive high-level waste (HLW) in repositories located in deep geologic formations is being stu died world-wide. The most credible release pathway is interaction betw een groundwater and nuclear waste forms, followed by migration of radi onuclide-bearing groundwater to the accessible environment. Under hydr ologically unsaturated conditions, vapor transport of volatile radionu clides is also possible. The primary components of repositories that m itigate release are the near-field and the far-field subsystems. The n ear-field encompasses the waste packages composed of engineered barrie rs (e.g., man-made materials, such as vitrified waste forms, corrosion -resistant containers), while the far-field includes the natural barri ers (e.g., host rock, hydrologic setting). Taken together, these two s ubsystems define a series of multiple, redundant barriers that act to assure the safe isolation of nuclear waste.1 In the U.S., the Departme nt of Energy (DOE) is investigating the feasibility of safe, long-term disposal of high-level nuclear waste at the Yucca Mountain site in Ne vada.2-3 The proposed repository horizon is located in non-welded tuff s within the unsaturated zone (i.e., above the water table) at Yucca M ountain. The purpose of this paper is to describe the source-term mode ls for radionuclide release from waste packages at Yucca Mountain site . The first section describes the conceptual release modes that are re levant for this site and waste package design, based on a consideratio n of the performance of currently proposed engineered barriers under e xpected and unexpected conditions. No attmept is made to assess the re asonableness nor probability of occurrence for any specific release mo de. The following section reviews the waste-form characteristics that are required to model and constrain the release of radionuclides from the waste package. The next section presents mathematical models for t he conceptual release modes, selected from those that have been implem ented into a probabilistic total system assessment code4 developed for the Electric Power Research Institute (EPRI). Most of these source-te rm models were initially developed by the researchers at the Universit y of California at Berkeley for the DOE. The final section summaries t he status of source-term models for Yucca Mountain and enumerates assu mptions/limitations to present models, as well as identifying key data and model enhancements that may be needed to improve the predictive r eliability of source-term calculations.