Integration of Postclosure Safety Analysis with Repository Design for the Yucca Mountain Repository through the Selection of Design Control Parameters

Abstract

The Yucca Mountain repository combines multiple barriers, both natural and engineered, which work both individually and collectively to limit the movement of water and the potential release and movement of radionuclides to the accessible environment. Engineered structures, systems and components (SSCs) are designed to function in the natural environment utilizing materials chosen to perform their intended functions in order to meet the postclosure performance objectives. Similarly, the features of the natural environment are expected to respond to the presence of the repository through geomechanical, hydrogeologic and geochemical changes. At Yucca Mountain, specific features, both engineered and natural have been identified as requiring design control during repository construction and operations. The integration between design and postclosure safety analysis is facilitated using design control parameters. The term “design control parameters” includes functions and performance requirements allocated to SSCs through the design process, as well as the attributes of SSCs that are developed during design (e.g., dimensions; weights; materials; fabrication and quality-control processes; and operating conditions). These control parameters provide an interface between the design and the analyzed postclosure safety bases, which needs to be maintained through the licensing process. Maintenance of the design is controlled through configuration management and procedural safety controls. The design control parameters serve three key purposes. First, they identify key aspects of the design that serve as the design bases for the designers of the SSCs of the facility. Second, they provide a useful input to the analyses of relevant postclosure features, events and processes (FEPs) and are used to either exclude FEPs from the postclosure safety analysis or as an input to models of included FEPs in the safety analysis. Finally, they provide important controlled interface constraints between the design and safety analyses organizations that are amenable to configuration management. Several examples of such design controls will be presented in this briefing. The first type of design controls relates to the location of the underground facility, including standoffs from faults and the ground surface. The second type of design controls relates to the configuration of the engineered features including the spacing of emplacement driftsand drip shield dimensions and characteristics. A third type of design controls relates to constraints on handling, loading and emplacing waste forms in canisters and waste packages in the emplacement drifts.