We are using an iterative process to develop preliminary concept descriptions for the Engineered Barrier System and waste-package components for the potential geologic repository at Yucca Mountain. The process allows multiple design concepts to be developed subject to major constraints, requirements, and assumptions. Involved in the highly interactive and interdependent steps of the process are technical specialists in engineering, metallic and nonmetallic materials, chemistry, geomechanics, hydrology, and geochemistry. We have developed preliminary design concepts that satisfy both technical and nontechnical (e.g., programmatic or policy) requirements.

The US Department of Energy is studying the suitability of Yucca Mountain (YM) as a potential nuclear waste repository site. Environmental conditions are important to engineered barrier system (EBS) design, materials testing, selection, design criteria, waste-form characterization, and performance assessment. Yucca Mountain is located in uninhabited desert which results in an environmental framework (unsaturated conditions, and sorptive properties of the rock materials) that is generally favorable for waste disposal. However, waste package (WP) and engineered barrier system (EBS) design concepts, including materials testing and selection, design criteria development, waste-form characterization, and performance assessments all require a specific and detailed understanding of the environmental conditions that will interact with the WP/EBS. Environmental conditions assessments from a series of laboratory and modeling studies have been conducted which provide the current understanding of the near-field environmental conditions at YM that not only exist now but will exist in the future. Because the environmental conditions can change with time, emphasis of the investigations were on processes and changed (not ambient) conditions.

A Viability Assessment (VA) for the Yucca Mountain Project is being completed for delivery in September of 1998. A major element of the VA is the design of a high level waste repository on the Nevada Test Site. The repository is made up of surface and subsurface facilities. The engineered barrier includes the man-made elements of the system that act to retard the migration of radionuclides from a geologic repository. They act in conjunction with the geologic barriers present at Yucca Mountain. The engineered barrier system (EBS) consists of the Waste Package and the underground facility. The focus of this paper is the status of the design of the underground facility portion of the EBS. In addition to a robust waste package, the EBS components in the reference design include a number of features that impede naturally occurring infiltration from reaching and corroding the waste packages. In addition, and as a defense-in-depth strategy, a number of other optional features are being considered. They include drip shields above the waste packages to intercept dripping water and granular backfill around the waste packages to form a diffusion barrier. Plans are being made to test a number of the EBS materials and structures. The Viability Assessment document will discuss the various EBS options and alternative designs and lay out a plan for determining those to be included in the License Application to the Nuclear Regulatory Commission (NRC) scheduled for completion in 2002.

One of the current major national environmental problems is the safe disposal of large quantities of spent nuclear fuel and high-level radioactive waste materials, which are rapidly accumulating throughout the country. These radioactive byproducts are generated as the result of national defense activities and from the generation of electricity by commercial nuclear power plants. At present, spent nuclear fuel is accumulating at over 70 power plant sites distributed throughout 33 states. The safe disposal of these high-level radioactive materials at a central disposal facility is a high national priority. This Reference Design Description explains the current design for a potential geologic repository that may be located at Yucca Mountain in Nevada for the disposal of spent nuclear fuel and high-level radioactive waste materials. This document describes a possible design for the three fundamental parts of a repository: a surface facility, subsurface repository, and waste packaging. It also presents the current conceptual design of the key engineering systems for the final four phases of repository processes: operations, monitoring, closure, and postclosure. In accordance with current law, this design does not include an interim storage option. In addition, this Reference Design Description reviews the expected long-term performance of the potential repository. It describes the natural barrier system which, together with the engineered systems, achieves the repository objectives. This design will protect the public and the environment by allowing the safe disposal of radioactive waste received from government-owned custodial spent fuel sites, high-level radioactive waste sites, and commercial power reactor sites. All design elements meet or exceed applicable regulations governing the disposal of high-level radioactive waste. The design will provide safe disposal of waste materials for at least a 10,000 year period. During this time interval, natural radioactive decay of the waste materials will result in fission products that pose a minimal radiological hazard to the public afterward. For example, after 100 years, the relative hazard from the waste fission products will have diminished approximately 90 percent. After 1,000 years, the hazard will have diminished 99 percent, and after 10,000 years it will have diminished 99.9 percent. The resulting radiological hazard after 10,000 years is minimal, being of the same order of magnitude as that posed by 0.2 percent uranium ore, which is equivalent to that which was used to originally produce the nuclear fuel. Because developing such a repository is extremely complex, the design will move forward in three stages: Site Recommendation, License Application, and Construction. This document presents the design as it will be submitted in the Site Recommendation Consideration Report; the design will be updated as the design process moves forward. As more cost-effective solutions, technical advancements, or changes to requirements occur, the design may evolve. The U.S. Department of Energy's (DOE) Office of Civilian Radioactive Waste Management is developing a system that includes this potential repository. This waste management system integrates acceptance, transportation, storage, and disposal of spent nuclear fuel and high-level radioactive waste. Acceptance and transportation will be handled by regional servicing contractors under contract to the DOE. The U.S. Nuclear Regulatory Commission will conduct an in-depth and thorough licensing review to determine the acceptability of the proposed waste management system. Eight sections of this document follow. Section 2 discusses the design requirements for the proposed repository. Section 3 describes the physical layout of the proposed repository. Section 4 describes the evolutionary phases of the development of the proposed repository. Section 5 describes the receipt of waste. Section 6 details the various systems that will package the waste and move it below ground, as well as safety monitoring and closure. Section 7 describes the systems (natural and engineered) that ensure continued safety after closure. Section 8 offers design options that may be adopted in the future, and Section 9 provides a summary statement on the repository.