Office of Civilian Radioactive Waste Management Nature and engineering working together for a safe repository If a repository were built at Yucca Mountain, it would rely on two different systems to prevent radioactive materials from escaping into the environment. These systems act as barriers to the movement of radionuclides (radioactive atoms). The first system involves natural barriers — characteristics of the rocks and the groundwater at Yucca Mountain. The second system includes man-made, or engineered, barriers that give the repository defense-in-depth and added safety margins. The two systems would work together to protect public health and safety and the environment. Yucca Mountain’s climate is very dry. The precipitation averages about 7.5 inches (190 mm) per year, most of which (more than 95%) either runs off, evaporates, or is taken up by the desert vegetation. The mountain’s water table is unusually far beneath the surface — on average, about 2,000 feet (600 m) underground. Yucca Mountain is located in the Death Valley hydrologic basin. Water in this basin does not flow into any rivers or oceans and is isolated from the aquifer systems of Las Vegas and Pahrump, the major nearby town, located about 40 miles (70 km) from the mountain. Natural barriers act together to slow the movement of radioactive particles Yucca Mountain has several natural characteristics that would work together to contain and isolate spent nuclear fuel and high-level radioactive waste. The most important natural barriers include the following: The surface soils and the natural physical shape and configuration of the mountain and its geologic environs (i.e., topography) — which limit the ability of water to infiltrate the surface Unsaturated rock layers above the repository level — which limit the ability of water to move down into the repository’s emplacement tunnels Unsaturated rock layers below the repository level — which limit transport of radionuclides that might escape from repository tunnels Volcanic rocks and water-deposited clay, silt, and sands (alluvial deposits) below the water table — which limit radionuclide transport in the saturated zone With very little water available to start with, the surface soils and topography of Yucca Mountain and its region limit the amount of water that can infiltrate the mountain’s surface. Perhaps the most important natural barrier, however, can be found in the rock layers and minerals of Yucca Mountain. The repository would be located about 1,000 feet (300 m) below the mountain’s surface and, on average, about 1,000 feet above the water table — in the unsaturated zone of rock. The unsaturated zone is the expanse of rock in which the microscopic pores are not completely filled with water. Water tends to move very slowly through such rock. At most locations within the mountain, it takes thousands of years for the small amounts of water that can infiltrate the surface to reach the level of the repository. It would then take thousands of additional years for the water to move through the next approximately 1,000 feet (300 m) of unsaturated rock to reach the water table. In addition, certain minerals within the rock actually strain radioactive particles from contaminated water, holding them in place in the rock. Of any particles that do reach the water table, the silts, rocks, and clays would slow, or capture, them. From there, any radioactive particles must then be transported more than 11 miles (18 km) through the rock in the saturated zone before reaching a location where the water is likely to be pumped to the surface and used by anyone. It would take many thousands of years for these processes to occur. Engineered barriers contribute to defense in depth By itself, the mountain would provide a high degree of protection to the public. To enhance the mountain’s natural barriers, scientists and engineers have devised a series of man-made, or engineered, barriers to augment the natural system. The major engineered barriers include the following: Drip shields — which limit the ability of water to contact the waste package Waste packages — which limit the water contacting the actual waste forms inside Cladding (corrosion-resistant metal tubes that contain the ceramic fuel pellets) — which limits the water contacting the commercial spent nuclear fuel portion of the waste Solid waste forms — which limit the rate of radionuclides picked up by any water that does contact the waste Inverts (the floors of stainless steel and crushed volcanic rock added to the emplacement tunnels) — which limit the rate of release of radionuclides to the natural barriers. The repository tunnels themselves also would serve as important engineered barriers to potential radioactive releases. The tunnels would be constructed away from large fractures in the rock because in unsaturated rock water moves fastest in large fractures. However, because of capillary forces, any water in small fractures near a larger opening, such as a tunnel, tends to stay in the fractures. The emplacement tunnels would also be designed so that any water that does enter them can drain, by gravity, out of the tunnels and away from any others. Natural and engineered barriers work together to provide necessary protection When designing disposal systems intended to last longer than recorded human history, scientists and engineers must consider the possibility that one or more barriers, natural or engineered, could fail to perform as expected. Waste packages may fail earlier than expected because of undetected defects. Unforeseen circumstances could cause more water than anticipated to seep into the tunnels. Fortunately, the repository’s ability to contain and isolate its contents would not depend on any single barrier, natural or man-made. Having a combination of barriers is called defense-in-depth, meaning if one barrier fails to perform as expected, other barriers will continue to function in a way that compensates for the unexpected failure. Each of the barriers would work with the others to support a system designed to protect the public’s health and safety and safeguard the environment. In fact, considering all the barriers working together, sophisticated computer calculations project that for at least 10,000 years after the repository is closed, the radiation a person could receive from the repository would be far below the radiation protection standards for public health and safety. U.S. Department of Energy Office of Civilian Radioactive Waste Management Yucca Mountain Project 1551 Hillshire Drive Las Vegas, NV 89134 1-800-225-6972 http://www.ocrwm.doe.gov DOE/YMP-0203 November 2003