The Engineered Barrier System: Performance Issues
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THE ENGINEERED BARRIER SYSTEM: PERFORMANCE ISSUES T. H. Pigford Department of Nuclear Engineering, University of California, Berkeley, CA 94720 ABSTRACT
The engineered barrier system (EBS) for geologic disposal of high-level waste may typically consist of the waste solid, a container, and a porous backfill. Whether the surrounding rock is part of the EBS is arguable, but the properties of the surrounding rock strongly affect the EBS performance. The EBS functions and performance issue are addressed. FUNCTIONS OF THE ENGINEERED BARRIERS The container is expected to delay the exposure of waste to ground water and air. Even after a container fails by some penetration, it can still act as an engineered barrier by affecting the chemical environment to which the radioactive waste is exposed. If the remainder of the container is intact the small penetrations that caused container failure may impede the release of radionuclides to the surrounding backfill and rock. The container can benefit repository performance if there is a broad time distribution of container failures, thereby reducing the average release rate of radionuclides throughout the repository. The waste form is expected to release radionuclides only slowly to the water or air within the container. In unsaturated rock decay heat in the waste can be used to keep the container dry, thereby delaying container failure. Backfill surrounding the container can delay the transport of radionuclides sufficiently for decay of short-lived species. Its chemistry can affect the transport process. It can filter colloids. It can protect the container from mechanical deformation by the surrounding rock. However, backfill does not necessarily reduce release rates to surrounding rock. Because clay backfill typically has far greater porosity than the rock, a waste package emplaced in a borehole with thick porous backfill can release radionuclides more rapidly than would occur if the container without backfill were placed in a smaller diameter borehole. The near-field rock must be considered part of the engineered barrier system because it affects container life and affects the release rates from waste package via liquid pathways. ADEQUACY OF PERFORMANCE CRITERIA
The fundamental performance criterion is that radionuclides released from the EBS and ultimately to the environment do not result in health effects exceeding some established limit. It is not necessary to predict the actual releases from a repository. Predictions from
Mat. Res. Soc. Symp. Proc. Vol. 294. c 1993 Materials Research Society
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simple bounding calculations are sufficient if the resulting health effects do not exceed the limit and if the calculations can be reliably shown to be bounding. Without adequate, well-defined, and sufficiently tested criteria for protection of public health and safety, much time and effort will be wasted in trying to guess what EBS performance is required. The U.S. program is particularly vulnerable. Current U.S. requirements depart considerably from those adopted heretofore in
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