Flow-Diversion Barriers for HLW and LLW Disposal under Partially Saturated Conditions
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FLOW-DIVERSION BARRIERS FOR HLW AND LLW DISPOSAL UNDER PARTIALLY SATURATED CONDITIONS M. J. APTED*, P. F. SALTER* AND J. L. CONCA** * Intera Information Technologies, Inc., Denver, Colorado 80235 USA ** UFA Laboratory, Washington State University, Richland, Washington 99352 USA ABSTRACT
An innovative EBS design of sand-gravel layers for waste disposal under partially saturated conditions is described. Advantages of this EBS are 1) as a hydraulic barrier to advective inflow from the surrounding rock, and 2) as a diffusion barrier to the transport of dissolved or colloidal radionuclides. This sand-gravel barrier design for unsaturated sites is analogous to EBS designs using compacted bentonite backfills/buffers in saturated repository sites. Percolation box experiments, field tests, and natural analogues supporting the advantageous performance of this flow-diversion design are described and estimates for source-term release are calculated. INTRODUCTION
Low-permeability backfills between packages of nuclear waste and encompassing saturated host rock have been adopted for many high-level (HLW) and low-level waste (LLW) sites. Inclusion of such a backfill effectively decouples the waste package (near-field) and geosphere (far-field) portions of the repository system. This decoupling is central to demonstrating a robust safety case. It assures that the source-term release of radionuclides from the engineered barrier system (EBS) of waste package and backfill will always be limited by readily predictable diffusional transport, regardless of uncertainties in future hydrological flow conditions of the site. Low-permeability backfill material such as compacted bentonite clay also functions as a colloid filter, eliminating any possible transport of colloids from the waste package to the host
rock. These capabilities simplify site characterization, easing issues with respect to site suitability. By decoupling the EBS from the site hydrology, the primary functions of the rock are reduced to providing stable mechanical and stable chemical conditions over a long time period, so that the long-term performance of the EBS can be reliably estimated [1, 2]. Highly conductive, flow-diversion ("hydraulic-cage") EBS designs have been proposed as alternatives to low-permeability backfills. Such designs are particularly relevant to unsaturated repository conditions in which backfill material such as compacted bentonite would draw water preferentially to the waste package, resulting in adverse EBS performance. This paper presents a refined EBS flow-diversion design that relies upon combined sand and gravel layers between the unsaturated host rock and nuclear waste packages. Ultracentrifuge tests [3], percolation box tests [4], field tests [5], and natural analogue studies [6] relevant to our proposed flow-diversion system are reviewed. Estimates of possible source-term performance for such a design are calculated. From these results, recommendations for future work are made. WATER MOVEMENT IN PARTIALLY SATURATED CONDITIONS
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