Reference Laboratory Testing for Backfill
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THE SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT Stephen V. Topp, editor
379
REFERENCE LABORATORY TESTING FOR BACKFILL
RILEY M. CHUNG AND FELIX Y. YOKEL National Bureau of Standards, Washington,
D.C.
20234
ABSTRACT Relatively high magnitude of swelling and low hydraulic conductivity are two of the performance requirements for the backfill placed around the radioactive waste package for the underground nuclear waste storage scheme. Some studies have been conducted in U.S. national laboratories and in other countries where the candidate backfill materials were tested under many different conditions to determine the expected range of these properties. This paper briefly examines the variables that were found to be significant in the evaluation of swelling and hydraulic conductivity and special emphasis is placed on the compaction method, compaction effort, and the moisture content at the time of compaction, which do not receive much consideration in ongoing test programs.
INTRODUCTION In the proposed underground scheme of the U.S. nuclear waste management program, a multi-barrier isolation system is considered to contain the radionuclides and thus to isolate them from reaching the biosphere over a long period of time. As a simplified example, the multi-barrier system consists of a glass waste form, a metal canister containing the waste form which is lowered into a borehole drilled from the floor of tunnels in the repository, backfill placed between the canister and the borehole wall, and the repository rock. Many material properties are desirable for the candidate materials to be considered as the backfill. These include sufficiently high swelling potential and plasticity, low hydraulic conductivity, adequate thermal conductivity, high sorption capability, adequate shear strength, and chemical stability over a wide range of temperatures and pressures. This paper focuses on the current status of laboratory testing and the variables affecting the determination of the first two material properties, i.e., swelling and hydraulic conductivity. Suggestions are also made as to how the tests can be conducted to promote exchange of test data determined from many research laboratories. CURRENT STATUS OF LABORATORY TESTING Relative high magnitude of swelling and plasticity of the backfill material is desirable to provide a self-sealing ability for material cracks within the backfill and localized cracks and flaws in the surrounding rocks in order to minimize the potential of groundwater instrusion. Backfill with low hydraulic conductivity, when placed around the waste package, may shield the waste canister from the intrusion of groundwater for a long time period and subsequently inhibit mass movement of molecules and thereby minimize the corrosion potential of the canister and delay leakage of radionuclides to the groundwater. As part of the NBS nuclear waste management program, data on backfill performance were presented in a workshop held at NBS in April, 1981. Further data were derived from the published literatu
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