Backfill barrier as a component in a multiple barrier nuclear waste isolation system

Abstract

Quantitative estimates of the potential effectiveness of backfill barriers based on a linear sorption node! are presented. Using getters such as clays (known sorbent3), a backfill approximately l-foot-thick can delay 4 5 \ 3 by 10 to 10 years the breakthrough of transuranics. A delay of 10 years is possible for major cationic fission products. These delays can be achieved provided that (1) the distribution coefficient (K , a measure of affinity for sot bed species) for the barrier maternal is equal to or greater than 2000 ml/g for transuranics and 200 ml/g for fission products; (2) the interstitial groundwater velocity through the barrier is limited to J ft/year or less; (3) the effective porosity of the barrier is equal to or less than 0.1; and (4) the physical integrity of the barrier is maintained (no channels or cracks!. Mixtures containing expanding clays such as smectites and other getters, are expected to 3atisfy these criteria.

Near-Field Mass Transfer in Geologic Disposal Systems: a Review

“…Previous backfill analyses 37,38,39 have calculated the "breakthrough" time for a stable species at a specified distance in a backfill of infinite thickness and have used that time to estimate the amount of radioactive decay that will occur during diffusion through finite backfill in a repository. This has led to considerable overestimates of the possible retention of radionuclides in backfill.…”

Development of backfill material as an engineered barrier in the waste package system- Interim topical report

“…Studies and performance testing on emplacement hole backfill materials have been in progress for several years in the United States, Canada, and Sweden Pusch 1977a;Sandia 1979;Fried and Friedman 1979;Nowak 1980a Sandia 1979) have focused on the testing and development of smectite clay and sand barriers for use in a salt repository. Screening studies have been completed on various clays, 4.1 natural soils, and zeolites as a function of solution salinity~ pH, flow rate, and temperature.…”

“…Plutonium and americium sorb very well (K d values about 2000 ml/gm) and europium, cesium, and strontium sorb moderately (K d values about 200 ml/gm). Existing data (Nowack 1979;Sandia 1979;Nowak 1980aNowak , 1980b suggest that a properly chosen~ 0.3-m-thick backfill barrier surrounding a waste container could delay the breakthrough (defined as one percent of initial concentration) of plutonium and americium through the barrier for periods of 10,000 to 100~000 years depending upon the interstitial brine flow rate. Concurrently, the breakthrough of cesium, strontium, and europium could be delayed for 1,000 to 10,000 years, which is sufficient time for nearly complete decay of those radionuclides.…”

“…One activity (PNL 1978) involves testing the radionuclide sorptive behavior of various rocks and minerals. Sorption has been shown to be an important potential barrier to nuclide transport (Nowak 1980a). The sorptive capacity of several minerals for anionic radioactive chemical species, such as iodine and technetium, is also being studied (Fried and Friedman 1979) along with the increased sorption of technetium under reducing conditions (Ewing and Haaker 1979).…”

“…Currently available data on emplacement hole backfill barrier performance Nowak 1980a;DOE 1979) particularly in regard to radionuclide sorptive characteristics, indicate that backfill materials can effectively contribute to the isolation of radioactive wastes in a geologic 4.2 repository in the presence of brines and other ground waters. Further work on these backfill barriers is in progress (Jenks 1979;Sandia 1979;Nowak 1980aNowak , 1980bEPRI 1979) for better characterization and engineering development.…”