PRELIMINARY PERFORMANCE ASSESSMENT OF THE ENGINEERED BARRIERS FOR A LOW-LEVEL AND INTERMEDIATE-LEVEL RADIOACTIVE-WASTE REPOSITORY

Radionuclide release from an engineered barrier in a low- and intermed iate-level waste repository is evaluated. The results of experimental studies conducted to determine the radionuclide diffusion coefficients and the hydraulic conductivities of calcium bentonite and crushed gra nite mixtures are presented. The hydraulic conductivity of the mixture is relatively low even at low dry density and clay content, and the p rincipal mechanism of radionuclide migration through the mixture is di ffusion. The measured values of apparent diffusion coefficients in cal cium bentonite with a dry density of 1.4 Mg/m(3) are of the order of 1 0(-13) to 10(-12) m(2)/s for cations and 10(-11) m(2)/s for iodine. Th ese values are similar to those in sodium bentonite. The radionuclide release rates from the engineered barrier composed of the concrete str ucture and the clay-based backfill were calculated. Carbon-14 and Tc-9 9 are the important nuclides; however, their maximum release rates are <10(-5) GBq/yr. To quantify the effect of uncertainties of input para meters on the radionuclide release rates, Latin Hypercube sampling was used, and the ranges of release rates were estimated statistically wi th a confidence level of 95%. The uncertainties of the assessment resu lts of the radionuclide release rate are larger in the case of the sor bing nuclides such as Cs-137. Finally, the sensitivity of the input pa rameter to release rare is also evaluated.