Studies on the radionuclide retarding characteristics of clay-based backfill materials

Abstract

The objective of this thesis is to investigate the radionuclide-retarding characteristics of clay-based materials. When the clay-based materials are used for the backfill of waste repository, the retardation of radionuclide release is dependent upon the following processes : water permeation, radionuclide sorption and diffusion. In this connection, a series of tests were conducted with domestic clay-based materials. The clay-based materials used involve a bentonitic clay taken from Younil area, Kyungbuk and the mixture of the bentonitic clay and crushed granite taken from Daeduk, Taejon. The water content and particle size distribution of the clay-based materials were adjusted to give a maximum dry density. In water permeation tests, the dimension of test cylinder was $5 × 10^{-2} m$ in diameter and $5 × 10^{-2} m$ in height, and the water, which is a synthetic ground water, was fed at the pressure of 1.3 Kg/㎠. The hydraulic conductivity of the clay-based materials decreased with increasing dry density as well as clay content. When the clay content was increased to 50 wt.%, the hydraulic conductivity is $3 × 10^{-10} m/sec$ even at the dry density of 1.5 Mg/㎥. Also, when the dry density was 1.5 Mg/㎥, the hydraulic conductivity significantly decreased and its value was $4 × 10^{-10} m/sec$ at the clay content of 30 wt.%. For the mixture of clay and crushed rock, the hydraulic conductivity was dominated by the clay and the relationship could be explained by means of the effective clay dry density concept. Sorption tests were performanced using a typical batch method. As the representative radionuclides of cation and anion were employed Sr-90 and I-125, respectively. The sorption isotherm was determined, and the sorption mechanism of radionuclides onto the clay was investigated using a sequential chemical extraction technique. The effects of radionuclide concentration, ionic strength, initial solution pH, solution-to-clay ratio, and the clay content of mixture on ...