Leaching study of Cs-137 from a cement matrix generated at nuclear power plant

Abstract

A diffusion model was developed to explain the leaching phenomena of Cs-137 from cement matrix considering various factors which affect the leachability. A finite-slab approximation method also was developed to predict long-term leachability. Experimental study for the leaching behavior of Cs-137 was carried out with simulated evaporator bottom product of PWR plant. As expected, the cumulative leaching fraction of Cs-137 varied with the square root of time. The method of leach test proposed by the International Atomic Energy Agency (IAEA) was followed, and part of the American Nuclear Society (ANS) method was also adopted. The effect of various factors, including sampling method, curing temperature, curing time, leachant temperature, vermiculite addition, volume-to-surface (V/S) ratio and leachant unoccupation time, have been considered. Diffusion model in semi-infinite slab gives very good agreement with the data obtained from 4-weeks cured specimens. The effective diffusion coefficient of a specimen which was cured at temperature 24℃ for 4 weeks was found to be $1.47×10^{-11} ㎠/sec$. With the derived effective diffusion coefficient, comparisons were made for the leachability of Cs-137 from a 55 Gal drum by using the finite-slab approximation method and the solution based on the cylindrical shape. These comparisons show that the leachability of the waste container can be predicted quite accurately by the finite-slab approximation method. In the case of Cs-137, the predicted amounts by the finite-slab approximation method are qual to or slightly greater than those by the solution of actual geometry. The estimated fraction of Cs-137 which remains in the environment, is found to be less than 0.25 percent of initial amount after 100 years. After about 25 years from the start of leaching, the fractional amount in the environment reach to maximum value which corresponds to 0.66 percent of initial amount.