Effects of Relative Surface Area and Leachant Composition on the 137 Cs Leach Rate from Cement Waste Forms 1
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EFFECTS OF RELATIVE SURFACE AREA AND LEACHANT COMPOSITION ON THE ' 37Cs LEACH RATE FROM CEMENT WASTE FORMS1 ALEKSANDAR D. PERIC, ILIJA B. PLECAS AND SNEZANA D. GLODIC Institute of Nuclear Sciences "Vinca", Belgrade, Yugoslavia
ABSTRACT Cement matrices, doped with "37Cs, were investigated to establish their physicochemical properties, mainly 137Cs leach-rate. Experiments were carried out in leachant media with different degrees of aggressiveness on the cement matrix, e.g., distilled water, Na2SO4 and NaCl solutions. A series of six cement samples, which were orthocylindrically-shaped, were treated in each of three leachant media. For each sample, the surface area exposed to leaching relative to the total surface area was increased from 1:6 to 6:6, at increments of 1/6 ` Cs leach-rates of the samples immersed in the of the total surface area. Expected higher 37 more aggressive environment show very specific behavior and functional relations with respect to the exposed surface area. Radionuclide leach-rate coefficients did not exhibit linear behavior. Instead, leach-rate values were best fit by exponential equations to describe cement matrix behavior in the leachants. Different empirical equations were established to describe 137Cs leach-rates in the various leachants as a function of the relative surface area of the cement matrix exposed to the leachant. INTRODUCTION A technique that is widely used in immobilizing low- and intermediate-level liquid radioactive wastes is cementation. Concepts and appropriate materials for the engineered trench system, as a final disposal method for radioactive wastes, were investigated at the Vin~a Institute [1]. Material formulations for trench system components (i.e., concrete for trenches, concrete containers, trench-filling materials, and mortar for immobilizing waste materials) were optimized with respect to the main physical, chemical and mechanical properties [1,2]. This paper deals with investigations of such characteristics, mainly leach-rates of radionuclides immobilized in the waste forms. In this study, mortar composition was not optimized because of the high water-to-cement ratio (W/C =0.36) and the absence of radionuclide sorbents in the experimental mixture formulation. Instead, the composition used in the experiments was designed to allow easy leaching of 13'Cs from the mortar monolith in a relatively short period of 292 days. The experiments were intended to investigate what might happen to the mortar mixture solid if, in a probabilistic scenario, the trench system was accidentally flooded, and the level of monolith immersion in the leachant gradually moves from the top to the bottom of the monolith [3]. Results of investigations on cement matrices doped with 13"Cs could show their capability to retain bound activity under the influence of leachant media with different degrees of aggressiveness towards matrix materials, e.g., corrosion effects of the first and the second class on cement. With the aid of experimental data, appropriate equations for `"Cs leach-rate cou
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