Mathematical Modelling of Transport Phenomena in Concrete Porous Media
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Mathematical Modelling of Transport Phenomena in Concrete Porous Media
Ilija B. Plecas and Slavko D. Dimovic Institute of Nuclear Sciences "Vinca" P.O.BOX 522, 11001 Belgrade, Serbia
ABSTRACT Two fundamental concerns must be addressed when attempting to isolate low-level waste in a disposal facility on land. The first concern is isolating the waste from water, or hydrologic isolation. The second is preventing movement of the radionuclides out of the disposal facility, or radionuclide migration. Particularly, we have investigated here the latter modified scenario.To assess the safety for disposal of radioactive waste-concrete composition, the leakage of 137Cs from a waste composite into a surrounding fluid has been studied. Leakage tests were carried out by original method, developed in Vinca Institute [1,2,3,4,5] . Transport phenomena involved in the leaching of a radioactive material from a cement composite matrix are investigated using three methods based on theoretical equations [6,10]. These are: the diffusion equation for a plane source an equation for diffusion coupled to a first-order equation, and an empirical method employing a polynomial equation.. The results presented in this paper are from a 25-year mortar and concrete testing project that will influence the design choises for radioactive waste packaging for a future Serbian radioactive waste disposal center. Keywords : radioactive waste, cement, concrete, transport phenomena, leakage rate INTRODUCTION Cement and concrete are widely used in low-level waste management both as a means of solidifying waste and for containment of dry or liquid wastes. At present there is also widespread interest in the use of near-surface concrete trench system for the disposal of radwaste materials. Typical concrete is a mixture of cement , sand, stone aggregate and water in various proportions that together determine the structural properties and tightness of the poured material. Water content is one of the critical parameters and must be carefully controlled during purring and setting; to a large extent it will determine the porosity of the resulting material. Engineered barriers are features of the disposal system made or altered by humans during the construction, operation and closure of a repository. Engineered barriers are intended to contribute to the overall performance of the disposal system by providing the level of containment required while the waste remains hazardous. In cases where the selected site or geological environment is not ideally suited for disposal, the repository can be heavily engineered so that, for meeting safety targets, reliance is placed primarily on the engineered barriers. Engineering trenches system provides three biological protection barriers: 1. Mortar for immobilizing the waste and filling the concrete containers. (enable its penetration into all cavities of solid radioactive waste and thus fix it permanently; permit no leakage of radionuclides- use of special cement or adsorbers and provide primary biological protection) 2. Concret
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