The Influence of Ligands and Precipitates on the Release of Nuclides from the Near Field under Natural Repository Condit
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The Influence of Ligands and Precipitates on the Release of Nuclides from the Near Field under Natural Repository Conditions L. Liu and I. Neretnieks Department of Chemical Engineering and Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ABSTRACT Once groundwater intrudes into a damaged canister and wets the spent fuel pellets, radiation emitted from the spent nuclear fuel splits nearby water into oxidizing and reducing species. This may lead to an oxidizing condition near the fuel pellets. As a result, uranium oxide that makes up the fuel matrix will become more soluble, and the incorporated radionuclides will be released more rapidly. The dissolution process is, however, a dynamic one that can be influenced by many factors. Of great importance are the radiation power of the fuel matrix, the concentration of ligands near the fuel surface, and the transport resistance of the near field. Consequently, the escape of nuclides from the damaged canister is dominated mainly by the intrusion of ligands, and the precipitation/dissolution of secondary phases within the fuel rods. To investigate the possible effects of ligands and precipitates, a coupled dissolution and transport model, which includes the barrier effect of the Zircaloy claddings, is developed. The application of the model to a SKB-specified reference scenario indicates that by far the largest fraction of the oxidized uranium will reprecipitate within the canister. This may significantly decrease the fuel surface available for oxidation and the water available for radiolysis. Subsequently, much less fuel matrix will be dissolved and much less of the other nuclides will be released. Simulations further identify that carbonate and silicate have the greatest influences on the formation of secondary phases, and on the release of nuclides, under natural repository conditions. INTRODUCTION It has been generally accepted that the confinement of nuclides within the spent fuel matrix to approximately 97 % UO2 is guaranteed if the oxidation state of the matrix does not exceed the upper limit of stability of the fluorite structure, UO2.33, corresponding to a nominal stoichiometry of U3O7 [1]. However, the spent fuel matrix itself constitutes a dynamic redox system after groundwater comes into contact with the fuel pellets. This results mainly from the fact that αradiolysis associated with the spent fuel may generate equivalent quantities of oxidizing and reducing species. The reducing species is predominantly hydrogen. The relatively low reactivity of hydrogen, coupled with a relatively high diffusivity, may allow hydrogen to escape from the system before it reacts with the more reactive oxidizing species, mainly oxygen and hydrogen peroxide [2]. This may well lead to an increased redox potential on the fuel surface, and the oxidative dissolution of the fuel matrix, as UO22+, may start to dominate the redox chemistry of the system [3]. As a result, the incorporated nuclides are released into the groundwater. The dissolved species may interact with o
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