Diffusion of Uranium in Compacted Sodium Bentonite
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DIFFUSION OF URANIUM IN COMPACTED SODIUM BENTONITE ARTO MUURINEN, KAIJA OLLILA, JARMO LEHIKOINEN, Technical Research Centre of Finland, Reactor Laboratory, P.O. Box 200, SF-02151 Espoo, Finland.
ABSTRACT In this study the diffusion of uranium dissolved from uranium oxide fuel was studied experimentally in compacted sodium bentonite (Wyoming bentonite MX-80). The parameters varied in the study were the density of bentonite, the salt content of the solution and the redox conditions. In the studies with non-saline water of total dissolved solids about 300 ppm, uranium was both in aerobic and anaerobic experiments as anionic complexes and followed the anionic diffusion mechanism. Anion exclusion decreased effective diffusion coefficients, especially in more dense samples. In the studies with saline water of total dissolves solids about 35000 ppm, uranium appeared in the aerobic experiments probably as cationic complexes and followed the cationic diffusion mechanism. Uranium in the saline, anaerobic experiment was probably U(OH) 4 and followed the diffusion mechanism of neutral species. INTRODUCTION The major factor controlling the mobilization of most fission products and actinides from spent fuel under disposal conditions is the rate of dissolution of the U0 2-matrix into groundwater. The progress of the dissolution process is controlled by the diffusion of the dissolved uranium through the buffer to groundwater. The chemistry of uranium is known to be very sensitive to the surrounding conditions. Change of valence affects the solubility and speciation. Uranium also forms carbonate and hydroxyl complexes, that affect solubility. Diffusion mechanisms, for example anion exclusion or sorption, depend on the speciation of the ions. The site-selection studies in Finnish crystalline bedrock have shown that the salt content in groundwater can vary from a few hundred ppm to tens of thousands of ppm. The salt concentration of water affects the solubility and speciation of uranium as well as the diffusion behaviour in bentonite. The effective diffusivity of anions is very sensitive to bentonite compaction as well as to the salt content of water [1]. The parameters varied in the study were the density of bentonite, salt content of the solution and redox conditions. The higher density values (1.7 g/cm3 ) corresponded about to those planned to be used in the repository [2], while the lower densities (0.7 g/cm3) were selected in order to see the anion exclusion effects. The salt concentrations in the waters corresponded about to the values found in the bedrock in Finland. Uranium was dissolved under aerobic conditions in order to simulate oxic conditions possibly caused by radiolysis in the repository. The experiments were then carried out both in aerobic and anaerobic conditions in order to see the baviour in different conditions (oxidizing/reducing) which possibly could appear.
EXPERIMENTAL The through-diffusion measurement method was used in the study [3]. The bentonite sample was compacted in a steel tube which was placed bet
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