Interactions Between Np-Doped Synroc and Boom Clay
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INTERACTIONS BETWEEN Np-DOPED SYNROC AND BOOM CLAY K. P. HART*, B. J. ROBINSON*, T. E. PAYNE*, P. VAN ISEGHEM** AND K. LEMMENS** *Australian Nuclear Science and Technology Organisation (ANSTO) Private Mail Bag 1, Menai, New South Wales, Australia, 2234. **S.C.K./C.E.N., Boeretang 200, B-2400 Mol, Belgium.
ABSTRACT Leaching experiments with deionised water at 70°C have been carried out to ascertain the effect of oxidising and anaerobic conditions and the presence of Boom clay on Np release from Synroc. The normalised solution leach rates of Np measured in these experiments are about a factor of 10 lower under reducing conditions than oxidising conditions and the presence of Boom clay was found not to enhance the leaching of Np from Synroc. Neptunium was found to be predominantly in the mobile fraction and was not removed from the leaching solutions significantly by filtration or ultrafiltration.
INTRODUCTION The interactions between geological materials and wasteforms in a HLW repository can affect the release and transport of radionuclides from the wasteform by altering the groundwater chemistry, by precipitating the radionuclides as secondary phases, or by sorbing radionuclides onto the mineral's surfaces. The extent of these interactions is critical for determining the amount of radioactivity that is available for transport from any HLW repository. The Boom clay formation at the Mol site in Belgium has been used as the reference case for repositories in argillaceous environments and is the candidate host rock in the Belgian repository concept. This study focuses on the effect of Boom clay on the leaching of Synroc in both deionised water and simulated clay pore water under reducing conditions. In addition, the sorption and desorption of Np onto Boom clay at different solid to liquid ratios, using both deionised water and simulated clay pore water under reducing conditions will also be compared. Under a reducing environment Np is expected to be in the highly insoluble tetravalent state. To assess the effect of oxygen content and redox conditions on the leaching of Np-doped Synroc, leaching tests are being carried out using deionised water and simulated clay pore water, individually and in the presence of Boom clay, under reducing conditions. The presence of humic materials in solution can enhance the release of radionuclides from wasteforms by complexing the ions in solution. The effect of humic materials, present in the Boom clay, on the release of Np from Synroc was studied by analysing Np in the filtrates from a number of filters of different pore sizes ranging from 1000 NMWL to 0.45 glm. As the majority of humic materials are expected to be in this size range, this procedure gives an indication of the possible importance of humic materials and other colloidal phases.
EXPERIMENTAL Samples were prepared from a batch of Synroc containing 1.14 wt% 2 37Np and simulated PW4b D waste. This batch of Synroc was synthesised following the methods described previously1 . Cores, 10 mm dia., were cut into wafers 1-2 mm t
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