Effect of Carbonate Concentration on the Sorption of Plutonium Onto Geological Materials
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EFFECT OF CARBONATE CONCENTRATION ON THE SORPTION OF PLUTONIUM ONTO GEOLOGICAL MATERIALS G.M.N. BASTON, J.A. BERRY, M. BROWNSWORD, D.J. WLETT, C.M. LINKLATER, C.J. TWEED AND M. YUI" AEA Technology plc, 220 Harwell, Didcot, Oxfordshire, UK *Japan Nuclear Cycle Development Institute, Tokai Works, Ibaraki, Japan ABSTRACT This paper describes the most recent work in a programme of generic experimental and modelling sorption studies undertaken to increase confidence in the performance assessment for a potential high-level radioactive waste repository in Japan. The sorption of plutonium onto three rock samples was studied as a function of carbonate concentration under reducing conditions. Geochemical modelling was used to assist with experimental design and interpretation of results. INTRODUCTION The work described here forms part of the Japan Nuclear Cycle Development Institute (JNC) programme of generic studies undertaken to increase confidence in the performance assessment for a potential Japanese high-level radioactive waste (HLW) repository. In a continuation of work reported previously [1-3], batch sorption experiments have been carried out to study the sorption of plutonium at different carbonate solution concentrations onto samples of basalt, mudstone and sandstone (representing generic rock types from Japan). One of the aims of the JNC programme is to generate data suitable for use in repository performance assessment calculations. Experimental conditions were therefore chosen in order to simulate, as closely as possible, those anticipated in the vicinity of a potential HLW repository in Japan. Thermodynamic chemical modelling was carried out to help to plan the experiments and to aid interpretation of the results. EXPERIMENTAL Batch sorption experiments were carried out in duplicate at room temperature (21±3'C). Sorption was studied from synthetic rock-equilibrated de-ionised water under a nitrogen atmosphere and also under nitrogen atmospheres containing 0.4% and 2.0% carbon dioxide. In addition, sorption from synthetic rock-equilibrated seawater was studied under a nitrogen atmosphere and with a nitrogen / 2.0% carbon dioxide atmosphere. Reducing conditions, to simulate those expected in-situ, were obtained by adding sodium dithionite. Samples of the geological materials and mineralogical descriptions were supplied by JNC [4]. The composition of the basalt was mainly plagioclase, feldspar and quartz. The mineralogy of the mudstone was dominated by quartz, muscovite, calcite and carbonaceous material, while that of the sandstone was predominantly quartz, plagioclase and chlorite. The methodology for the preparation of rock samples, and synthetic rock-equilibrated de-ionised water and seawater has been described previously [1-3]. The liquid : solid ratio was 5:1 (100 cm 3 : 20g). The de-ionised water solutions were of low ionic strength, with carbonate concentrations of -3 x 10-4 M in sandstone- and mudstone-equilibrated water and below detection limit for basalt-equilibrated water (
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