Static Dissolution of Uo 2 in Interstitial Boom Clay Water
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U0
2
IN INTERSTITIAL BOOM CLAY WATER
K. LEMMENS, C. CACHOIR, P. VAN ISEGHEM SCK.CEN, Boeretang 200, B-2400 Mol, Belgium.
ABSTRACT Static dissolution experiments were performed with unirradiatcd U0 2 in Boom Clay water. The objectives were (1) to measure the solubility of uranium species in Boom Clay water, with U0 2 as the solid phase, and (2) to assess the impact of dissolved organic matter and carbonate concentration on this solubility. The tests were supported by calculations with geochemical codes to indicate possibly solubility controlling solid phases. The tests were performed in anoxic and reducing conditions, at 20 and 25°C. The following conclusions could be drawn: (I) Within 2 months in anoxic conditions, the uranium concentrations appear to approach saturation. (2) The "near-saturation" concentrations are between 2.4 and 7.8x 10 - M. (3) The influence of the carbonate concentration and humic acids on the uranium concentration was apparently small, but the interpretation is hampered by pH and El, variations. (4) The concentrations tend to be higher than the concentrations found in literature for similar Eh and/or pH conditions; this can probably be explained by small differences in experimental conditions. (5) The measured "near steadystate" uranium concentration in the real clay water agrees relatively well with the solubility calculated for uraninite. (6) Addition of sulfide species reduced the rcdox potential, but not the uranium concentrations, except in real Boom Clay water.
INTRODUCTION To assess the performance of the possible final disposal of spent fuel in the Belgian Boom Clay layer, it is important to know the maximum uranium concentrations in the interstitial Boom Clay water upon contact with spent fuel. The objectives of the proposed experimental programme are (1) to measure the uranium solubility in real Boom Clay Water, with unirradiated U0 2 as the solid phase and (2) to assess the impact of dissolved organic matter and carbonate concentration on this solubility. The measured uranium solubility may be representative for the dissolution in reducing conditions of spent fuel with an alpha activity that has decayed to the level where the fuel dissolution kinetics is chemnically controlled.
EXPERIMENT Depleted UO 2 powder with a particle diameter between 50 and 100 p m was immersed in Teflon® containers with real and synthetic clay waters for various test durations, at a target SA/V (= U0 2 surface area/leachant volume) of 1000 m-1. The specific surface area of the UO 2 powder 2 was estimated to be 0.044 m /g, based on BET measurements. Four types of clay water were used: real Boom Clay water ("RIC", sampled in the underground laboratory), synthetic Boom Clay water without humic acids ("SCW"), synthetic Boom Clay water with humic acids ("SCWHA") and synthetic Boom Clay water with humic acids, and with low carbonate content ("SCWHA\C"). To prepare the SCWHA and SCWHA\C, humic acids, extracted from real clay water, were used. This extract is a concentrate of dissolved organic matter ("DOM")
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