Absorption Study of the Tc(IV) on Rocks and Minerals Under Simulated Geological Conditions
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Absorption Study of the Tc(IV) on Rocks and Minerals Under Simulated Geological Conditions Takayuki ANAYA, Wataru KOBAYASHI, Kazunori SUZUKI JGC CORPORATION, Oaralmachi, Ibaraki-Pref., 311-13 Japan
ABSTRACT The absorption of the technetium on rocks and minerals have been studied by using freshly quarried rocks to provide reductg conditions, without the addition of reduchive reagents. The initial Tc concentration in this study was about lx1O- mol/l (0.62 Bq/ml). Technetium was not absorbed on tuff or granite under oxidizing conditions, but higher Kd values of Tc were obtained for granite under reducing conditions. The Tc absorbed on granite was not desorbed under reducing condition. This indicates the absorption reaction Is irreversible. INTRODUCTION 99
Tc Is a beta nuclide with a half-life of 2.11x10 5 y and a thermal neutron fission yield of 6.13%. As Tc Is a major radionuclides of the High Level Waste(HLW), it is important for the safety evaluation of HLW disposal to understand the migration behavior of this element. Technetium is a representative redox-sensitive radionuclide [1,2]. Under oxidizing conditions, technetium exists as the anionic species TcO4[3] in solution, and shows high solubility and little sorption on naturally occurring materials [4,51. Under reducing conditions, Tc exists as TcO in solution, shows low solubility [3,8-10] and the strong absorption [6,71 on granite.
Chemical agents, such as metals and Na2 S2 04 etc, were often used in previous studies to produce reducing conditions in bulk solutions. However, it has recently been suggested that the reducing condition in the micro area on the surface of the rock is more important than that of bulk solution [2,7] in the absorption reaction. Therefore, precisely simulated conditions are Important to understand the actual absorption reaction that occurs in geological media. In this study, only freshly quarried rocks were used in the inert atmosphere glove box to simulate geological conditions. No reductive reagents were used. The absorption behavior of technetium under both geological and oxidising condition Is reported. EXPERIMENTAL Preparation of Simulated Underground Water (SUW) All laboratory work was performed in an inert atmosphere glove box, which was filled with argon gas. The gas contained less than 1 ppm 02 and CO2 . Two species of rock, granite and tuff, were obtained directly from quarry sites, Inada in Ibaraki Prefecture and Oya In Tochigi Prefecture, respectively. These rocks were rinsed with argon gas to remove atmospheric oxygen from their surfaces and stored in the inert glove box to avoid air contamination. The rocks were dry-crushed and dry-sieved to make powders with parti0
Mat. Res. Soc. Symp. Proc. Vol. 353 1995 Materials Research Society
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cle sizes less than 100 um. These rock powders were used to prepare simulated underground water (SUW) from degassed water. The SUW was prepared for each rock. One liter of degassed water was mixed with 100 g of rock powder in 1 liter polyethylene bottle. This was agitated slowly fo
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