Kinetics of Aqueous Alteration of P0798 Simulated Waste Glass in the Presence of Bentonite
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Kinetics of Aqueous Alteration of P0798 Simulated Waste Glass in the Presence of Bentonite K.Yamaguchi, Y.Inagaki, T.Saruwatari, K.Idemitsu, T.Arima Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 812-8581, Japan H.Yoshikawa, M.Yui Waste Isolation Research Division, JNC, Ibaraki 319-1194, Japan ABSTRACT Static aqueous alteration tests were performed with a Japanese simulated HLW glass, P0798, in the presence of bentonite in order to understand the effects of bentonite on the glass alteration kinetics and on the associated Cs release. Analogous alteration tests were performed in 0.001M NaOH solution without bentonite for comparison. The results indicated that; 1) at the initial stage of alteration up to 50 days, no remarkable difference was observed in the alteration rate between both cases “with” and “without” bentonite, 2) at the later stage beyond 50 days, however, the rate in the case “with” bentonite was larger than that in the case “without” bentonite. These results on the alteration rate were analyzed by use of a water-diffusion model. In the case “without” bentonite, a good agreement was observed between the model analysis and the experimental results at the initial stage of alteration up to 50 days, however, the model analysis deviated from the experimental results at the later stage beyond 50 days. In the case “with” bentonite, on the other hand, a good agreement was observed even at the later stage to give the value of the apparent diffusion coefficient, Di of 3.5x10-21 m2 /s. The comparison between both cases suggests that the alteration rate is controlled by the water diffusion in both cases “with” and “without” bentonite, however, the rate is depressed in the case “without” bentonite probably by the protective effects of the alteration layer developing at the glass surface. In the case “with” bentonite, on the other hand, the alteration layer is expected to be less protective. Cesium desorption tests performed for the altered glass and bentonite indicated that most of the cesium dissolved from the glass is retained in the secondary phase of smectite developing in the precipitated layer by sorption with ion-exchange in the case “without” bentonite. In the case “with” bentonite, however, it is likely to be sorbed at bentonite surface. INTRODUCTION A large number of studies have been done on nuclear waste glass performance in the presence of clay or buffer materials [1-5], and the enhancing effects of them on the glass dissolution/alteration rate have been observed by various authors. The following reaction processes have been proposed for explaining the effects: 1) sorption of silica dissolved from the glass on the clay or buffer materials decreases the silica concentration in solution, which increases the driving force for further dissolution of glass, 2) the transfer of relatively insoluble glass components (Si,Al,etc) from the glass surface to the clay makes the alteration layer less protective [2,4,5]. The reaction processes, however, have been not verified su
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