Measurement of initial dissolution rate of P0798 simulated HLW glass by using micro-reactor flow-through test method

Abstract

We applied a new type of flow-through test method using micro-reactor consisting of a simple test apparatus with compact size to measurement of the dissolution rate of a Japanese type of simulated waste glass (P0798 glass). In this test method, a solution flows through a micro-channel (20 mm length, 2 mm width, 0.16 mm depth) in contact with a face of coupon shaped glass specimen, and the output solution is retrieved at certain intervals to be analyzed for determination of the glass dissolution rate. By using this test method the initial dissolution rate of glass matrix or forward dissolution rate was measured as a function of pH (3 to 11) and temperature (25°C to 90°C). The present test results indicated that the initial dissolution rate has ‘V-shaped’ pH dependence, and the effect of pH on the dissolution rate decreases with increase in temperature similar to the results measured by using the Single-pass flow-through (SPFT) method. The present test results also indicated that the dissolution of B is controlled by diffusion process and that of Si is controlled by surface reaction process.

Initial dissolution rate of a Japanese simulated high-level waste glass P0798 as a function of pH and temperature measured by using micro-channel flow-through test method

“…For the test at any pH, the dissolution rate decreased slightly with time at the initial test period of up to 20 h, and then the rate reached at each constant value. According to our previous study [7,8], the relatively higher dissolution rate at the initial test period is suggested to be caused by mainly the larger surface area of initial glass specimen originated from the surface roughness by polishing, and the roughness becomes smoothed as the glass dissolution progresses to provide a constant dissolution rate at the test period of beyond 20 h. With respect to the chemical affinity of solution, the concentrations of Si in the output solutions were measured to be around 7 6 10 76 M (0.2 ppm) at pH 3, and much less than that value at other pH. Since these values of Si concentration are much smaller than those of both solubility of SiO 2 (am) (around 2 6 10 73 M at pH lower than 9) and the saturated Si concentration for P0798 glass dissolution (around 10 73 M at pH 9) [12], any solution condition examined in the present tests can be evaluated to be far from the saturation.…”

“…Figure 1 shows schematics of the MCFT test apparatus. The test apparatus has been originally developed by Okuyama et al in order to measure both diffusion and sorption coefficients of rock medium [9,10], and it has been modified to suit measurement of the glass dissolution/alteration kinetics by Inagaki et al [7,8]. The test apparatus consisted of three parts: an injection syringe pump, a micro-channel reactor unit, and an automatic sampler, being connected with each other with PTFE (polytetrafluoroethylene, Teflon 1 ) tubing.…”

“…Aqueous dissolution tests using MCFT method were performed for P0798 glass as a function of solution pH (pH 3-11) and temperature (25-908C) for test duration of up to 100 h with each sampling period of 2-5 h. The solution flow rate was selected to be 5 ml/ min according to our previous studies [7,8] where the glass dissolution rate at 258C has been confirmed to be independent of the solution flow rate in the range from 2 ml/min to 20 ml/min. The apparent S/V ratio was evaluated to be 6400 m 71 based on the dimensions of micro-channel (20 mm length, 2 mm width, 0.16 mm depth) shown in Figure 1 on the assumption that the inner volume of PTFE tube can be ignored.…”

“…Quantitative comparison between solution analysis data and reacted glass surface analysis data: Normalized elemental mass loss of Si, NL(Si), and average normalized dissolution rate of Si, NR(Si) ave , determined from solution data and from dissolution depth data. the glass dissolution tests with a higher solution flow rate are proposed to be an effective method to depress the pH change leading to the decrease in glass dissolution rate along the solution flow, though our previous studies confirmed that the glass dissolution rate is independent of the solution flow rate in the range from 2 ml/min to 20 ml/min when the glass dissolution rate is low at 258C [7,8]. Therefore, additional dissolution tests with a higher solution flow rate of 20 ml/min, instead of the normal flow rate of 5 ml/min, were performed at 908C for both pH 10 and pH 3.…”

“…Consequently, the glass dissolution rate measured by using SPFT method may remain some uncertainties for further evaluation of the dissolution kinetics in an aspect of the preciseness and consistency. Therefore, we have applied a newly developed flowthrough test method using micro-channel reactor to precise and consistent measurement of the glass dissolution kinetics [7,8]. In this test method named ''micro-channel flow-through (MCFT) method,'' a coupon shaped glass is used as a specimen, which allows accurate determination of the glass surface area actually reacting with solution.…”