| 1. |
Yaohiro Inagaki, Aqueous dissolution rate of nuclear waste glasses as a function of environmental parameters, Sum glass 2023, 2023.09, Over the last few decades, understanding the kinetics of nuclear waste glass dissolution/alteration in aqueous media has advanced through a large number of the experimental and atomistic simulation studies performed by a large number of researchers in the world, which has contributed successfully to an advance of modeling the glass dissolution/alteration for the geological disposal. For assessing the long-term glass performance in geological disposal with reliability, however, we need a greater understanding of the glass dissolution/alteration kinetics under the actual repository conditions in consideration of the potential long-term evolution of the repository conditions. The greater understanding of the dissolution/alteration kinetics is also required for parameterization of a mechanistic model used to determine the glass dissolution/alteration rate with the radionuclide release rate as a function of key environmental variables such as the solution composition, pH, Eh, temperature, and time.
For the understanding of glass dissolution/alteration kinetics, we need experimental data on the glass dissolution rate measured precisely, consistently and systematically under well-constrained test conditions. Along with the data on the dissolution rate, which is usually determined from solution analysis, we also need the data on the altered glass surface properties, such as morphology of alteration layers, chemical composition, elemental depth profile, crystallinity, stability, density, porosity, permeability of water and glass constituent elements, etc., analyzed for the glass altered under well-constrained test conditions. For the last few decades, a large number of dissolution tests have been performed for the waste glasses, however, there have existed only a few data available for evaluation of the dissolution kinetics, because of difficulties in precise and systematic measurement. With respect to pH dependence of the glass dissolution rate, for example, we have only a few data available for the systematic evaluation, because the test conditions such as the solution composition, pH, and the glass surface area can change easily during the test period against the expectations as a consequence of the nature of the current standard test methods such as MCC-1 and PCT tests. Therefore, we have developed a new test method, “Micro-channel flow-through (MCFT) method”, to measure the glass dissolution rate precisely and systematically under various well-constrained test conditions, and applied it to measurement of the glass dissolution rate as a function of environmental parameters such as the solution composition, pH, temperature.
The MCFT method has been applied first to measurement of the initial dissolution rate, r0, as a function of pH and temperature for an international reference glass ISG and a Japanese reference glass P0798. The r0 has been measured successfully to provide the systematic data of r0 as a function of pH, where ISG showed a “V-shaped” pH dependence with a minimum at pH4, while P0798 glass shows a “U-shaped” pH dependence with a minimum at pH6. While the systematic data of r0 as a function of temperature at each constant pH has provided the apparent activation energy, Eact, for the initial dissolution. The Eact was evaluated to be 60-70 [kJ/mol] at acidic to weakly basic pH for ISG, which suggests a surface-reaction controlled-dissolution mechanism. For P0798, the Eact was evaluated to be 60-70 [kJ/mol] at neutral to weakly basic pH as well as ISG, however, the Eact at acidic pH was evaluated to be 50 [kJ/mol], which suggests a diffusion -controlled dissolution mechanism.
Currently, we have been trying to measure the glass dissolution rate as a function of solution concentration of Si by using the MCFT method and Si-isotopes. The concentration of Si dissolved in solution has been considered to be one of fundamental factors dominating the glass dissolution kinetics, and the rate equation based on the first-order dissolution rate law of pure amorphous silica has been proposed to be applied to modelling the glass dissolution/alteration as follows,
r = r0 (1-[H4SiO4]/K) (eq.1)
where r is the glass dissolution rate, r0 is the initial dissolution rate, [H4SiO4] is the activity of orthosilicic acid at the glass surface, and K is the equilibrium constant (which equals the
activity of orthosilicic acid at saturation).
We have been accumulating the data on the glass dissolution rate as a function of solution concentration of Si for ISG and P0798, and some of the interesting results will be. |
| 2. |
Yaohiro. Inagaki, R. Kato, R.Egami, K. Idemitsu, T. Arima, Dissolution rate of simulated nuclear waste glass ISG as a function of solution concentration of Si and pH, GOMD2022, American Ceramic Society, 2022.05, Dissolution rate of simulated nuclear waste glass ISG was measured at 70 °C by using Micro-Channel Flow-Through test method as a function of solution concentration of Si at constant pH of 4, 7 and 9, respectively. Si-29 isotope was used for preparing the reaction solution with the Si concentration from 0 to 120ppm in order to distinguish between Si dissolved from glass and Si dissolved in the original solution, and the glass dissolution rate was determined by measuring the dissolution rate of Si-28, major Si isotope (92.2%) of natural Si being consisted of ISG.
The test results showed that the glass dissolution rate decreases drastically with an increase of solution concentration of Si at both pH7 and pH9, where the dependency of solution concentration of Si cannot be accounted by the ordinal first-order dissolution rate law based on SiO2dissolution. At pH4, however, the tendency of glass dissolution rate was different from that at pH7 and pH9, where the solution concentration of Si had no remarkable effects on the glass dissolution rate. These results suggested the glass dissolution kinetics change depending on solution pH sensitively.. |
| 3. |
Evaluation of HLW glass dissolution as a function of dissolved silica concentration under various pH conditions. |
| 4. |
Yaohiro Inagaki, How to Integrate Various Research Fields for Reasonable and Reliable Radwaste Management, 6th East Asia Forum on Radwaste Management Conference, 2017.11. |
| 5. |
D.Akiyama, N.Sato, A.Kirishima, Y.Inagaki, T.Arima, Properties of Glass formed by Vitrification of Radioactive Cs sorbed Zeolite and its Cs Evaporation Behavior, 12th Pacific Rim Conference on Ceramic and Glass Technologu including Glass & Optical Materials Division Meeting, 2017.05. |
| 6. |
Y.Inagaki,T.Arima, K.Idemitsu, D.Akiyama, N.Sato, A.Kirishima, Optimum Conditions for Vitrification of Cs-Sorbed Zeolite Waste Generated from Decontamination of Effluents at Fukushima Dai-ichi NPP, 12th Pacific Rim Conference on Ceramic and Glass Technologu including Glass & Optical Materials Division Meeting, 2017.05. |
| 7. |
稲垣 八穂広, 松本 遼, 出光 一哉, 有馬 立身, Dissolution kinetics of a simplified nuclear waste glass, ISG, in MgCl2 solution, Goldschmidt 2016, 2016.06, ISG (International Simple Glass) is a six component alumina-borosilicate glass developed as reference bench-mark glass for six nations collaborating study on high-level nuclear waste glass dissolution kinetics [1]. In order to evaluate effects of environmental Mg on the dissolution kinetics of ISG, aqueous dissolution tests were performed at 90°C with MgCl2 solution at a constant pH (8.2) as a
function of MgCl2 concentration by using Micro-Channel Flow-Through (MCFT) method [1]. The dissolution rate was measured for each glass constituent element as a function of reaction time,
and the reacted glass surface was analyzed by use of SEM/EDX and XRD.. |
| 8. |
Yaohiro Inagaki, Taiki Kikunaga, Kazuya Idemitsu, tatsumi arima, Aqueous corrosion rate of ISG under silica-saturation condition measured by using micro-channel flow-through test method, American Ceramic Society GOMD2014, 2014.05, For assessing long-term performance of high-level nuclear waste (HLW) glass in geological disposal, kinetics of the aqueous corrosion under near silica-saturation conditions is one of fundamental subjects to be evaluated. In the present study, aqueous corrosion tests were performed for International Simple Glass (ISG) by using Micro -Channel Flow-Through (MCFT) method to evaluate the corrosion rate under silica -saturation condition. The ISG is a six-component alumino-borosilicate glass developed as a reference benchmark glass for six nations collaborating study on HLW glass dissolution/corrosion mechanism. The corrosion tests were conducted at the temperature range of 25-70 °C with the solution saturated with respect to amorphous silica at pH9. The test results showed that the corrosion rate based on boron release under the silica-saturation condition is slower than that under silica-free condition or the initial dissolution rate by a factor of only up to 10. The presentation will focus on the corrosion mechanism of the ISG in comparison with other simulated HLW glasses.. |
| 9. |
Yaohiro Inagaki, Micro-Channel as a New Tool to Investigate Glass Dissolution Kinetics, 2nd International Summer School on Nuclear Glass Wasteform, 2013.09, In this lecture, several test methods currently applied to measurement of the glass dissolution kinetics are introduced with their advantages and disadvantages, and problems to be solved for a greater understanding of the glass dissolution kinetics are discussed.. |
| 10. |
Yaohiro Inagaki, Yuki Yamamura, Kazuya Idemitsu, tatsumi arima, Seiichiro Mitsui, Kenji Noshita, Dissolution rate of P0798 simulated HLW glass in MgCl2 solution measured by using micro-channel flow-through test method as a function of pH, American Ceramic Society GOMD2013, 2013.06, Magnesium is one of major elements dissolved in groundwaters, and is expected to affect dissolution behavior of HLW glass in geological disposal. Some of previous studies have reported that the presence of magnesium in solutions can accelerate the glass dissolution by precipitation of magnesium-silicates consuming dissolved silica in solutions. While some studies have reported that the presence of magnesium can depress the glass dissolution under certain conditions. Comparison of these previous studies suggests that the effects of magnesium on the glass dissolution rate can change sensitively depending on the dissolution conditions such as pH, temperature, solution composition, S/V and time, and we need precise and consistent experimental data measured systematically under various well-constrained conditions for greater understandings of kinetics of the glass dissolution in the presence of magnesium. In the present study, therefore, aqueous dissolution/alteration tests were performed for P0798 simulated HLW glass in MgCl2 solution by using “Micro-channel flow-through test method” to measure the dissolution rate precisely as a function of pH. On the basis of the test results including solid phase analyses of the reacted glass surface, this presentation discusses the kinetics of the glass dissolution in the presence of magnesium.. |
| 11. |
Evaluation of residual dissolution rate of HLW glass by using micro-channel flow-through test. |
| 12. |
A new type of flow-through test method using micro-reactor was developed and applied to measurement of dissolution kinetics for a Japanese simulated HLW glass, P0798. In this method, a coupon shaped glass is placed on a Teflon plate with a micro-channel (20mm x 2mm x 0.16mm), and a solution is injected into the inlet of the channel at a constant flow rate. The outlet solution, which reacted with glass through the channel, is retrieved to be analyzed for determination of the dissolution rate. By using this method we measured the initial dissolution rate as a function of pH and temperature. The results showed that the initial dissolution rate has “V-shaped” pH dependence at any temp. from 25 to 90°C similar to that for R7T7 glass evaluated by CEA, France. However, a certain difference was observed between them in the temp. dependence at high pHs. Based on the results and comparison, we discussed the dissolution kinetics.. |
| 13. |
Sorption of silica on corrosion products of iron under reducing condition. |
| 14. |
A new type of flow-through test method using micro-reactor was developed and applied to measurement of the dissolution/alteration kinetics for a Japanese type of simulated HLW glass, P0798.. |
| 15. |
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).. |
| 16. |
Dissolution rate of HLW glass as a function of pH measured by micro-rector flow-through test. |
| 17. |
Several kinetic models have been proposed to predict the aqueous dissolution/ alteration rate of nuclear waste glass for long-term. However, reaction processes controlling the long-term rate are much more subjected to controvert. Temperature dependence of the alteration rate is one of essential issues to be evaluated for an understanding of the rate controlling processes. In the present study, the static alteration tests were performed with a Japanese simulated waste glass P0798 as a function of temperature from 60。C to120。C, and the temperature dependence of the alteration rate was evaluated to understand the rate controlling processes. The tests were performed in 0.001M NaOH solution to maintain a constant solution pH of around 10 during the test period and to provide smectite forming conditions where smectite forms as the major secondary phase without zeolite formation. From the test results on dissolution of boron, the alteration rate at each temperature was analyzed by use of a water-diffusion model. The water-diffusion model used is based on a simple assumption; the glass alteration proceeds being controlled by water diffusion with ion-exchange between water (hydronium ion: H3O+) and soluble elements(B, Na, Li, etc) at the glass surface layer with the apparent diffusion coefficient of Di. A good agreement was observed between the model analysis and the test results, and the value of Di was evaluated to be 1.2 x 10-22 m2/s at 60。C to 1.8 x 10-21 m2/s at 120。C. The Arrhenius plot of Di showed a good linearity to give the activation energy of 49 kJ/mol, which value is much lower than that for the initial dissolution rate of French waste glass(71 kJ/mol) by Delage[1] and Techer[2], but is very close to that for ion-exchange in sodium aluminosilicate glass by McGrail[3]. These results suggest that water diffusion with ion-exchange can be the dominant process controlling the alteration rate under smectite forming conditions. At elevated temperatures(100。C and 120。C), however, the model analysis deviated from the test results, which suggests that the precipitated layer developing at the glass surface can be protective against the diffusion to depress the alteration rate.. |
