Evaluation of long-term durability of engineered barrier system (EBS) of bentonite and cementitious materials by migration technique K. Nakarai* , M. Watanabe*, T. Sugiyama**, Y. Tsuji* * Gunma University, Japan ** Hokkaido University, Japan
Evaluation of long-term durability of engineered barrier system (EBS) of bentonite and cementitious materials by migration technique K. Nakarai* , M. Watanabe*, T. Sugiyama**, Y. Tsuji* * Gunma University, Japan ** Hokkaido University, Japan
Contents of this presentation 1. Introduction Introduction 1. Concept of EBS for LLW in Japan Degradation of EBS at cement/bentonite interaction 2. Test method Test method 2. Acceleration test by electrical migration technique 3. Experimental procedures and results Experimental procedures and results 3. 1. Investigation of effect of dry density on degradation of EBS at cement/bentonite interaction 2. Investigation of effect mixed NaHCO 3 on degradation of EBS at cement/bentonite interaction 4. Conclusions Conclusions 4.
Contents of this presentation 1. Introduction Introduction 1. Concept of EBS for LLW in Japan Degradation of EBS at cement/bentonite interaction 2. Test method Test method 2. Acceleration test by electrical migration technique 3. Experimental procedures and results Experimental procedures and results 3. 1. Investigation of effect of dry density on degradation of EBS at cement/bentonite interaction 2. Investigation of effect mixed NaHCO 3 on degradation of EBS at cement/bentonite interaction 4. Conclusions Conclusions 4.
EBS for LLW in Japan Underground cavern type disposal facility to isolate low-level radioactive waste (http://www.enecho.meti.go.jp/rw/gaiyo/gaiyo03-3.html) Low conductivity buffer [Bentonite material] Low diffusion layer [Cementitious material] 50~100m under the ground Repository concept of LLW in Japan Extremely long-term stability for several tens thousands years is required.
Durability problem of EBS for LLW in Japan To evaluate extremely long-term stability during tens thousands of years Low diffusion layer Low conductivity buffer [Cementitious material] [Bentonite material] Ca 2+ Ca 2+ Ca 2+ Ca 2+ water Ca 2+ Ca 2+ Repository concept Increase in Ca leaching of LLW in Japan Ca & pH Degradation Degradation Increase in porosity Decrease in swelling capacity >>>Increase in diffusivity >>>Increase in conductivity Na Ca -type -type
Two experimental plans in this study Problem of EBS To evaluate stability during tens thousands of years Degradation Low diffusion layer Leaching Ex) Increase [Cementitious material] in diffusivity Low conductivity buffer Degradation Increase in Ex) Increase in [Bentonite material] Ca & pH conductivity Repository concept of LLW in Japan Approach I Reduce negative cement/bentonite interaction Increase in dry density of bentonite Exp I Approach II Control cement/bentonite interaction for increasing stability Mixing NaHCO 3 for creating additional layer of CaCO 3 Exp II
Next topic 1. Introduction Introduction 1. Concept of EBS for LLW in Japan Degradation of EBS due to cement/bentonite interaction 2. Test method Test method 2. Acceleration test by electrical migration technique 3. Experimental procedures and results Experimental procedures and results 3. 1. Investigation of effect of dry density on degradation of EBS due to cement/bentonite interaction 2. Investigation of effect mixed NaHCO 3 on degradation of EBS due to cement/bentonite interaction 4. Conclusions Conclusions 4.
Acceleration test by electrical migration technique To accelerate ion transport by applying electric potential gradient Several studies have been reported. (e.g. Saito et al. 1997) Direct current voltage (15V) Direct current voltage (15V) Calcium solution Calcium solution Saturated Saturated Ca 2+ Sand Mixture Sand Mixture Bentonite Bentonite Concrete Concrete Steel mesh Steel mesh Steel mesh Steel mesh ＋ － Ca 2+ Cement Bentonite Ca 2+ Anode Cathode Ca 2+ Ca 2+ 25 25 10 10 Side view 65 65 Ca leaching Degradation
Next topic 1. Introduction Introduction 1. Concept of EBS for LLW in Japan Degradation of EBS due to cement/bentonite interaction 2. Test method Test method 2. Acceleration test by electrical migration technique 3. Experimental procedures and results Experimental procedures and results 3. 1. Investigation of effect of dry density on degradation of EBS due to cement/bentonite interaction 2. Investigation of effect mixed NaHCO 3 on degradation of EBS due to cement/bentonite interaction 4. Conclusions Conclusions 4.
Exp I: Influence of dry density (Specimen) [Cementitious material] Concrete Calcium solution Calcium solution Saturated Saturated Sand Mixture Sand Mixture Bentonite Bentonite Cement: OPC Concrete Concrete Steel mesh Steel mesh Steel mesh Steel mesh W/C = 55% [Bentonite material] Bentonite sand mixture Bentonite: Kunigel V1 Bentonite:Sand = 7:3 25 25 10 10 65 65 4 specimens of bentonite sand mixture Name Dry Density Water Content Compaction times of specimen g/cm 3 % / Layer Bt16 1.6 28.6 11 Bt17 1.7 26.1 19 Bt18 1.8 22.6 38 Bt19 1.9 15.0 150, 200
Exp I: Measurement after electrical migration test Dry density, g/cm 3 No. Water content, % Calcium solution Calcium solution Saturated Saturated Sand Mixture Sand Mixture Bentonite Bentonite Bt16 1.6 28.6 Concrete Concrete Steel mesh Steel mesh Steel mesh Steel mesh Bt17 1.7 26.1 Bt18 1.8 22.6 Bt19 1.9 15.0 25 25 10 10 65 65 Cathode Anode After 13.0kC accumulated electrical charge Swelling capacity Bentonite TGA Concrete Cation concentration sand mixture Ca(OH) 2 , EPMA (Bt19)
Exp I: Measurement after electrical migration test Dry density, g/cm 3 No. Water content, % Calcium solution Calcium solution Saturated Saturated Sand Mixture Sand Mixture Bentonite Bentonite Bt16 1.6 28.6 Concrete Concrete Steel mesh Steel mesh Steel mesh Steel mesh Bt17 1.7 26.1 Bt18 1.8 22.6 Bt19 1.9 15.0 25 25 10 10 65 65 10 5 5 5 5 10 10 ・・・ (mm) Cathode Anode 陰極 After 13.0kC accumulated 1 2 3 4 5 6 7 Center Bt-side (Layer) electrical charge Swelling capacity Bentonite TGA Concrete Cation concentration sand mixture Ca(OH) 2 , EPMA (Bt19)
Result of TGA: Degradation of concrete Ca(OH) 2 : leach from concrete first Measurement of residual Ca(OH) 2 Bt17 Bt18 Bt19 Residual Ca(OH) 2 (%) Residual Ca(OH) 2 (%) Residual Ca(OH) 2 (%) 25 25 25 20 20 20 75% 75% Percentage of Percentage of Percentage of 15 15 15 10 10 10 5 5 5 0 0 0 Center Bt-side Center Bt-side Center Bt-side 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 Bt17 Bt18 Bt19 Center > Bt-side Percentage of residual Ca(OH) 2 : Decrease in Bt-side was not observed Calcium leached from Bt-side of concrete to bentonite Reduce degradation
Result of EPMA: Cations in bentonite (Bt19) surface analysis and profile high high high Ca Na Cathode Cathode Cathode Cathode Concrete Concrete Concrete Concrete Concrete Concrete Concrete Concrete Cathode Cathode Cathode Cathode Na Ca low low low Profile of Sodium Profile of Calcium Sodium Ion: Calcium Ion: Migrated to cathode Migrated from concrete Alteration of bentonite progress from the interface
Result of cations and swelling capacity of bentonite Bt16 Bt17 Bt18 Bt19 Cation ratio (Ca/Na): Ion ratio of calcium to sodium ions 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 10 10 10 10 8 8 8 8 Ca/Na Ca/Na 6 Ca/Na Ca/Na 6 6 6 4 4 4 4 2 2 2 2 0 0 0 0 Swelling capacity ratio: Swelling capacity divided by mean value of swelling 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 capacities of 6th and 7th layers Number of Layer Number of Layer Number of Layer Number of layer 1.2 1.2 1.2 1.2 Swelling capacity ratio Swelling Capacity ratio Swelling Capacity ratio Swelling Capacity ratio 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Number of Layer Number of Layer Number of Layer Number of layer
Result of cations and swelling capacity of bentonite Bt16 Bt17 Bt18 Bt19 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 10 10 10 10 8 8 8 8 Ca/Na Ca/Na 6 Ca/Na Ca/Na 6 6 6 4 4 4 4 2 2 2 2 0 0 0 0 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Number of Layer Number of Layer Number of Layer Number of layer 1.2 1.2 1.2 1.2 Swelling capacity ratio Swelling Capacity ratio Swelling Capacity ratio Swelling Capacity ratio 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Number of Layer Number of Layer Number of Layer Number of layer Concrete side: Swelling capacities decrease with increase in Ca/Na Change to Ca-type
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