The Scientific Basis for Nuclear Waste Management symposium

1. Behaviour of sand-bentonite buffer material of deep geological repository under high confining pressure Janaka J. KUMARA and Takeshi KODAKA Department of Civil Engineering, Meijo University, Nagoya, Japan. The Scientific Basis for Nuclear Waste Management symposium Sydney, Australia (29 Oct – 3 Nov 2017)

2. Overview • Introduction • Objectives • Methodology • Results and Discussion • Conclusions

3. Introduction (1/4) • Radioactive wastes are classified into 6 categoriesby the IAEA. • High-level radioactive wastes are stored indeep geological repositories. • Leakage of nuclear wastes into the surrounding environment could be very serious,therefore, they should be prevented by a sealing material. • Sand-bentonite is widely used as buffer material for nuclear waste repositories. • Deep geological repositories are constructed in deep ground. Thus, the buffer material is subjected to a high confining pressure. • Unsaturated buffer material subjects to local groundwater flow during the operation, hence, becomes (partially) saturated. • Suction properties of buffer material is very important to prevent any leaks from nuclear wastes. • Thus, the behaviourof buffer material could change with time.

4. Introduction (2/4) Radioactive waste classification

5. Introduction (3/4) Deep geological repository

6. Introduction (4/4) A deep geological repository before and after decommission Before decommission After decommission

7. Objectives • To study strength properties of sand-bentonite buffer material under high confining pressure and various degree of saturation. • To study volumetric behaviour of sand-bentonite buffer material under high confining pressure and various degree of saturation. • To study how strength and volumetric behaviour are affected by suction properties.

8. Methodology (1/3) • Undrained and unconsolidated test condition. • 0.5 and 2.0 MPaof confining pressures. • Volume change measured by a burette attached to a double-cell type triaxial apparatus. 1. Triaxial compression tests Burette

9. Methodology (2/3) Notes • 30 sand & 70% bentonite mixture. • 1600 kg/m3dry density. • 15 (IC), 30, 50, 70 and 90% of degree of saturation. 1.1 Sample preparation Capacity: 100 kN, Stroke: 120 mm, Pressure: 68.9 MPa Mixing bentonite and sand Adding water Trimming the ends to avoid inhomogeneity A prepared specimen (35 x 70mm) Filling materials into the mold (3 layers) Compact specimen hydraulically Precautions • Bentonite absorbs water quickly. So, water should be spread uniformly and mix sand and bentonite quickly.

10. Methodology (3/3) 2. Suction measurements • Suction-only specimens and tested specimens from triaxial compression test. • Total suction is measured. Matric suction + Osmotic suction Cup Specimen height ≤ 1/2 of cup height • After equilibrium state between the chamber and specimen reaches, suction is measured. • Equilibrium time varies based on moisture content of the sample (e.g., 15 ~ 50 min). WP4C uses the chilled-mirror hygrometer technique

11. Results and Discussion (1/8) Stress-strain behaviour • Specimens exhibit strain-hardening behavior when degree of saturation exceeds 50%. • Stressreduces largely when degree of saturation exceeds 70%. Under a small confining pressure (0.5MPa)

12. Results and Discussion (2/8) Stress-strain behaviour cont. • All specimens exhibit strain-hardening behaviour. • It also indicates stresses reduce heavily when degree of saturation exceeds 70%. • Thus, under less degree of saturation, stress-strain behavior is changed from strain-softening into strain-hardening by confining pressure. Under a high confining pressure (2.0MPa)

13. Results and Discussion (3/8) Volumetric strain behaviour • The specimens of 50% or more of Srproduce continuous volumetric compression. • The specimen of less water content (Sr of less than 30%) producevolumetric expansion in the post-failure. • The specimens exhibiting volumetric contraction reduces the magnitude of volumetric strain with degree of saturation. Under a small confining pressure (0.5MPa)

14. Results and Discussion (4/8) Volumetric strain behaviour cont. • All specimens exhibit continuous volumetric compression. • The magnitude of volumetric strainreduces with degree of saturation. • The magnitude of volumetric compression reduces with confining pressure when water content is high (i.e., Sr ≥ 70%). Under a high confining pressure (2.0 MPa)

15. Results and Discussion (5/8) Strength properties with the degree of saturation • On average, both cohesion and friction angle decrease with the degree of saturation. • Near the quasi-saturation state, a large reduction in cu and fu are seen. • Also, near its full saturation, sand-bentonite material becomes a frictionless material.

16. Results and Discussion (6/8) Compressive strength with the degree of saturation • The influence of confining pressure on compressive strengthreduces with the degree of saturation.

17. Results and Discussion (7/8) Variation of total suction with the degree of saturation After shearing is finished. • The confining pressure applied in shearing does not affect the total suction.

18. Results and Discussion (8/8) Strength properties with the total suction • Total suction increases both cohesion and frictional properties. • When total suction approaches around 1 MPa, the sand-bentonite material becomes frictionless material.

19. Conclusions • Stress-strain behavior is affected by the degree of saturation, and confining pressure, particularly strain-softening behaviourchanges into strain-hardening by confining pressure when degree of saturation is small. • Volumetric expansion is dominant in less saturated specimen (i.e., Sr ≤ 30%) under a small confining pressure (i.e., 0.5 MPa). Under a high confining pressure (i.e., 2.0 MPa), all the specimens yield volumetric compression. • The magnitude of volumetric strain reduces with the degree of saturation. • Compressive strength reduces with degree of saturation, and increases with confining pressure, but less effects of confining pressure under high degree of saturation. • When total suction approaches 1.0 MPa, sand-bentonite material behaves as a frictionless material.

20. Thank You Very Much for Your Attention ! Acknowledgement The research funding comes from the JSPS Postdoctoral Fellowship (P17074), which is greatly appreciated. Assoc. Prof. Ying Cui of Yokohama National University, Japan and Mr. K. Takeuchi, a former graduate student of Meijo University are also acknowledged for their supports during laboratory experiments. Meijo University, Department of Civil Engineering, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, 468-8502 JAPAN

21. Additional data Particle size distribution of silica sand

22. Additional data The preparation of the pre-defined degree of saturation Water content Particle density Degree of saturation Void ratio Dry density

23. Additional data Calculation of the volumetric strain Volume change of specimen Initial volume of specimen Volumetric strain Volume change in burette Water volume in the inner cell replaced by loading rod

24. Additional data Calculation of Total suction Universal gas constant (i.e., 8.314 J/mol.K) Absolute temperature in Kelvin Unit weight of water in kg/m3 RT is relative humidity Molecular weight of water (i.e., 18.016 kg/kmol)

25. Additional data Stress-strain behavior 0.1 MPa of confining pressure

26. Additional data Volumetric strain behavior 0.1 MPa of confining pressure

27. Additional data Definition of compressive strength, qc Peak stress Deviator stress Deviator stress Maximum stress Axial strain Axial strain Strain-softening behaviour Strain-hardening behaviour

28. Additional data Mohr stress circle 15% (IC) of Sr 90% of Sr

29. Additional data Comparisons of Mohr stress circles Sr = 30% Sr = 50% Sr = 15% Sr = 70% Sr = 90%