1. Cement waste matrix evaluation and modelling of the long-term stability of cementitious waste matrices

2. Thermodynamic modelling 2 Lisa Almkvist and Börje Torstenfelt Swedish Nuclear Fuel and Waste Management Co (SKB) Peter Cronstrand Vattenfall Power Consultant (VPC) Thermodynamic modelling

3. Thermodynamic modelling 3 Cement waste matrix development (presented at the RCM in Moskow) Solidification of operational intermediate-level waste –Ion exchange resins –Evaporator concentrates Laboratory test programs studying: –Waste load –Water-to-cement ratio –Type of cement –Additives (liquid, solid) –Type of storage (dry, wet (deionised, salt or chalk water))

4. Thermodynamic modelling 4 A 1D cell-divided representation of a nuclear repository The state of the engineered barriers is simulated through multi-component diffusive transport followed by thermo-dynamical calculations of relevant mineral species for each transport step. Integrity of the repository concrete structure; short-term and long-term (presented at the RCM in Moskow) Computational model Cement encapsu-lated waste Cl - SO 4 2- CO 3 2- Na + bentonite ShotcreteConcrete wall Ambient water with time-dependent ionic composition

5. Thermodynamic modelling 5 Reactive-transport modelling - uncertainties Transport: Diffusion, (Ficks law or MCD) Advection Thermodynamic database Rates Representation of CSH, (variable log k vs. solid solution) Composition of the cement paste Composition of infiltrating water Reaction: Thermodynamic equilibrium Kinetics Transport Reaction Uncertainties Flows Diffusivities Porosity evolution Porosity-diffusivity-relation

6. Thermodynamic modelling 6 Uncertainty assessment Uncertainties can initially be characterized in simplified leaching models before entering the full scale degradation scenario. + Easy to visualize the result + Easy to identify and isolate the influence of a specific input parameter + Easy to compare with experiments Degradation indicators: Dissolution of Ca(OH)2 Decalcification of CSH

7. Thermodynamic modelling 7 Leaching models Reactive properties Thermodynamic database Log k vs. solid solution Water composition Performed on crushed cement Transport properties Porosity-diffusivity relations Flows Diffusivities Porosity evolution Performed on solid samples

8. Thermodynamic modelling 8 Database

9. Thermodynamic modelling 9 Log k vs. Solid solution

10. Thermodynamic modelling 10 Water intrusion

11. Thermodynamic modelling 11 Porosity-diffusion relation Archies law:

12. Thermodynamic modelling 12 Recipe for a conservative - yet realistic- estimate Databases yielding high dissolution rates (PCHatches or Thermoddem) Log k approach can reproduce the result obtained by a solid solution approach. Water: Low calcium, high NaCl. Carbonates have a non-trivial and twofold effect; enhance dissolution rates, but the precipitation of calcite reduces the porosity. Porosity-diffusion relation: Case-dependent (although a conservative estimate can always be achieved by choosing a sufficiently high diffusivity)

13. Thermodynamic modelling 13 Full scale scenario - the Silo at SFR: initial state

14. Thermodynamic modelling 14 Full scale scenario - the Silo at SFR: 100 000 years

15. Thermodynamic modelling 15 Full scale scenario - monitoring some parameters NotationDiffusivityWater composition Temperature AFixed B Varying CFixedVaryingFixed DVaryingFixed EVarying

16. Thermodynamic modelling 16 Full scale scenario - porosity distribution

17. Thermodynamic modelling 17 Full scale scenario - pH distribution

18. Thermodynamic modelling 18 Some remaining uncertainties - clogging Introduces both a mesh-and time-step-dependency Is there a residual diffusivity (through the gel-pores) even in perfectly clogged material? Can remaining non-hydrated clinker materials lead to fractures? Are there types of waste that can accelerate the degradation process?

19. Thermodynamic modelling 19 Some remaining uncertainties - fractures The effect within the fracture Pure advectionAdvection and diffusion to adjacent pores Dual porosity approach Only evaluates degradation in the fracture itself The effect on the overall sample.