An overview on radioactive waste repository - role of cementitious materials

Transcription

1 An overview on radioactive waste repository - role of cementitious materials Tổng quan về kết cấu chôn rác thải phóng xạ - vai trò của bê tông xi măng Danang, 22 February 2016 Quoc Tri Phung Faculty of Road & Bridge Engineering, Danang University of Technology, Vietnam EHS Institute, Belgian Nuclear Research Centre (), Belgium qphung@sckcen.be

2 Contents Waste disposal in Belgium Concept for geological waste disposal Role of concrete engineered barriers : carbonation and leaching 2/47

3 Long-term Types of waste Multi-barrier concept Safety management Waste disposal in Belgium SCK-CEN Heat! 3/47

4 Long-term Types of waste Multi-barrier concept Safety management Low level and cemented waste (A+B) Bitumenised waste (B) Vitrified high level waste (C) Spent fuel (C) No industrially mature solutions (A+B) Waste disposal in Belgium SCK-CEN 4/47

5 Long-term Types of waste Multi-barrier concept Safety management Waste disposal in Belgium Volume of radioactive waste based on 40 years of exploitation Cat. B Cat. C With reprocessing Cat. A = m³ Cat. B = m³ Cat. C = 600 m³ SCK-CEN Without reprocessing Cat. A = m³ Cat. B = m³ Cat. A Cat. C = m³ estimation /47

6 Waste disposal in Belgium Type of waste Long-term management Multi-barrier concept Safety SCK-CEN What after interim storage? Radiological risks over long time periods: Cat. A short lived => several 100 years Cat. B & C long lived / high level => several years Challenge: to find a stable and safe management system that protects man and environment during these time periods 6/47

7 Type of waste Long-term management Multi-barrier concept Waste disposal in Belgium Safety SCK-CEN Category A waste: surface disposal in Dessel 7/47

8 Waste disposal in Belgium Type of waste Long-term management Multi-barrier concept Safety SCK-CEN Category B & C waste: research on geological disposal internationally recommended solution potential rock types: crystalline rocks, salt or clay examples: Finland (granite); Sweden (granite); France (clay); Switzerland (clay); Germany (salt and clay); Belgium (clay) 8/47

9 Waste disposal in Belgium Type of waste Long-term management Multi-barrier concept Safety SCK-CEN Example: Geological disposal Waste forms Engineered barrier (near field) Natural barrier (far field, most important) Natural barrier: deep clay layer isolation from biosphere and potential disturbance retardation of radionuclide migration HOW can clay do this? almost impermeable for water plastic behaviour retardation of radionuclides by minerals 9/47

10 Type of waste Long-term management Multi-barrier concept Waste disposal in Belgium Safety SCK-CEN 3 safety functions to protect man & environment through passive safety Contain (Engineered Barriers) Prevent contaminants dispersion 1000 to many 1000 s a Isolate (Geology, Natural Barrier) Ensure stable conditions Reduce likelihood of inadvertent human intrusion up to 1 Ma Retard (Engineerd Barriers + Natural Barrier) Limit contaminants release from waste Limit water flow Retard contaminant migration up to 1 Ma 10/47

11 Type of waste Long-term management Multi-barrier concept Waste disposal in Belgium Safety SCK-CEN EURIDICE R&D Waste Packages R&D Disposal Performance assessment Nuclear Science & Technology Biosphere impact studies Crisis Management & Decision support Microbiology Decommissioning & Decontamination Low level radioactivity measurements Radiation protection dosimetry & calibration Radiobiology 11/47

12 Type of waste Long-term management Multi-barrier concept Waste disposal in Belgium Safety SCK-CEN Ethics & Social Aspects Geology Hydrogeology Safeguards & Monitoring Material Science Performance Assessment Radiochemistry Biosphere impact Geomechanics Geomicrobiology Geochemistry 12/47

13 Type of waste Long-term management Multi-barrier concept Waste disposal in Belgium Safety SCK-CEN Underground infrastructure HADES, operated by EIG EURIDICE Major expertise: Excavation and construction techniques Thermo-Hydro-Mechanical modelling Instrumentation Praclay experiment: long-term heater experiment 13/47

14 Role of concrete Concrete in radioactive waste disposal: Main material in engineered barriers Concrete present for: Encapsulation, backfill, Construction Low level waste Role of concrete: Limiting water flow through disposal facility, High and long-lasting sorption of radionuclides 14/47

15 Role of concrete Concrete in radioactive waste disposal: Main material in engineered barriers High level waste (Supercontainer concept) Concrete used for: Gallery lining, buffer End-plug, backfill HLW repository section LILW-LL repository section Role of concrete: Shielding, geochemical conditions Sorption of radionuclides 15/47

16 Role of concrete Challenges for engineered barrier concrete durability assessment: Long-term performance Coupled effects Limited experimental timeframe available to capture these processes Multiple processes [NIROND-TR E] 100s 1000s 100,000s Lifespan (years) 16/47

17 Role of concrete Challenges for engineered barrier concrete durability assessment: Long-term performance Coupled effects Limited experimental timeframe available to capture these processes Under service conditions, the disposal facilities undergo chemical degradation processes (carbonation, leaching) very slow but important for the long-term durability assessment. Sound zone Leached zone Alteration of chemical properties changes in microstructure, mechanical performance and transport properties (permeability, diffusion) 17/47

18 Role of concrete s 1000s 100,000s years Sulfate attack EFt Chloride attack ASR Leaching Carbonation 90 years Leached zone Sound zone Carbonation and Ca-leaching have been identified as the most important degradation mechanisms in waste disposal facilities. Low High Chemical degradation 18/47

19 Leaching Carbonation Modelling Results Ca-leaching: process of Ca extraction from solid phases due to disequilibrium of cement matrix Sound zone Leached zone Detrimental effects: Change in cement mineralogy: ph decrease rebar corrosion Reduction of mechanical properties Increased porosity, connectivity more permeable for aggressive species ingress Extremely slow process 19/47

20 Leaching Carbonation Modelling Results Near surface Dessel, Belgium Relevant for some important applications: Extreme long lifetime: nuclear waste disposal system Hydro structures: dam, bridge, water tank Structures in aggressive environments: acid, flowing water, water containing aggressive agents (chloride, sulfate) combined chemical attack Degradation of concrete bridge pylon in the Pimpama River, Queensland, caused by combination of leaching and sulfuric acid attacks 20/47 state I state II state III state IV

21 Leaching Carbonation Modelling Results Accelerated Ca-leaching using NH 4 NO 3 Ammonium nitrate solution 6M: Aggressive solution reacts with most of main phases in cement paste (CH, C-S-H, Afm, AFt) to form Calcium nitrate Calcium nitrate soluble salt increase Ca ion concentration 21/47

22 Leaching Carbonation Modelling Results New carbonation method under controlled CO 2 pressure, coupled diffusion and advection Valve 4 Moisture trap Valve 5 Atmosphere Quick coupling Outlet Outlet mass flowmeters Specimen Inlet Valve 3 O ring Bypass line Pressure controller CO2 source Valve 1 Valve 2 Inlet mass flowmeters 22/47

23 Leaching Carbonation Modelling Results 1. Modelling carbonation under controlled CO 2 pressure One dimensional model for accelerated condition Based on macroscopic mass balance for CO 2 in gaseous and aqueous phases Transport mechanisms of CO 2 : diffusion and advection (due to pressure gradient) Account for the evolution of RH Outputs: Carbonation degree, porosity change, ph, CO 2 uptake, CO 2 profile, portlandite content, changes in transport properties 23/47

24 Leaching Carbonation Modelling Results 2. Modelling leaching in aggressive NH 4 NO 3 solution One dimensional model for accelerated condition Based on macroscopic mass balance for Ca in solid and aqueous phases Transport mechanisms: diffusion of both nitrate and Ca Outputs: leaching depth, porosity change, leached Ca amount, Ca profile, portlandite content, changes in transport properties Calcium Nitrate u Ca u us De t x x u t D u x 1 NO3 1 e 0 Ca concentration in solid phases f e d C Solubility increase B A Ca from CH Ca from C-S-H D a Ca ion concentration b c 24/47

25 Leaching Carbonation Modelling Results Role of limestone fillers in leaching: Limestone filler replacement slightly affects the leaching rate of cement pastes. Effects of w/p ratio and limestone filler replacement on leaching rate (expressed as depth/day 0.5 ) 25/47

26 Leaching Carbonation Modelling Results Role of limestone fillers in carbonation: Limestone filler replacement results in different carbonation mechanisms. Carbonation products are precipitated around limestone Portlandite is covered by carbonation products C-S-H Portlandite Limestone Pore solution CaCO3 CO Ca 2- CO3 2- CO3 2- CO3 2- CO3 CO CO3 2- CO3 2- CO3 2- CO3 2- CO3 Ca CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 2- CO3 (a) Sample with limestone filler (b) Sample without limestone filler 26/47

27 Leaching Carbonation Modelling Results Role of limestone fillers in carbonation: Limestone filler replacement results in different carbonation mechanisms. Without limestone filler With limestone filler 27/47

28 Leaching Carbonation Modelling Results Contribution of C-S-H to degradation processes: C-S-H is significantly dissolved under accelerated conditions, especially during leaching (portlandite is the main degraded phase). % contribution to formed CaCO CH contribution C S H contribution S3C: 0 3 mm S3C: 3 6 mm S4C: 0 3 mm S4C: 3 6 mm Without limestone filler With limestone filler Atomic Ca/Si ratio Phenolphthalein Measured Moving average C-S-H leaching Depth, mm Carbonation Ca-leaching 28/47

29 Leaching Carbonation Modelling Results Contribution of C-S-H to degradation processes: C-S-H carbonation results in opening gel pores. Monolayer water Interlayer water Calcium silicate sheets opening gel pore - N2 accessible Small gel pore - N2 inacessible CaCO3 CaCO3 Large gel pore - N2 accessible After carbonation Large gel pore - N2 accessible 29/47

30 Leaching Carbonation Modelling Results Carbonation and leaching significantly change the microstructure of cementitious materials but in opposite ways. Finer microstructure for carbonated materials Coarser microstructure for leached materials C-S-H degradation significantly contributes to both processes Microstructural parameters Porosity Pore size Leaching Carbonation Porosity, % Carbonation Leaching Ref. Leached Carbonated S. Surface area Pore diameter, µm 30/47

31 Leaching Carbonation Modelling Results Carbonation and leaching significantly change the microstructure of cementitious materials but in opposite ways. Sound Leached Leaching increases the pore sizes, porosity and Portlandite percolation. 31/47

32 Leaching Carbonation Modelling Results Carbonation and leaching significantly change the microstructure of cementitious materials but in opposite ways. Sound Carbonated Carbonation slightly decreases the porosity, portlandite is carbonated but still observable after carbonation. 32/47

33 Leaching Carbonation Modelling Results Changes in microstructure are significantly linked to mineralogical changes. 1. Leaching SEM BE image of transient zone of leached sample (topleft); Ca/Si ratio along the red arrow (bottom-left); spatial distribution of Ca (right-top) and element (Si, Ca, Al, Fe) mapping generated by x-ray imaging field width of 500 µm 33/47

34 Leaching Carbonation Modelling Results Changes in microstructure are significantly linked to mineralogical changes. 1. Leaching Atomic Ca/Si ratio Phenolphthalein Measured Moving average Depth, mm Leaching induces the decrease in atomic Ca/Si ratio over the sample depth 34/47

35 Leaching Carbonation Modelling Results Changes in microstructure are significantly linked to mineralogical changes. 2. Carbonation % mass Portlandite in carbonated sample Initial portlandite Calcite in carbonated sample Initial calcite Depth, mm XRD patterns of reference and carbonated samples Changes in portlandite and calcite contents as a function of depth averaged over 3 mm depth intervals of the carbonated sample. 35/47

36 Leaching Carbonation Modelling Results Modification in transport properties is linked to the changes in microstructure and mineralogy (degradation stage). Intrinsic permeability, m 2 1E 17 1E 18 1E 19 1E 20 Leached S3L Ref. S Immersed time, days Change in intrinsic permeability of leached sample due to leaching in 6 mol/l NH 4 NO 3 as a function of immersed time 36/47

37 Leaching Carbonation Modelling Results Modification in transport properties is linked to the changes in microstructure and mineralogy (degradation stage). Sample w/p ls/p ᶲ ref., % k ref. /k leached Leached S /6 Leached S /236 Leached S /25 Leached S /18 Leached S /8 Fully leached S /600 Increase in permeability after leaching sample fully leached S5 was measured after 142-day leaching when the sample was fully decalcified, the others were measured after 28-day leaching. Sample w/p ls/p k ref. /k carbonated Carbonated S Carbonated S Carbonated S Reduction in permeability after 28-day carbonation 37/47

38 Leaching Carbonation Modelling Results Modification in transport properties is linked to the changes in microstructure and mineralogy (degradation stage). Series model to compute permeability of carbonated zone Flux J Carbonated zone: k 1, J 1, d 1 d Sound zone: k 2, J 2 = J 1, d 2 Composite permeability Sample w/p ls/p k ref. /k carbonated k ref. /k car. zone Carbonated S Flux J Carbonated S Carbonated S Carbonated zone 38/47

39 Leaching Carbonation Modelling Results New conceptual models accounting for changes in mineralogy and microstructure and C-S-H degradation enable to predict the evolution of the microstructure and related transport properties of cementitious materials on the long-term. 1. Ca-leaching model Comparisons of experimental and modelling results: leached depth (a) and leached calcium amount (b): S1 w/c = 0.325, LS = 0; S2 w/c = 0.425, LS = 0.2; S3 w/c = 0.425, LS = 0; S4 w/c = 0.375, LS = 0.1; S5 w/c = 0.325, LS = /47

40 Leaching Carbonation Modelling Results 1. Ca-leaching model Normalized portlandite content of samples without (a) and with (b) limestone fillers after 28-day leaching in ammonium nitrate solution The portlandite contents of samples S3 and S5 are compared with quantitative XRD results. 40/47

41 Leaching Carbonation Modelling Results 1. Ca-leaching model 41/47

42 Leaching Carbonation Modelling Results 1. Ca-leaching model 42/47

43 Leaching Carbonation Modelling Results 2. Carbonation 43/47

44 Leaching Carbonation Modelling Results 2. Carbonation 44/47

45 Leaching Carbonation Modelling Results 2. Carbonation 45/47

46 Summary The scientific community agrees that carbonation/leaching change the transport properties of cement-based materials but to what extent is still questionable. This study allows for a better understanding of the alteration degree of transport properties due to chemical degradation by: Establishing the links between mineralogy, microstructure and transport properties; Clearly describing the degradation processes/mechanisms by the combination of experiments and modelling. This work consists three main parts: Development of novel methods; qualitative and quantitative study of the carbonation and leaching under accelerated conditions; and development of new phenomenological models. 46/47

47 Thank you! - PLEASE NOTE! This presentation contains data, information and formats for dedicated use ONLY and may not be copied, distributed or cited without the explicit permission of the. If this has been obtained, please reference it as a personal communication. By courtesy of. Studiecentrum voor Kernenergie Centre d'etude de l'energie Nucléaire Belgian Nuclear Research Centre Stichting van Openbaar Nut Fondation d'utilité Publique Foundation of Public Utility Registered Office: Avenue Herrmann-Debrouxlaan 40 BE-1160 BRUSSELS Operational Office: Boeretang 200 BE-2400 MOL