The Material Science for a UK Geological Disposal Facility

Transcription

1 The Material Science for a UK Geological Disposal Facility, Claire Corkhill, Susan Bernal and John Provis Immobilisation Science Laboratory, & Engineering The University of Sheffield. n.c.hyatt@sheffield.ac.uk IGDTP, Manchester UK, June 2014 The views expressed in this talk are the personal opinion of the speaker and do not necessarily reflect those of sponsors or funding

2 UK Geological Disposal Concepts

3 Case study I. Understanding the sorption behaviour of a cement backfill How is transport of anionic radionuclides retarded by a cement backfill?

4 Case study I. Understanding the behaviour of cement backfill Cementitious backfill in a UK hard rock scenario: Nirex Reference Vault Backfill (NRVB) Mixture of OPC / Ca(OH) 2 / CaCO 3 Condition ground water to high ph Minimise actinide solubility and enhances actinide sorption How does it behave towards anionic radionuclides such as 99 Tc? 99 Tc (t 1/2 = 211,000 y) Tc(VII) highly soluble, insoluble as Tc(IV) Although in a nominally reducing environment, models suggest that some Tc(VII) may be present and will poorly sorb to cement material No reliable data to support this assumption

5 Case study I. Understanding the behaviour of cement backfill gamma camera collimator head 99m Tc injection 99m Tc injection point flow in direction of flow duplicate flow cells pump in- and out-flow solutions 10cm flow out Calibrated (non-destructive) quantitative gamma imaging Uses a common medical isotope, 99m Tc Images are taken every 30s for 160 min Calibration to relate counts directly to activity Camera corrects for decay (t 1/2 6.02hr) Build a 2D model of transport through opaque porous media

6 Case study I. Understanding the behaviour of cement backfill Normalised concentration, C/Cin 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 Nirex Reference Vault Backfill Material 2.80 cm 3.08 cm 3.36 cm 3.64 cm 3.92 cm 0 0,2 0,4 0,6 0,8 1 Time after injection, PV 20 min 0.3 PV 40 min 0.6 PV 60 min 0.9 PV 80 min 1.2 PV Convection dispersion model: R = D 2 C x2 v μμ Velocity (x10-5 ) (m s -1 ) Dispersivity (λ) x10-5 (m) Retardation (R) NRVB ± ± ± 0.38 NRVB ± ± ± 0.28 Summary: Value for retardation derived from model was 1 99m Tc(VII) was conservatively transported Corkhill et al. Environmental Science and Technology (2013), 47,

7 Case study I. Understanding the behaviour of cement backfill Redox functionality in BFS additive to cement greatly reduced the transport of 99 Tc partial reduction of Tc(VII) to Tc(IV) Development of functional cement backfill ongoing MBq L PV 1.2 PV 2 PV OPC/BFS NRVB γ-imaging mm OPC OPC/PFA OPC/BFS NRVB Tc(VII) pre-edge μ-xanes Normalised 99 T concentration 1,2 1 0,8 0,6 0,4 0,2 0 NRVB OPC OPC/BFS OPC/PFA Time (days) μ-xrf mapping Corkhill et al. Journal of Cement and Concrete Science, in review.

8 Case study II. HLW glass dissolution under hyperalkaline conditions How could carbonation impact the long term performance of a cementitious backfill?

9 Case study II. Understanding the carbonation behaviour of cement backfill This side has been redacted, please contact the author for details.

10 Case study II. Understanding the carbonation behaviour of cement backfill This side has been redacted, please contact the author for details.

11 Case study III. HLW glass dissolution under hyperalkaline conditions How might the ILW hyperalkaline plume in a co-disposal scenario impact HLW glass dissolution mechanism?

12 Case study III. HLW glass dissolution under hyperalkaline conditions Dissolution of 25% Magnox simulant : 50 C in saturated Ca(OH) 2 SA/V = 10,000 m -1 or 10 m -1 Up to 168 days of dissolution Dissolution in the presence of Ca(OH) 2 was an order of magnitude lower than in initially pure water. NL B (g m -2 ) 8,0E-3 7,0E-3 6,0E-3 5,0E-3 4,0E-3 3,0E-3 2,0E-3 1,0E-3 0,0E+0 U CSS Water Ca(OH) Time (days) Observed the formation of M-S-H phases on the surface of monoliths result of high Mg composition of UK HLW glass Also observed neoformed C-S-H phases Corkhill et al. International Journal of Applied Glass Science, 4, (2013)

13 Case study III. HLW glass dissolution under hyperalkaline conditions Solution saturation w.r.t. Si occurred rapidly for MW25 in water Dissolution of Si did not occur in Ca(OH) 2 until the solution concentration of Ca dropped <200mg/L. A thick alteration layer comprising Ca, Si, Al and Mg was developed Ca concentration (mg L -1 ) 1000 Corkhill et al. International Journal of Applied Glass Science, 4, (2013) Time (days) Ca(OH) 2 Blank CSS CSS Blank

14 Case study III. HLW glass dissolution under hyperalkaline conditions A range of C-S-H phases were predicted to form The Ca / Si ratio decreased with time as Ca was incorporated into the alteration layer. Corkhill et al. International Journal of Applied Glass Science, 4, (2013)

15 Case study III. HLW glass dissolution under hyperalkaline conditions The rate limiting step of glass dissolution in Ca-rich solutions is Ca-Si equilibrium. Dissolution occurs in 3 regimes: 1. Incubation regime: Incorporation of Ca into the hydrated glass surface and precipitation of M-S-H phases 2. Intermediate regime: Precipitation of C-S-H phases, including a range of compositions in the C-(N)-(A)-S-H and M-S-H systems; rapid Si dissolution 3. Residual regime: Precipitation of lower Ca/Si ratio C-S-H phases; steady concentration of Ca; slower dissolution of Si Corkhill et al. International Journal of Applied Glass Science, 4, (2013)

16 Summary A toolkit of advanced wasteforms has been developed to address complexity of UK nuclear waste Through careful characterisation of materials chemistry processes we are able to: Expand the range of materials consigned to the GDF Give confidence in the materials that are being considered for consignment in the GDF Develop a mechanistic understanding of wasteforms and their interaction with conceptual disposal environments Support the disposal system safety case

17 Acknowledgements University of Sheffield Jonathan Bridge, Xiaohui Chen, Steve Thornton, Maria Romero-Gonzalez, Steven, Banwart, Nathan Cassingham, Paul Heath, Martin Stennett. Sheffield Teaching Hospital NHS Trust Phil Hillel Brookhaven National Laboratory Bruce Ravel, Ryan Tappero National Nuclear Laboratory Martin Dutton, Ed Butcher British Geological Survey Tony Milodowski.