A new approach for comprehensive modelling of molten salt properties

Transcription

1 A new approach for comprehensive modelling of molten salt properties Anna L. Smith Delft University of Technology Radiation, Science & Technology Department Mekelweg JB Delft The Netherlands 1

2 Background and context of research MSFR Liquid fuel 232 Th 90 Thorium 232 Th fuel cycle Physico-chemical properties - requirements NEUTRONIC Small neutron capture cross section CHEMICAL and THERMAL Chemical stability; High solubility of actinides Compatibility with structural materials Low melting point and vapour pressure High heat capacity TRANSPORT Appropriate viscosity and thermal conductivity 2

3 Molten fuel salt - composition REFERENCE fuel for the MSFR 7 LiF-ThF UF 4 ( mol%) 7 LiF-ThF 4 - enr UF 4 -PuF 3 ( mol%) FISSION PRODUCTS Salt soluble fission products Metallic precipitates Gas CORROSION PRODUCTS Ni-based alloy as structure material Complex multi-component system Non-ideal thermodynamic behaviour 3

4 Species fraction (%) Molten fuel salt - structure 3LiF:BeF 2 LiF is a strongly ionic liquid Li + and F - species ThF 4 is a molecular liquid [ThF 7 3- ], [ThF 8 4- ], [ThF 9 5- ] complexes Network of corner-sharing & edge-sharing polyhedra LiF:BeF 2 BeF 2 is a polymeric liquid Li +, [BeF 4 2- ], F - species [Be 2 F 7 3- ], [Be 3 F ], [Be 4 F ] units with increasing BeF 2 concentration Flibe 2LiF-BeF 2 BeF 2 x(thf 4 ) Water [1] M. Salanne et al., J. Phys. Chem. B 110 (2006)

5 Structure property relation Dissociated ionic melt, molecular species, polymerization Structure 7 LiF-ThF 4 -UF 4 -PuF 3 molten salt liquid fuel Flibe 2LiF-BeF 2 Thermodynamic stability EXPERIMENTS COUPLED MODELLING CFD codes Physico-chemical properties Water 5

6 Outline Local structure experimental studies Set-up developed at TU Delft Results on LiF-ThF 4 Molecular dynamics simulations The polarizable ion model Comparison with EXAFS data Coupled thermodynamic-structural model The quasi-chemical model Modelling of the LiF-ThF 4 system 6

7 Outline Local structure experimental studies Set-up developed at TU Delft Results on LiF-ThF 4 Molecular dynamics simulations The polarizable ion model Comparison with EXAFS data Coupled thermodynamic-structural model The quasi-chemical model Modelling of the LiF-ThF 4 system 7

8 Local structure studies in-situ EXAFS sample X-ray Absorption Spectroscopy (XAS) at KARA Intense and tunable X-ray beam High temperature in-situ experiments Th-L 3 edge (16.3 kev) (excitation of 2p 3/2 level) X-rays I 0 I 1 =I 0 e -μ(e)l Short-range technique Adapted to liquids such as molten salts (highly disordered) Challenges: Hygroscopic, radioactive, corrosive salts 8

9 Local structure studies in-situ EXAFS BN measurement cell (1 st barrier) Airtight, leak tight BN compatible with salt Pure salt: pellets (~100 μm) Mapping of Th concentration profile (Th 17.0 kev) (NaF:ThF 4 ) samples after melting 9

10 Local structure studies in-situ EXAFS BN measurement cell (1 st barrier) Airtight, leak tight BN compatible with salt Pure salt: pellets (~100 μm) Furnace set-up (2 nd barrier) Heating coil Thermocouple O-ring (Silicon) Mo heating shields Water cooling 10

11 Local structure studies in-situ EXAFS 3 rd ionization chamber 2 nd ionization chamber Kapton window Furnace Kapton window Fluorescence detectors X-ray beam 1 st ionization chamber ThF room temperature (monoclinic, C2/c) 11

12 T( o C) Local structure studies LiF-ThF 4 Phase diagram LiF-ThF 4 10% 50% Measurements o C above the liquidus and at RT after cooling and solidification 25% Results: (LiF:ThF 4 )=(0.9:0.1) LiF x(thf 4 ) ThF 4 12

13 Local structure studies LiF-ThF 4 Fitting with standard EXAFS equation Results (LiF:ThF 4 ) = (0.5:0.5) R = 2.33(1) Å N = 7.3(3) σ 2 = 0.019(1) 13

14 Outline Local structure experimental studies Set-up developed at TU Delft Results on LiF-ThF 4 Molecular dynamics simulations The polarizable ion model Comparison with EXAFS data Coupled thermodynamic-structural model The quasi-chemical model Modelling of the LiF-ThF 4 system 14

15 Molecular dynamics PIM model Polarizable Ion Model Developed by Madden et al. in the last 20 years [2] Particularly well-adapted to ionic systems LiF-BeF 2, AF-ZrF 4, AF-LaF 3 (A=Li,Na,K,Cs), LiF-ThF 4 Semi-classical approach Interaction potentials Charge-charge Dispersion Repulsion Polarization [2] P.A. Madden, M. Salanne, Thorium Molten Salts: Theory and Practice, Proceedings of the ThEC13 Conference,

16 Molecular dynamics LiF-ThF 4 data Local structure (Li,Th)F x liquid solution Treated as mixture of Li +, ThF 4+ and F - ions Complete picture by MD (not obtainable by EXAFS) [ThF 6 2- ], [ThF 7 3- ], [ThF 8 4- ], [ThF 9 5- ], and [ThF ] coexisting Distribution of [ThF n ] 4-n complexes calculated from RDF [3] Dewan et al., J. Nucl. Mater. 434 (2013) [4] Liu et al. J. Phys. Chem. B 118 (2014) [5] Dai et al., Journal of Molecular Liquids 211 (2015)

17 Molecular dynamics LiF-ThF 4 data Local structure (Li,Th)F x liquid solution Network of corner-sharing and edge-sharing polyhedra Network becomes sparser with [LiF] and free F - anions LiF:ThF 4 = (0.9:0.1) LiF:ThF 4 = (0.5:0.5) ThF 4 17

18 Molecular dynamics LiF-ThF 4 data Comparison MD simulated spectra with EXAFS data MD trajectories used as input for FEFF8.40 Accumulation of atomic configurations to reproduce the effect of Debye-Waller factor and anharmonic vibrations Th-F distances underestimated by MD Need to fine-tune interaction potentials 18

19 Molecular dynamics LiF-ThF 4 data Local structure Density Thermal expansion Thermodynamic properties Mixing enthalpies (see next section) Heat capacity etc Transport properties Viscosity Diffusion Electrical conductivity [3] Dewan et al., J. Nucl. Mater. 434 (2013)

20 Outline Local structure experimental studies Set-up developed at TU Delft Results on LiF-ThF 4 Molecular dynamics simulations The polarizable ion model Comparison with EXAFS data Coupled thermodynamic-structural model The quasi-chemical model Modelling of the LiF-ThF 4 system 20

21 Modelling CALPHAD (CALculation of PHAse Diagrams) 21

22 Modelling The quasi-chemical model Modified quasi-chemical model in quadruplet approximation [6] Formalism well-adapted to ionic liquids Two sublattices (Li +, Th 4+, cations, ) (F -, Cl -, anions) Basic unit = quadruplet composed of 2 anions and 2 cations Li + F - SNN FNN F - Li + Th 4+ F - 2 FNN SNN F - Th 4+ Li + F - SNN FNN F - Th 4+ Optimized excess parameters linked to SNN exchange reaction 22

23 Coupled structural-thermodynamic model 23

24 Coupled structural-thermodynamic model T(K) Phase diagram equilibrium data ThF 9 5- ThF 7 3- ThF 8 4- X(ThF 4 ) Thermodynamic data Fusion enthalpy ThF 4 : 41.8 kj mol -1 (41.9 ±2.0 kj mol -1 [7]) Fusion enthalpy Li 3 ThF 7 : 59.2 kj mol -1 (58.4 ±0.2 kj mol -1 [8]) [7] E. Capelli et al., J. Chem. Thermodyn. 58 (2013) [8] R.A. Gilbert, Journal of Chemical & Engineering Data. 7 (1962)

25 Coupled structural-thermodynamic model Mixing enthalpy data HEATING ThF 4 LiF Nickel separator Li x Th 1-x F solution T = 1121 K DSC [7] E. Capelli et al., J. Chem. Thermodyn. 58 (2013)

26 Coupled structural-thermodynamic model Complexes distribution 26

27 CONCLUSION X-ray Absorption Spectroscopy Structure studies Local structure Molecular Dynamics Transport properties Thermodynamic properties Thermodynamics CALPHAD modelling based on quasichemical model with quadruplet approximation Advanced model Calorimetry 27

28 Acknowledgements Netherlands Organisation for Scientific Research KIT light source for provision of instruments at the INEbeamline Institute for Beam Physics and Technology for operation of the KARA storage ring TU Delft (Delft, The Netherlands) John Vlieland, Dick de Haas, Malte Verleg, Jaen Ocadiz-Flores, Elisa Capelli, Rudy Konings KIT-INE (Karlsruhe, Germany) Joerg Röthe, Kathy Dardenne CEA (Marcoule, France) Philippe Martin UPMC Université Paris 06 (Paris, France) Mathieu Salanne Ecole Polytechnique (Montreal, Quebec) Aimen Gheribi 28