1

A new approach for comprehensive modelling of

molten salt properties Anna L. Smith

Delft University of Technology Radiation, Science & Technology Department Mekelweg 15 2625 JB Delft The Netherlands a.l.smith@tudelft.nl

2

Background and context of research

Liquid fuel

232Th fuel cycle

Th 232

Thorium

90

MSFR

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

3

REFERENCE fuel for the MSFR

• 7LiF-ThF4-233UF4 (77.5-20.0-2.5 mol%)

• 7LiF-ThF4-enrUF4-PuF3 (77.5-6.6-12.3-3.6 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

Molten fuel salt - composition

4

Spe

cies

frac

tion

(%)

x(ThF4)

Molten fuel salt - structure LiF is a strongly ionic liquid

• Li+ and F- species

ThF4 is a molecular liquid • [ThF7

3-], [ThF84-], [ThF9

5-] complexes • Network of corner-sharing & edge-sharing polyhedra

BeF2 is a polymeric liquid

• Li+, [BeF42-], F- species

• [Be2F73-], [Be3F10

3-], [Be4F135-] units with

increasing BeF2 concentration

BeF2

LiF:BeF2

3LiF:BeF2

[1] M. Salanne et al., J. Phys. Chem. B 110 (2006) 11461–11467

Water

Flibe 2LiF-BeF2

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Structure – property relation

7LiF-ThF4-UF4-PuF3 molten salt liquid fuel

Thermodynamic stability

Physico-chemical properties

Structure

EXPERIMENTS

COUPLED MODELLING

Dissociated ionic melt, molecular species,

polymerization

CFD codes Water

Flibe 2LiF-BeF2

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Local structure experimental studies • Set-up developed at TU Delft • Results on LiF-ThF4

Molecular dynamics simulations • The polarizable ion model • Comparison with EXAFS data

Coupled thermodynamic-structural model

• The quasi-chemical model • Modelling of the LiF-ThF4 system

Outline

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Local structure studies – in-situ EXAFS X-ray Absorption Spectroscopy (XAS) at KARA

• Intense and tunable X-ray beam • High temperature in-situ experiments • Th-L3 edge (16.3 keV) (excitation of 2p3/2 level)

“Short-range” technique • Adapted to liquids such as molten salts (highly disordered)

Challenges: Hygroscopic, radioactive, corrosive salts

sam

ple

X-rays I0 I1=I0e-μ(E)L

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Local structure studies – in-situ EXAFS BN measurement cell (1st barrier)

• Airtight, leak tight • BN compatible with salt • Pure salt: pellets (~100 μm)

Mapping of Th concentration profile (Th fluorescence @ 17.0 keV)

(NaF:ThF4) samples after melting

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Local structure studies – in-situ EXAFS BN measurement cell (1st barrier)

• Airtight, leak tight • BN compatible with salt • Pure salt: pellets (~100 μm)

Furnace set-up (2nd barrier)

Mo heating shields

Heating coil

Thermocouple

O-ring (Silicon) Water cooling

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Local structure studies – in-situ EXAFS

1st ionization chamber

Fluorescence detectors

Furnace

2nd ionization chamber

3rd ionization chamber

Kapton window

Kapton window

X-ray beam

ThF4 @ room temperature (monoclinic, C2/c)

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Phase diagram LiF-ThF4 Measurements 50-100oC above the liquidus and at RT after cooling and solidification Results: (LiF:ThF4)=(0.9:0.1)

Local structure studies – LiF-ThF4

10%

25%

50%

LiF ThF4

T(o C

)

x(ThF4)

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Local structure studies – LiF-ThF4 Fitting with standard EXAFS equation Results (LiF:ThF4) = (0.5:0.5)

R = 2.33(1) Å N = 7.3(3) σ2 = 0.019(1)

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Local structure experimental studies • Set-up developed at TU Delft • Results on LiF-ThF4

Molecular dynamics simulations • The polarizable ion model • Comparison with EXAFS data

Coupled thermodynamic-structural model

• The quasi-chemical model • Modelling of the LiF-ThF4 system

Outline

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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-BeF2, AF-ZrF4, AF-LaF3 (A=Li,Na,K,Cs), LiF-ThF4 • 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, 2013

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Molecular dynamics – LiF-ThF4 data

Local structure (Li,Th)Fx liquid solution • Treated as mixture of Li+, ThF4+ and F- ions • Complete picture by MD (not obtainable by EXAFS) • [ThF6

2-], [ThF73-], [ThF8

4-], [ThF95-], and [ThF10

6-] coexisting

Distribution of [ThFn]4-n complexes calculated from RDF

[3] Dewan et al., J. Nucl. Mater. 434 (2013) 322-327 [4] Liu et al. J. Phys. Chem. B 118 (2014) 13954-13962 [5] Dai et al., Journal of Molecular Liquids 211 (2015) 747-753

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Local structure (Li,Th)Fx liquid solution • Network of corner-sharing and edge-sharing polyhedra • Network becomes sparser with [LiF] and free F- anions

Molecular dynamics – LiF-ThF4 data

LiF:ThF4 = (0.9:0.1)

LiF:ThF4 = (0.5:0.5)

ThF4

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Molecular dynamics – LiF-ThF4 data

Comparison MD simulated spectra with EXAFS data • MD trajectories used as input for FEFF8.40 • Accumulation of 25000 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

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Molecular dynamics – LiF-ThF4 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) 322-327

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Local structure experimental studies • Set-up developed at TU Delft • Results on LiF-ThF4

Molecular dynamics simulations • The polarizable ion model • Comparison with EXAFS data

Coupled thermodynamic-structural model

• The quasi-chemical model • Modelling of the LiF-ThF4 system

Outline

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Modelling – CALPHAD (CALculation of PHAse Diagrams)

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Modelling – The quasi-chemical model Modified quasi-chemical model in quadruplet approximation [6]

• Formalism well-adapted to ionic liquids • Two sublattices

(Li+, Th4+, cations, …) (F-, Cl-, anions)

• Basic unit = quadruplet composed of 2 anions and 2 cations

Optimized excess parameters linked to SNN exchange reaction

Li+ F-

F- Th4+ FNN

SNN

Li+ F-

F- Li+ FNN

SNN

Th4+ F-

F- Th4+ FNN

SNN 2

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Coupled structural-thermodynamic model Phase diagram equilibrium data Thermodynamic data

• Fusion enthalpy ThF4: 41.8 kJ·mol-1 (41.9 ±2.0 kJ·mol-1 [7]) • Fusion enthalpy Li3ThF7: 59.2 kJ·mol-1 (58.4 ±0.2 kJ·mol-1 [8])

ThF73-

ThF84-

ThF95-

[7] E. Capelli et al., J. Chem. Thermodyn. 58 (2013) 110–116 [8] R.A. Gilbert, Journal of Chemical & Engineering Data. 7 (1962) 388–389

X(ThF4)

T(K

)

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T = 1121 K T = 1121 K

Mixing enthalpy data

Coupled structural-thermodynamic model

DSC

ThF4

LiF LixTh1-xF solution

HEATING

Nickel separator

[7] E. Capelli et al., J. Chem. Thermodyn. 58 (2013) 110–116

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Coupled structural-thermodynamic model Complexes distribution

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X-ray Absorption Spectroscopy

CONCLUSION

Molecular Dynamics

Thermodynamics

Structure studies

CALPHAD modelling based on quasi-

chemical model with quadruplet

approximation Advanced model

Calorimetry

Transport properties

Local structure

Thermodynamic properties

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Netherlands Organisation for Scientific Research

KIT light source for provision of instruments at the INE-beamline

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

Acknowledgements