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PHYSICO-CHEMICAL PROPERTIES OF THORIUM FUELS

O. Beneš, R. J. M. Konings, J. Somers, D.Staicu, D. Manara

European Commission – Joint Research Centre Institute for Transuranium Elements (ITU), Karlsruhe, Germany

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Institute for Transuranium Elements

One of the 7 research institutes

of the European Commission‘s

Joint Research Centre

~ 300 staff members

Location: Germany, Karlsruhe

6 Scientific Units

4 pillars of the ITU work programme:

- Basic actinide science and application

- Safety of the nuclear fuel cycle

- Safeguards and nuclear forensics

- Education and user facilities

http://itu.jrc.ec.europa.eu/

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Th-containing fuel

2 main research lines at ITU

solid oxide fuels

(Th-U)O2, Th-MOX

molten salt fuels

F-, Cl- salts

- basic actinide research

- better understanding of physico-chemical properties

to increase the safety of neclear reactors.

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solid oxide fuels

Systematic study of the Th-U-Pu-O system

(possibly Am addition)

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solid oxide fuels

Heat capacity determination - by drop calorimetry, PPMS, adiabatic calorimetry

- 0.5 – 1800 K

Melting temperature determination - using a laser flash technique

- self-containing method – above 4000 K

Vapour pressure measurements - Knudsen cell coupled with MS

- up to 2800 K

Thermal conductivity measurements

- laser flash technique

- up to 3000 K (tungsten heater)

Irradiated fuel investigation (hot cells, γ-tight GB)

- fuel synthesis and fabrication

- P-C properties, topography

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solid oxide fuels

1. Heat capacity determination

drop calorimeter

400 - 1800 K

dH measured first Cp

automatic

motor

dropping tube

crucible

crucible

28 TCs

Al2O3 tube

heating element

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solid oxide fuels

Heat capacity determination of (Th,U,Pu)O2 mixtures

heat capacity functions obtained by derivation

Enthalpy increments measurments

ThO2-PuO2 system: 3%, 15%, 30% of Pu

ThO2-UO2 system: 20%, 40%, 60%, 80% of Pu

UO2-PuO2 system: 25%, 50%, 75% of Pu

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solid oxide fuels

2. Melting temperature determination

Experimental set-up of laser flash method

Advantages:

• Fast experiments (from ms to s)

• Containerless conditions (no contact between liquid sample holders)

Disadvantages

• Emissivity needs to be known

• Thermal stresses

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solid oxide fuels

Melting temperature determination

0 20 40 60 80 100

2000

2400

2800

3200

3600

Solidification point

Te

mp

era

ture

/ K

Time / ms

RLS first derivative

Fast pyrometer thermogram (= 0.83)

0

500

1000

1500

2000

2500

3000

Laser power

La

se

r P

ow

er

/ W

0,15 0,16 0,17 0,18 0,19

2600

2800

3000

3200

3400

3600

0,150 0,155 0,160 0,165

2940

3000

3060

Lyon et al.Riley

Kato et al.

Peak laser power

630 W

675 W

720 W

Tem

pe

ratu

re / K

time / s

This work

(3017 28) K

Melting temperature measurement of PuO2

Measurement of PuO2 and NpO2 performed recently at ITU observed melting points significantly higher than

reported in the literature.

Measurement of ThO2 confirmed the literature value

It is planned to make systematic study of solidus-liquidus equilibria in the ThO2-UO2 and ThO2-PuO2 systems

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solid oxide fuels

3. Vapour pressure determination

scheme of Knudsen cell with MS

Knudsen cell glove box

W/Ir set-up – up to 2800 K

contained in shielded glove box

- fundamental studies of An-containing

systems

- studies of fission product release at

high temperatures of irradiated fuels

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solid oxide fuels

Ionization potential determination of UOx

8.0

8.3

0.5

+

+

+

+

UO

10.8

5.92

7.85

7.81

U U

UO

UO2 UO2

UO3 UO3

5.4

5.6

6.15

11.3

19.3

13.9

13.4

21.7

27.6

21.6

15.7

units in eV

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solid oxide fuels

Vapour pressure determination of PuO2

PuO2 (g)

PuO (g)

Pu (g)

1 2 3

Ordinary fuel

structure Rim

structure

Cladding+corrosion

products

500 1000 1500 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

3

2

136 X

e A

bso

lute

rele

ase n

orm

alised

Temperature (K)

1

Low

Tirr

High

Tirr

136Xe release from irradiated fuel

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solid oxide fuels

4. Thermal conductivity determination

ΔTmax

t1/2

α = 0.13885 L2/t1/2

Parker‘s model

scheme of laser flash technique

Experimental set-up

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solid oxide fuels

Thermal conductivity determination of (Th,Pu)O2 solid solution

Cozzo C., et al. J. Nucl. Mater. 416 (2011)135

ThO2 and PuO2 pure compounds measured

(Th0.97Pu0.03)O2, (Th0.92Pu0.08)O2 and

(Th0.7Pu0.3)O2 compositions measured for

500 – 1500 K

at low temperatures the addition of Pu into

the ThO2 matrix influence significantly (almost

by one half) the thermal diffusivity

at higher temperatures this influence is of

much lower order – due to increase of phonon-

phonon scattering mechanism, becoming

predominant compared to lattice strains.

(Th,U)O2 system is planned to be investigated

synthesis of Th80%, Th60%, Th40% and Th20%

is being performed now.

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molten salt fuels

Reference system: LiF – ThF4 – UF4 (AlkF (Alk=Na, K, Rb),

BeF2, AnF3(An=Pu,MA))

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molten salt fuels

Work carried out at ITU

Experimental part Thermodynamic modelling

Heat capacity determination

- pure solid compounds

- liquid solutions (systematic study)

Equilibrium data determination (phase diagram)

- melting temperatures

- solidus, liquidus points

Heats of transition and fusion determination

Enthalpy of mixing of liquid solutions

extremely strong tool to predict behaviour

of any multi-component system

ITU‘s molten salt database (2002-2011)

contanining 39 binary systems

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Calorimetry at ITU Total temperature range covered from 0.5 K to 1750 K

2 low temperature adiabatic calorimeters (Cp determination)

(4.2 K – 350 K, from 0.5 K coupled with PPMS)

2 Drop calorimeters (Cp and fusion

enthalpy determin.)

(400 K – 1750 K)

1 DSC (equilibrium data

fusion enthalpy)

(400 K – 1650 K)

molten salt fuels

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molten salt fuels

Low temperature heat capacity measurement of UF3

S298 = 126.8 J.K-1.mol-1

TC = 1.59 K

Benes O., et al. Inorg. Chem. doi.org/10.1021/ic2010453

further measurements of Cp in

magnetic field and magnetization

confirmed ferromagetic nature

of the transition.

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molten salt fuels

High temperature studies

Encapsulation necessary to avoid F- vapour release

Drop calorimetry

nickel crucible

emergency button

optical scope

laser inlet

vacuum pump

turning device

moveable platform

vacuum chamber

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molten salt fuels

High temperature studies

Encapsulation necessary to avoid F- vapour release

Differential Scanning Calorimetry (DSC)

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molten salt fuels

High temperature heat capacity measurement of molten salts

systematic study of LiF-AlkF systems: LiF-NaF, LiF-KF, LiF-RbF and LiF-CsF

will be followed by An-containing systems, LiF-ThF4, UF4-ThF4 ...

Very important observation as untill now all

assessed systems were treated ideally in terms

of the heat capacity behaviour.

X (LiF)

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molten salt fuels

Phase diagram investigation of molten salt systems

NaNO3-KNO3, RbF-CsF measured as first systems (also to proof the encapsulation technique)

Th-containing systems: CaF2-ThF4, LiF-ThF4, UF4-ThF4 (next planned)

CaF2-ThF4 system

preliminary version

LiF-ThF4 system

(purification of ThF4 was required)

- our data

- ORNL data

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molten salt fuels

Heats of transition of fluoride salts

from DSC and Drop technique: ∆fusH of CsF recently evaluated

by DSC by Drop

∆fusH = 22,190 J.mol-1 ∆fusH = 21,550 J.mol-1

Literature: ∆fusH (Dworkin et al.) = 21,700 J.mol-1

∆fusH (Macleod) = 14,850 J.mol-1

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molten salt fuels

Future plans:

To establish a suitable technique to measure the thermal conductivity of MS

requires encapsulation – more complex model necessary

To set-up the Raman spectroscopy to high temperatures

- To determine the structure of these liquids

- what clusters are formed

- to couple these results with MD studies (UPMC collaboration)

- improve ITU‘s thermodynamic database which considers cluster formation

- To determine solubilities of AnFx in various fluoride matrixes

- e.g. PuF3 solubility in LiF-ThF4 melt/eutectic (EVOL project)

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furnace

sample inside

laser generator

detector

2 methods:

- Macro line

higher sensitivity

- Superhead

more flexible for

manipulation,

easier focusing

microscope

Raman - molten salt fuels

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molten salt fuels

Thermodynamic modelling as a tool to

optimize a fuel composition of the MSR

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MSFR reactor concept (French concept) (Molten Salt Fast Reactor) (considered in EVOL EU-project) Initial MSFR fuel composition: X(LiF) = 77.45 mol% X(ThF4) = 20 mol% (LiF-ThF4 eutectic) X(UF4) = 2.55 mol%

Criteria:

X (ThF4) = ~20 mol%

X (UF4) = 2.55 mol%

Temperature low as possible

(to avoid freezing)

MSFR pre-conceptual design,

GIF Annual Report 2009: (MSR)

molten salt fuels

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7LiF – NaF – ThF4 – UF4

Matrix components

Fertile material Fissile material

Fuel for Molten Salt Fast Breeder Reactor (MSFR)

232Th + 1n 233Th 233Pa 233U

fertile fissile

β- decay β- decay

22 min 27 days

Th / U fuel cycle

molten salt fuels

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The aim: to make a full thermodynamic decription of a molten salt

reactor fuel (LiF-NaF-ThF4-UF4) optimize the MSR

fuel composition

The way: 1. TD assessment of all binary sub-systems

LiF – NaF LiF – ThF4 LiF – UF4

NaF – ThF4 NaF – UF4 ThF4 – UF4

2. extrapolation to higher order systems

molten salt fuels

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X (ThF4)

T /

K

0.0 0.2 0.4 0.6 0.8 1.0

400

600

800

1000

1200

1400

LiF - NaF LiF – ThF4 LiF – UF4

NaF – ThF4 NaF – UF4 ThF4 – UF4

X UF4

T(K

)

0.0 0.2 0.4 0.6 0.8 1.0

400

600

800

1000

1200

1400

X (ThF4)

T /

K

0.0 0.2 0.4 0.6 0.8 1.0

400

600

800

1000

1200

1400

X (NaF)

T /

K

0 .2 .4 .6 .8 1

600

700

800

900

1000

1100

1200

1300

X (UF4)

T /

K

0.0 0.2 0.4 0.6 0.8 1.0

400

600

800

1000

1200

1400

X (UF4)T

/ K

0.0 0.2 0.4 0.6 0.8 1.0

1200

1240

1280

1320

1360

1400

molten salt fuels

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Lowest eutectic:

T = 775 K (502 °C)

x (ThF4) = 0.221

x (UF4) = 0.0255

x (LiF) = 0.521

x (NaF) = 0.23

Tinlet = 825 K (552 °C)

12 Invariant eq.:

2 Eutectics

9 Quasi-peritectics

1 Peritectic

Criteria:

X (ThF4) = ~20 mol%

X (UF4) = 2.55 mol%

Temperature low as possible

(to avoid freezing)

molten salt fuels