Nuclear-Related Research at the

Department of Chemical Engineering

Fluoride-Salt-Cooled High-Temperature Reactors (CNE)

(Plus: Advanced Thermohydraulic Measurements;

Thermodynamic Cycles/Waste-Heat Utilization)

CN Markides, M Millan-Agorio, GF Hewitt

Fluoride High-Temperature Reactor (FHR)

• New (!) nuclear reactor that uses a fluoride

molten-salt as a coolant

• Operation at low pressures and high

temperatures (~700-800 oC) for increased

safety and higher efficiency

• Promising technology, but its

development stagnated in the last 50

years and more research is needed

• Preliminary FHR designs feature FLiBe as

the coolant, TRISO fuel microspheres

and a Brayton power-generation cycle

TRISO fuel microsphere

Tritium generation in FHRs

The reactor concept has some unresolved issues, one of which is the

formation of Tritium in the cooling salt by neutron absorption of Lithium-6:

Li��

� n → H�

� He��

� 4.8MeV

If tritium is not removed from the reactor, it will permeate the structural materials and find its way out to the environment.

Graphite has been proven to have the ability to adsorb tritium at high temperature,and thus, it can be considered as a tritium removal technology. The graphite constituent of the TRISO microspheres could be used as a tritium sink.

Investigation of tritium absorption into graphite and other carbon materials is

essential to study the feasibilty of such solution.

Tritium generation in FHRs

An experimental system has been designed and commissioned that allows the measurement of H2 adsorption into carbon materials immersed in a molten salt. The amount of hydrogen adsorbed is calculated by monitoring the change in pressure.

• Vessel volume: 200 mL

• Working temperature:

500 – 700 oC

• Working pressure:

5 – 12 bar

• Molten salt used:

FLiNaK (LiF-NaF-KF,

Tmelt = 454 oC)

H2 Ar

P

T

Vacuum

Pump

Hot Section

Molten salt

Stainless steel

vessel

Carbon sample Nickel crucible

TTo extraction

PT

Sample cylinder

V-1

V-3

V-2V-4 V-5

Evolution of the hydrogen adsorption on activated carbon and graphite immersed in FLiNaK for different working temperatures:

H2 adsorption on AC and graphite in FLiNaK

0 2000 4000 6000 8000 100000.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

500 oC 600 oC 700 oC

∆nH

2, mol

Time, s

0 2000 4000 6000 8000 10000 120000.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

0.00014

500 oC

600 oC

700 oC

∆nH

2 (m

ol)

Time (s)

Activated carbon

Graphite

Temperature

(oC)

KAC,H2

(molH2/gAC)

Kgraphite,H2

(molH2/ggraphite)

500 4.9E-04 1.0E-04

600 4.6E-04 1.0E-04

700 2.5E-04 5.7E-05

Characterization of fresh and spent activated carbon

SEM and EDX of Activated Carbon (AC)

Cu ClO

Si

Cl

C

0 2 4 6 8 10 12 14 16 18 20

keVFull Scale 4919 cts Cursor: 2.953 (40 cts)

Spectrum 1

F

K

C

K

0 2 4 6 8 10 12 14 16 18 20

keVFull Scale 4947 cts Cursor: 2.953 (57 cts)

Spectrum 1

Cr FeCr FeNa Al KFe

Cr

OF K

Si

C

K

0 1 2 3 4 5 6 7 8 9 10

keVFull Scale 1752 cts Cursor: 0.000

Spectrum 1

K

C

K

0 1 2 3 4 5 6 7 8 9 10

keVFull Scale 3134 cts Cursor: 0.000

Spectrum 2

F KK

C

K

0 1 2 3 4 5 6 7 8 9 10

keVFull Scale 4152 cts Cursor: 0.000

Spectrum 4

2

3

5

1

4

Fre

sh a

ctivate

d

carb

on

Spent

activate

d

carb

on

b C peaks only

K peaks

F peaks

Na peaks

Molten salts as heat transfer fluids

Advantages of molten salts as heat transfer fluids:- High heat capacity/thermal conductivity- Low reactivity- Low vapour pressure- Very high boiling point (FLiBe > 1400 oC)

Preliminary design of molten-salt pebble-bed reactor.

A variety of compact and thermally efficient reactor designs are possible. The thermohydraulicbehaviour of such design is however not obvious and experiments are difficult to conduct.

Region of interest for molten-salt pebble-bed reactor

We use CFD simulations to predict the heat transfer and pressure drop correlations for molten fluorides in a pebble bed reactor.

Approach currently adopted to model molten salt pebble-bed reactor.

Direct reactor auxiliary cooling system modelling

Intrinsic safety is a key feature of molten salt reactors. Passive safety systems such as Direct Reactor Auxiliary Cooling System (DRACS) can be used to remove decay heat in case of accident.

Preliminary design of a molten salt pebble bed reactor

including the DRACS passive safety system.

One of the disadvantages and possible modes of failure of molten salt reactors is the freezing of the salt due to its high melting point (FLiBe = 459 oC)

A quasi-steady-state model was developed to simulate the salt freezing process. The model was validated against experimental results.

Comparison between experimentally measured freezing time of water

(circles) and the results of our model (lines). The experimental data were

taken from McDonald et al. (2014) who measured the freezing time of

water in a cylindrical geometry as a function of the outside temperature.

Direct reactor auxiliary cooling system modelling

We modelled the feasibilty of molten

salt DRACS passive safety system

under Loss of Forced Circulation.

Sketch of DRACS passive system. During normal

operations a diode valve prevents the coolant from

circulating in the DRACS heat exchanger. In case of

accident, the coolant flows by natural circulation

through the DRACS heat exchanger in the direction

allowed by the diode. A second natural circulation loop

transports the waste heat to an outside air Inlet.

A critical behaviour of DRACS under

accident is the freezing of salt in the

molten salt/air heat exchanger

Transient flow-rates of DRACS primary and secondary loop during loss of forced circulation. For

the higher value of the molten salt/air HX heat transfer coefficient the salt freezes obstructing the

flow in the secondary loop (first picture on the left).

Measurements of thermophysical properties of molten salts

Thermal conductivity is a key property when modelling the thermohydraulic behaviour of molten salts. Few reliable data are however available in the literature and none for the salt shortlisted for nuclear applications.

We developed a novel method for measuring the thermal conductivity of liquid salts through a thin quartz capillary filled with Galinstan.

Advanced thermohydraulic measurements

High-efficiency cycles/waste-heat utilization

ORC

TE

Condenser Evaporator

ExpanderGenerator

Pump