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