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September 5, 2006
1
Neutronic Design of a Liquid Salt-cooledPebble Bed Reactor (LSPBR)
PHYSOR-2006 Conference, Vancouver, Canada
S.J. de Zwaan, J.L.Kloosterman, D.Lathouwers & B.Boer
Faculty of Applied Sciences (TNW)Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
September 10-14, 2006
2
Introduction• Improve High Temperature Reactors by replacing Helium with a liquid
salt-coolant. Benefits are:• Ambient pressure operation• Increased power density without compromise to safety• Lower fuel temperatures & higher outlet temperature• Better decay heat removal
• Oak Ridge National Laboratories design: AHTR / LS-VHTR• TU Delft design: Liquid Salt-cooled Pebble Bed reactor (LSPBR)
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
AHTR / LS-VHTR LSPBR
• hexagonal matrix fuel • offline refueling• wide range in volume fractions
• pebble bed fuel geometry • online refueling• fixed coolant volume fraction (~39 %)
Main differences:
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
3
Selection of Liquid Salt Coolant
Criteria for selection of liquid salt coolant:• Good heat transfer coefficients• Reasonably low melting points• Compatible with moderator and structural materials• Chemically inert, Low toxicity
Neutronics and Liquid Salt Coolant• Salts moderate and absorb neutrons• Voiding of salt reduces: - moderation (reactivity decreases)
- absorption (reactivity increases)• Liquid salt may not lead to positive voiding or temperature
reactivity effect!
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
4
Liquid Salt Coolant CandidatesRelevant physical properties of 7 candidate salts (Forsberg, ORNL)
• Heat capacity at 700 °C• Lithium is highly enriched with 7Li, 6Li concentration ~ 0.0007 %
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Fluoride Salt Melting point(°C)
heat capacity (kJ kg-1 K-1)
Moderating Ratio ξΣs / Σa
Li-Be 458 2.38 63.0
Na-Be 360 2.18 9.8
Li-Na-K 454 1.88 1.7
Na-Zr 510 1.17 6.7
Na-Zr-K 385 1.09 2.9
Li-Na-Zr 460 1.47 12.5
Na-NaB 385 1.51 12.9
Liquid salt, source: ORNL
5
Results Salt Selection Simulations (1/4)
• Voiding of salt introduces large positive reactivity, except for Li-Be salt (FLIBE)• FLIBE has largest negative uniform temperature coefficient• All salts have a negative porosity reactivity coefficient, except FLIBE
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Fluoride salt
k∞ Complete voiding reactivity ($)
Uniform temperature reactivity coefficient (pcm/K)
Porosity reactivity coefficient (pcm/ % porosity)
Li-Be 1.39 -2.30 -7.68 +70
Na-Be 1.11 21.5 -2.53 -860
Li-Na-K 0.71 87.9 8.14 -1290
Na-Zr 1.10 23.0 -0.465 -870
Na-Zr-K 0.81 65.1 5.42 -1310
Li-Na-Zr 1.15 17.7 -1.53 -730
Na-NaB 0.86 56.2 8.32 -1250
Helium 1.36 -0.11 -8.58 +30
Reactivity coefficients for pebbles containing 12 g of uranium with 10% enrichment. All coolants at ambient pressure except Helium (7 MPa)
6
Results Salt Selection Simulations (2/4)
• For FLIBE with a fuel loading less than ~8.5 g per pebble, voiding leads to an increase of k∞
• All other salts have behaviour similar to Li-Na-Zr Fluoride salt
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
The k∞ as a function of the fuel loadings per pebble for FLIBE (left) and Li-Na-Zr(right) combined with the complete voided case
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 0% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 10% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 20% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 30% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 40% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 50% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1 /(H M per pebb le) (1 /g)
k-in
finity
k - in fin ity as func tion o f 1 /H M w ith 60% vo id ing
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 70% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 80% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 90% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 100% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 0% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 10% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 20% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 30% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1 /(H M per pebb le) (1 /g)
k-in
finity
k - in fin ity as func tion o f 1 /H M w ith 40% vo id ing
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 50% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 60% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 70% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 80% voiding
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1 /(H M per pebble) (1 /g)
k-in
finity
k -in fin ity as func tion o f 1 /HM w ith 90% vo id ing
0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1/(HM per pebble) (1/g)
k-in
finity
k-infinity as function of 1/HM with 100% voidingLiF-BeF2 voided LiF-NaF-ZrF4 voided
7
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-12
-10
-8
-6
-4
-2
0
2
4
6
8
dk/d
T (
pcm
K-1
)
1/HM per pebble (g-1)
I II III
Salt voiding due to dT
Total Temperature Effect
Doppler effect
Three zones are identified:I. The Doppler reactivity coefficient and
the coolant temperature feedback reinforce each other
II. Coolant temperature is positive but Doppler coefficient is negative and dominant
III. Coolant temperature (voiding) reactivity coefficient has become dominant
Results Salt Selection Simulations (3/4)
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
The Uniform Temperature Coefficient (pcm/K) as a function of the fuel loadings per pebble for FLIBE
8
Results Salt Selection Simulations (4/4)
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
The Porosity Reactivity Coefficient (pcm/ % porosity) as a function of the fuel loadings per pebble for FLIBE and Li-Na-Zr salts compared to Helium
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-2000
-1500
-1000
-500
0
500
1/(HM per pebble) (1/g)
poro
sity
rea
ctiv
ity c
oeffi
cien
t (pc
m /
%po
rosi
ty)
Helium
LiF-BeF2
LiF-NaF-ZrF4
Loss of forced cooling might lead to floating of fuel pebbles:
• For a limited range of fuel loadings an increase in porosity can lead to increase in k∞ for FLIBE
• Top reflector could be poisoned to avoid increase in k∞
• Neutron leakage is expected to increase (decrease in keff)
9
Conclusions Salt SelectionFLIBE is best candidate for application in LSBPR
• Best moderating quality• Highest k∞ values• Strongly negative temperature reactivity coefficients
Disadvantages FLIBE• Possible floating of the pebbles and effect on k• Cost • Toxicity
FLIBE was selected as primary coolant in LSPBR
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
10
Parameter Design of 2500 MWth LSPBR• Pressure drop calculated with Ergun Relation:
• Mass flow coolant salt is given by :
• P = 2400 MWth, ΔT = 100 K, cp = 2.38 kJ kg-1 K-1 m = 10478 kg s-1
• Two different core shapes have been investigated: Cilindrical & Annular• Resultant pressure drop is less than 1 bar in both cases, pumping power
less than 0.05 % of total power
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
( )2
3
1 170 1 1.75p p
A H mpmd d A
ε μ εε ρ
⎛ ⎞− ⎛ ⎞Δ = − +⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠
p
Pmc T
=Δ
i
.
11
Parameter Design of 2500 MWth LSPBRRelevant dimensions of two core geometries
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Parameter Cylindrical core Annular core
Core height (m) 7.5 7.5
Core outer diameter (m) 3.6 3.7
Inner reflector diameter (m) n.a. 2.0
Core volume (m3) 305.4 299.0
Average estimated power density (MW/m3) 8.19 8.36
Vessel diameter (m) 9.0 9.2
Vessel height (m) 16.6 16.6
Vessel thickness (m) 0.1 0.1
Outer reflector thickness (m) 0.8 0.8
Top and bottom reflector thickness (m) 1.5 1.5
12
Steady State Operation (1/2)Results of steady state calculations for annular and cylindrical core
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Parameter Cylindrical core
Annular core
Power level (MWth) 2500 2500
Average power density (MWth m-3) 8.19 8.36
Maximum power density (MWth m-3) 16.8 14.7
Peak factor 2.05 1.75
Average velocity of Salt (m s-1) 0.36 0.37
Reynolds number in pebble bed 15700 16100
Coolant inlet temperature (°C) 900 900
Coolant outlet temperature (°C) 1000 1000
Maximum coolant temperature (°C) 1051 1028
Maximum fuel Temperature (°C) 1190 1152 0 50 100150 200250300 360400
500
600
700
800
900
1000
1100
Powerprofile cylindrical core (MW/m3)
radius (cm)
heig
ht (c
m)
4
6
8
10
12
14
16
0 50 100150200250300 370400
500
600
700
800
900
1000
1100
Powerprofile annular core (MW/m3)
radius (cm)
heig
ht (c
m)
4
6
8
10
12
14
16
Power density profiles in cylindrical (left) and annular core (right)
• Compared to cylindrical core the annular core has: lower peak factor & lower corresponding maximum power density
13
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1900
950
1000
1050
1100
1150
1200
Relative Axial position (top-to-bottom)
Tem
pera
ture
(o C
)
Axial temperature profiles of fuel and coolant
max fuel temp Cy lindermax cool temp Cy lindermax fuel temp Annularmax cool temp Annular
Steady State Operation (2/2)Maximum axial fuel and coolant temperatures of the 2500 MWth LSPBR
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
• Maximum fuel temperatures:Cylinder LSPBR ~1190°CAnnular LSPBR ~1151°C
• Due to lower peaking in the annular core, maximum fuel and coolant temperatures are lower
• Calculations were performed with homogeneous core, continuous refueling moves maximum power density to top of core (the cooler region)
14
Various power levels were simulated to determine maximum power level without exceeding limits on fuel and coolant temperature
• Fuel failure at 1600 °C; FLIBE boiling at 1430 °C
• Cylindrical geometries simulated for 40 hrs of real time
• Two situations were simulated:- Pebble bed core- Pebble bed core with
additional salt plenum
Decay Heat Removal Calculations
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
refl
core
plenum
15
Decay Heat Removal Results (1/2)Maximum fuel temperatures as a function of time with initial power as parameter, geometry without additional salt plenum
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
0 5 10 15 20 25 30 35 40900
1000
1100
1200
1300
1400
1500
1600
1700
time(h)
Tem
pera
ture
T(o C
) boilingtemp salt
max allowable fuel temp 2500 MW
2100 MW
2000 MW
1500 MW
• After initial increase in temperatures, natural convection flow develops
• With increase in natural convection flow, convective heat transfer increases
• Then coolant and fuel gradually cool
• Maximum power without additional salt plenum: 2000 MWth
16
Decay Heat Removal Results (2/2)Maximum fuel temperatures as a function of time with initial power as parameter, geometry with additional salt plenum
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
0 5 10 15 20 25 30 35 40900
1000
1100
1200
1300
1400
1500
1600
time(h)
Tem
pera
ture
( o C
)
boiling temperature salt
2000 MW
3000 MW
4000 MW
5000 MW
With the salt plenum:
• The total volume of salt is larger by a factor 3.5
• The thermal inertia of the reactor is increased
• The outer surface of the reactor is increased
• Maximum power withadditional salt plenum: 4000 MWth
17
Conclusions
• From the 7 liquid salt candidates considered, the best choice for the LSPBR is Li-Be fluoride salt (FLIBE)
• The height of the pebble bed was not restricted by the pressure drop (< 1 bar)
• Because of lower maximum fuel and coolant temperatures, the annular core shape has preference for the LSPBR
• Maximum allowable nominal power is 2000 MWth without salt plenum and 4000 MWth with additional 7.5 m high salt plenum
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
18
Further work
• Burnup analysis; simulation of on-line refuelling; other fuel types and fuel cycles
• Improved heat transfer modeling • Improved modeling of floating pebbles • Detailed transient analysis, including effects of floating
pebbles, boiling salt etc.• Possibility of natural convection salt-cooled reactor• Possibility of other salts (less toxic, cheaper, etc)
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
19
End of presentation
Thank you for your attention
Questions?
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
20
Steady State Operation Thermal Hydraulics (THERMIX) and Neutronics (EVENT) Coupling
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Guessed Temperature Field T(r,z)
k-eff =1? New Guessednormalization a
EVENTGEMXSlibrary
THERMIXTn(r,z)
P (r,z)
σi,j (T ) σi,j
n-times
Ψ (r,z)
Guessed normalization a
INPUT
no
yes
Tn(r,z) -Tn-1(r,z) < θ?
T(r,z) k-effn
yes
no
THERMIX-EVENT coupling
Found Steady State Solution!
P = aΨ
Guessed Temperature Field T(r,z)
k-eff =1? New Guessednormalization a
EVENTGEMXSlibrary
THERMIXTn(r,z)
P (r,z)
σi,j (T ) σi,j
n-times
Ψ (r,z)
Guessed normalization a
INPUT
no
yes
Tn(r,z) -Tn-1(r,z) < θ?
T(r,z) k-effn
yes
no
THERMIX-EVENT coupling
Found Steady State Solution!
P = aΨ
21
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
Difference between THERMIX and HEAT
THERMIX - written for gas-cooled reactors
HEAT - written for fluidized bed applications
fuel pebbles
liquid salt coolant
reflector
qC
heat conduction
fuel pebbles
liquid salt coolant reflector
qC
convective heat transfer
22
Salt Selection SimulationsTo asses the effect of salt candidates on neutronics, simulations were performed on an infinite array of fuel pebbles with salt coolant:
• Effect of salt voiding on k∞• Effect of temperature on k∞• Effect of pebble packing fraction on k∞
Calculations performed with SCALE code system using JEF2.2 data.
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
TRISO Fuel Pebble
coolant region
graphite layer
homogenized fuel region
23
In a Loss of Forced Cooling incident (LOFC) the fission decay heat is not removed by the coolant
• Decay Heat Power is 7 % directly after shutdown…
• Decay Heat must be removed by:
• To examine temperature distribution during a LOFC with SCRAM, simulations were performed with code HEAT
Passive Decay Heat Removal
Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)
Introduction Selection-of-Salt Dimensioning Steady-State Decay-Heat-Removal Conclusions
core refl & vessel air
natural convection
convective heat transfer
conduction
thermal radiation
Decay heat power during transients