core physics characteristics and issues for the …1 core physics characteristics and issues for the...

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Core Physics Characteristics and Issues for the Advanced High-Temperature

Reactor (AHTR)

D.T. Ingersoll, E. J. Parma,C.W. Forsberg, J.P. Renier

ARWIF 2005Advanced Reactors With Innovative

Fuels (and Coolants)

February 16-18, 2005

OAK RIDGENATIONAL LABORATORYOAK RIDGENATIONAL LABORATORY

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AHTR ≠ MSR(Solid fuel; salt coolant) (Fuel dissolved in salt)


3

Passively Safe Pool-Type Reactor Designs

High-Temperature Coated-Particle

Fuel

The AdvancedHigh-Temperature

Reactor Combining Existing

Technologies in a New WayGeneral Electric

S-PRISM

High-Temperature, Low-Pressure

Molten-Salt CoolantBrayton Power Cycles

GE Power Systems MS7001FB


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AHTR Fills the High-Temperature, High-Power Need

0

800

1000

Tem

pera

ture

(°C

)

200

400

600

0 1000 2000

Light Water Reactor (High Pressure)

Liquid Metal Fast Reactor(Low Pressure)

Range of Hydrogen Plant Sizes

European Pressurized-Water

Reactor

Helium-Cooled VHTR (High-Pressure)

Brayton (Helium or Nitrogen)

Thermo-chemical Cycles

Rankine (Steam)

Electricity Hydrogen

Application

Liquid Salt Systems (Low Pressure)• Heat Transport Systems• Advanced High-Temperature Reactor (Solid Fuel)• Molten Salt Reactor (Liquid Fuel)• Fusion Blanket Cooling

General Electric ESBWR

Electricity (MW)


03-243R2

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2400 MW(t) AHTR Nuclear Island Has Similar Size To 1000 MW(t) GE S-PRISM

• Similar vessel size (9 m dia)− Space for 2400 MW(t)

AHTR core with low power density

• Similar equipment size due to larger volumetric heat capacity of liquid salt

• Higher capacity decay heat removal system due to higher vessel temperature

• Higher electrical output− S-PRISM: 380 MW(e)− AHTR: 1200 MW(e)


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The AHTR Uses Coated-Particle Graphite-Matrix Fuel Elements

• Same fuel as used in gas-cooled high-temperature reactors

• Peak operating limit: 1250ºC

• Failure temperature: 1600ºC

• Graphite blocks provide neutron moderation and heat transfer to coolant


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Models of Conceptual AHTR Design

Reactor CavityCooling Ducts

Reactor Core

MS-MS Heat Exchanger

Spent Fuel Storage

Turbine Hall With MS-Gas HX


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AHTR 9.0m Vessel Allows 2400 MW(t) Core

Elev. 0.0 m

Elev. -2.9 m

Elev. -9.7 m

Elev. -19.2 m

Elev. -20.9 mElev. -21.1 m

Reactor Closure

Cavity Cooling Channels

Floor Slab

Cavity Cooling Baffle

Cavity Liner

Guard Vessel

Reactor VesselGraphite Liner

Outer Reflector

Reactor Core

Inner Reflector

Coolant Pumps

Control Rod Drives

Siphon Breakers

102 GT-MHR fuel columns222 Additional fuel columns324 Total fuel columns

Power density = 8.3 MW/m3


03-155

9

AHTR Fuel Block (standard GT-MHR block)

216 Fuel channels (12.7 mm diam)

108 Coolant channels (9.53

mm diam)Block Height:793 mm

360 mm Fuel handling hole

19.0 mm


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AHTR And GT-MHR Have Similar Neutronics

10% enriched U

Fuel Volume Fraction0.0 0.1 0.2 0.3 0.4 0.5 0.6

K e

ffect

ive

0.9

1.0

1.1

1.2

1.3

1.4

1.5

GT-MHR

• Excess reactivity similar for given core loading

• Neutron lifetime ~1ms• keff increases with higher

moderator to fuel ratio (undermoderated in design region)

• Large negative temperature feedback due to Doppler effects (~ -$0.01/K)

• Similar fuel burnup/ fuel cycle behavior


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AHTR Burnup Predictions for Different Fuel Enrichments (3 Zone Core)

Time (days)0 360 720 1080

k ef

fect

ive

0.9

1.0

1.1

1.2

1.3

1.4Heterogeneous C moderator0.27 fuel vol frac, 10%enr U-235Three Radial Regions2400 MW, core vol = 290 m3Initial Loading

Shift and ReloadOuter at 540 days



10% Enrichment (shuffle time ~240 d) 20% Enrichment (shuffle time ~540 d)

Time (days)0 360 720 1080 1440 1800 2160 2520 2880 3240

k ef

fect

ive

0.9

1.0

1.1

1.2

1.3

1.4

1.5Heterogeneous C moderator0.27 fuel vol frac, 20%enr U-235Three Radial Regions2400 MW, core vol = 290 m3Initial Loading





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Key Difference: AHTR Void CoefficientDepends on Salt Composition and Core Configuration

10% enriched U

Fuel Vo lum e Fraction

0.0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6

Rea

ctiv

ity -

Com

plet

e Vo

idin

g ($

)

-2-10123456789

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50% N aF - 50% ZrF4F libe - 66% LiF - 34% B F2

with BoronPoison ing

Graphite Matrix

Fuel Compact

Coolant Fraction = 10%Fuel Fraction = 50%

Coolant Channel Radius = 0.4 cmFuel Radius = 1.265 cmPitch = 3.407 cm

Fuel Particle Packing Fraction = 0.3

Coolant Channel


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Void Coefficient vs. Salt Choice SNL Model With No Burnable Poisons; Pure 7Li in Salt

Salt Total Void Reactivity Effect ($)

BeF2

LiF/BeF2 (66/34)MgF2/BeF2 (50/50)-------------------------LiF (Li-7)

ZrF4/BeF2 (50/50)ZrF4/LiF (52/48)-------------------------NaF/BeF2 (57/43)ZrF4

NaF/ZrF4 (25/75)NaF/ZrF4 (50/50)NaF/ZrF4 (75/25)NaF

-1.46

-0.47-0.49

-------------------------+0.16

+0.43+1.25

-------------------------+1.82+1.41+1.88+2.64+3.82+7.05

• Example for 10% coolant fraction, 50% fuel fraction and complete core voiding

• Moderation benefit dominates for lower-Z elements in salt

• Absorption dominates for higher-Z elements in salt

Ranking (best to worst)Be, Li-7, Mg, Zr, Na


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Impact of Burnable Poisons and 7Li Purity on Void Coefficient – ORNL Model

• 2LiF-BeF2 Salt• 1 mol% VF3 Buffer• 102-column core (600MW)• 14 BR rods per assembly• 14 wt% 235U enrichment

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25Erbium Loading In BP Rods (g)

Void

Coe

ffici

ent (

$)

Pure 7Li

0.01% 6Li in lithium


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Variation of Void Coefficient With Fuel Fraction and Enrichment

Fuel Volume Fraction0.0 0.1 0.2 0.3 0.4 0.5 0.6

Rea

ctiv

ity-C

ompl

ete

Void

ing

($)

-2-10123456789

1011

50%NaF - 50%ZrF4

Flibe - 66%LiF (0%Li-6) 34%BeF2

Flibe - 66%LiF (0.01%Li-6) 34%BeF2

10% enriched U

U Enrichment0.00 0.05 0.10 0.15 0.20

Rea

ctiv

ity-C

ompl

ete

Void

ing

($)

-10123456789

1011

50%NaF - 50%ZrF4

Flibe - 66%LiF (0.01%Li-6) 34%BeF2

Flibe - 66%LiF (0%Li-6) 34%BeF2

0.27 Fuel Volume Fraction


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AHTR Transient Behavior With Competing Feedback Effects

Example: Na-Zr Salt (worst salt) with 20% Flow Blockage: +$0.40 Instantaneous Reactivity Insertion

Time (s)0 10 20 30 40 50 60 70 80 90 100

Pow

er (M

W)

2000

2500

3000

3500

4000

4500

5000

Cor

e Av

erag

e Te

mpe

ratu

re (o C

)

900

925

950

975

1000

1025

1050

Reactivity Addition = +$0.4Neutron Lifetime = 1.4 msTemp. Feedback = -0.009/K2400 MW, core vol = 90 m3

Power

Temperature

• Core power increases but is mitigated by increase in fuel temp of ~60oC

• Slow transient (10’s of seconds)

• Core reaches lower equilibrium power

• Concern is heat-up of blocked fuel columns (~9 oC/s)


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Conclusions on Void Coefficient• Decreases with increasing uranium loading and

increasing burnable poison loading• Depends on the neutron spectrum – decreases with

increasing U/C ratio• Is very sensitive to 7Li isotopic purity in 2LiF-BeF2 salt• Increases with increasing coolant hole diameter• Relatively insensitive to fuel burnup• Options for reducing:

− higher fuel loading (volume fraction or enrichment)− higher burnable absorber loading− poisoning the graphite blocks − Different fuel/coolant geometry

• Need substantial neutronics analysis to evaluate options


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Lithium Purity Considerations

• Large inventory of 99.99% 7Li is available

• Enriching to 99.999% 7Li (0.001% 6Li) will be very expensive

• 6Li level will eventually reach equilibrium at 0.001%− Burnout of initial 6Li “contamination”− Production of 6Li primarily from Be(n,α) reaction − Will take a few years to reach equilibrium

• Need to develop acceptable design with 4-9s 7Li (maybe 0.99995)


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Heterogeneous Fuel Designs May Help Ensure A Negative Void Coefficients

Homogeneous Fuel Heterogeneous Fuel

18.1 mm

360 mm

Block Height:793 mm

Coolant Channels(108)

Fuel Handling Hole

Fuel Channels(116)

Coolant Channels

Fuel Channels

OAK RIDGENATIONAL LABORATORYOAK RIDGENATIONAL LABORATORY 04-037

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Future Physics Investigations

• Control rods (number and location)• Reserve shutdown mechanism• Power density• Power peaking• Decay heat• Modeling the fuel double heterogeneity• Validation of methods

Now in progress


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