The High-Temperature HTV Graphite Irradiation Capsule for the High Flux Isotope Reactor at Oak Ridge National Laboratory

J.L. McDuffee, T.D. Burchell, K.R. ThomsSeptember 18, 2013

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Purpose

• The key data to be obtained from the graphite specimens are– Dimensions, volume– Mass, density

• Data are critical to the design of the NGNP and the high-temperature graphite irradiation creep capsule (AGC-5) planned for irradiation in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL)

• Supports ongoing work in the area of model development; e.g., irradiation effects models such as dimensional change, structural modeling, and fracture modeling

• Used to underpin the American Society of Mechanical Engineers (ASME) design code currently being prepared for graphite core components.

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Specimens

5.33 cm(0.210 in)

7.62, 8.89, 10.16 mm(0.300, 0.350, 0.400 in)

2.1 mm(0.082 in)

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Specimens• PCEA (15 samples)

– Supplied by Graftech International– Country of origin: Germany/France– Petroleum coke, extruded, medium grain e

• NBG-18 (14 samples)– Supplied by SGL Carbon– Country of origin: Germany/France– Pitch coke, vibrationally molded, medium grain

• IG-110 (14 samples)– Supplied by Toyo Tanso– Country of origin: Japan– Petroleum coke, isostatically molded, fine grain

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Specimens• NBG-17 (9 samples)

– Supplied by SGL Carbon– Country of origin: Germany/France– Pitch coke, vibrationally molded, medium grain

• Grade 2114 (13 samples)– Supplied by Mercen– Country of origin: USA– Nonpetroleum coke, isostatically molded, super fine grain

• H-451 (7 samples)– Supplied by SGL Carbon– Country of origin: USA– Petroleum coke, extruded, medium grain, no longer in production

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Overall Design• 1 capsules with 8 subcapsules

– Each subcapsule has one design temperature– 9 specimens per subcapsule– 64 specimens total

• HTV capsule will be irradiated for 2 cycles (3.2 dpa)

• Design goal is to distribute specimens as evenly as possible across fluence and temperature

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Graphite Specimen Distribution in the HTV Capsule

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The High Flux Isotope Reactor• Pressurized, light-water-cooled and –

moderated, flux-trap-type reactor

• HEU fuel — U3O8 dispersed in aluminum

• Two annular fuel elements

• Center cylindrical flux trap, 12.70 cm diameter

• Nominally 6 cycles/year, with a 25 day cycle length

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Irradiation Capsule Design

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Irradiation Subcapsule Design

Centering thimble

Specimen

POCO graphite sleeve

Thermometry (SiC or Graphite

Nb1Zr holder

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Irradiation Capsule Design• Subcapsule separators

– Stack of grafoil wafers held together with a molybdenum tube & washer

– Dosimetry is located in cutouts in separators– Grafoil provides axial insulation between

subcapsules

• Nb1Zr centering thimble– Contains specimens inside holder– Radial prongs center holder inside outer housing– Small contact surface area minimizes heat loss

• Critical for 1500 ºC capsules

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Irradiation Capsule Design

• Subcapsule holder– Nb1Zr holder is tapered from the

middle to each end to compensate for axial heat losses

– POCO graphite liners (~0.5 mm thick) prevent potential sticking between the specimens and the Nb1Zr due to prolonged exposure at high temperature

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Relative Importance of Modeling InputsInitial gas gap sizeHeat generation rateThermophysical properties

· Modeling approach is also a significant contributor

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Heat Generation Rate in Materials

2013 — Design for Irradiation Experiments

fission neutrons

fission photons

fission product photons

n, reactions

decay

Nb1Zr Graphite

Core fission neutrons <1% 11%

Core fission photons 64% 57%

Core fission product photons 36% 31%

Local decay — —

Relative Contributions to the Total Heat Generation Rate

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Heat Generation Rate in HFIR• Axial profile is strong, but relatively

independent of material

• Radial profile in the flux trap is weaker, but material dependent

• Radial profile in the reflector can be large

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0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

-25 -20 -15 -10 -5 0 5 10 15 20 25

Peak

ing

Fact

or

Location from Horizontal Midplane (cm)

EOC-TRRH-Water

EOC-TRRH-Steel

EOC-TRRH-V4Cr4Ti

EOC-TRRH-Al6061

EOC-PTP-Water

EOC-PTP-Steel

EOC-PTP-V4Cr4Ti

EOC-PTP-Al6061

EOC-HT-Water

EOC-HT-Steel

EOC-HT-V4Cr4Ti

EOC-HT-Al6061

EOC correlation

s+ = 31.9 cms- = 31.8 cm

Axial Peaking Factor

Radial Heat Generation Factor

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Thermal Modeling• Temperature is controlled by the size and composition of the fill gas

– Inert gases are most common: helium, neon, and argon

Temperature is controlled by the outer gas gap

0.00000.00050.00100.00150.00200.00250.00300.00350.00400.00450.0050

300 500 700 900 1,100 1,300 1,500Temperature (°K)

Ther

mal

con

duct

ivity

(W/c

m-°

K)

Helium

Neon CO2

Argon

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Thermal Modeling With Finite Elements• Small gap modeling

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Specimen/holder region

Gas gap

Housing1100°C over 0.33 mm = 3-4°C/µmThermal expansion ≈ 35 µm

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Conduction Through a Small Gas Gap• Thermal jump condition

– Important at small gap sizes typical in irradiation experiments

– Accounts for inefficiency in energy transfer between the gas molecules and the solid surface• especially important when

MWgas ≠ MWwall

– Modifies Fourier’s Law by adding a small extra conduction length on each side

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gs2

gs1

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Conductance Between Parts in Contact

• In real contact, two surfaces never truly conform at the microscopic level

• To simplify analysis, the effective heat transfer coefficient is divided into two parts:– Solid spot conductance, hs

• Represents conductance at the solid-solid interface points

– Gap conductance, h

• Represents conductance through the interstitial gap

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Preliminary Temperature Modeling

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Summary• Objective is to provide design data for NGNP relevant graphites

– Neutron dose range of 1.5 to 3.2 dpa– Irradiation temperatures of 900°C, 1200°C, and 1500°C

• Specimens are PCEA, NBG-17, NBG-18, IG-110, 2114, and H-451 (for reference)

• High temperatures are achieved through thermal barriers between subsections and profiled gas gaps

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Questions?