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

1. 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. Thoms September 18, 2013

2. 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.

3. Specimens 7.62, 8.89, 10.16 mm (0.300, 0.350, 0.400 in) 2.1 mm (0.082 in) 5.33 cm (0.210 in)

4. 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

5. Specimens • NBG-17 (9samples) • 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

6. Overall Design • 1 capsuleswith 8 subcapsules • Each subcapsule has one design temperature • 9specimens 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

7. Graphite Specimen Distribution in the HTV Capsule

8. 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 The High Flux Isotope Reactor

9. Irradiation Capsule Design

10. Irradiation Subcapsule Design Centering thimble Specimen POCO graphite sleeve Thermometry (SiC or Graphite Nb1Zr holder

11. 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

12. 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

13. Relative Importance of Modeling Inputs Initial gas gap size Heat generation rate Thermophysical properties • Modeling approach is also a significant contributor

14. 2013 — Design for Irradiation Experiments Heat Generation Rate in Materials fission neutrons n, reactions fission photons fission product photons  decay Relative Contributions to the Total Heat Generation Rate

15. 2013 — Design for Irradiation Experiments Heat Generation Rate in HFIR Radial Heat Generation Factor • 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 Axial Peaking Factor

16. 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 2013 — Design for Irradiation Experiments

17. 2013 — Design for Irradiation Experiments Thermal Modeling With Finite Elements • Small gap modeling Specimen/holder region Gas gap 1100°C over 0.33 mm = 3-4°C/µm Housing Thermal expansion ≈ 35 µm

18. 2013 — Design for Irradiation Experiments Conduction Through a Small Gas Gap gs1 • 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  gs2

19. Conductance Between Parts in Contact • In real contact, two surfaces never truly conform at the microscopic level d • 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, hd • Represents conductance through the interstitial gap

20. Preliminary Temperature Modeling

21. Summary • Objective is to provide design data for NGNP relevant graphites • Neutron dose range of 1.5 to 3.2dpa • Irradiation temperatures of 900°C, 1200°C, and 1500°C • Specimens are PCEA, NBG-17, NBG-18, IG-110, 2114, and H-451 (forreference) • High temperaturesareachievedthrough thermal barriersbetweensubsectionsandprofiled gas gaps

22. Questions?