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

INGSM 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.

INGSM Specimens 5.33 cm (0.210 in) 7.62, 8.89, mm (0.300, 0.350, in) 2.1 mm (0.082 in)

INGSM 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

INGSM 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

INGSM 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

INGSM Graphite Specimen Distribution in the HTV Capsule

INGSM The High Flux Isotope Reactor Pressurized, light-water-cooled and – moderated, flux-trap-type reactor HEU fuel — U 3 O 8 dispersed in aluminum Two annular fuel elements Center cylindrical flux trap, cm diameter Nominally 6 cycles/year, with a 25 day cycle length

INGSM Irradiation Capsule Design

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

INGSM 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

INGSM 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

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

INGSM Heat Generation Rate in Materials 2013 — Design for Irradiation Experiments fission neutrons fission photons fission product photons n,  reactions  decay Nb1ZrGraphite Core fission neutrons<1%11% Core fission photons64%57% Core fission product photons36%31% Local  decay —— Relative Contributions to the Total Heat Generation Rate

INGSM Heat Generation Rates Beginning of CycleEnd of Cycle MaterialDecay Prompt Fission Fission Product DecayTotal Prompt Fission Fission Product DecayTotal Design Graphite Nb1Zr Molybdenum Al Q fp_decay /Q prompt ≈ for Nb1Zr, Moly, Al = 0.45 for Graphite

INGSM 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 2013 — Design for Irradiation Experiments Axial Peaking Factor Radial Heat Generation Factor

INGSM 2013 — Design for Irradiation Experiments 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

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

INGSM 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 MW gas ≠ MW wall – Modifies Fourier’s Law by adding a small extra conduction length on each side 2013 — Design for Irradiation Experiments g s2 g s1 

INGSM 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, h s Represents conductance at the solid-solid interface points – Gap conductance, h  Represents conductance through the interstitial gap

INGSM Preliminary Temperature Modeling

INGSM 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

INGSM Questions?