Specialized SiC Components for Flow Channel Insert Applications R.J. Shinavski Hyper-Therm HTC Huntington Beach, CA  714-375-4085 robert.shinavski@htcomposites.com FNST Meeting UCLA, Los Angeles, CA August 12, 2008 Hyper-Therm High-Temperature Composites, Inc.

Introduction SiC fiber-reinforced silicon carbide matrix (SiC/SiC) composites combine the attributes of high temperature mechanical strength and toughness with relative dimensional stability under high neutron fluence that address the primary requirement of survivability in application as a flow channel insert SiC/SiC composites composed of near-stoichiometric SiC fibers (Hi-Nicalon Type S, Tyrannos SA3) and a CVI SiC matrix have shown promise for use in high neutron flux environments and have been termed nuclear grade SiC/SiC Mechanical, thermal, and electrical properties for nuclear grade SiC/SiC composites will be discussed with regards to the flow channel insert application Designs for a nuclear grade SiC/SiC flow channel insert will also be discussed Hyper-Therm High-Temperature Composites, Inc.

SiC/SiC Composites SiC matrix produced by isothermal/isobaric CVI MLSiC fiber coating 0º Fibers 90º Fibers Hi-Nicalon Type S CVI SiC SiC matrix produced by isothermal/isobaric CVI Composite bulk densities 2.7 g/cm3 Hi-Nicalon Type S fiber selected due to greater existing database on this fiber showing radiation resistance Fiber coating is Hyper-Therm HTC’s MLSiC fiber coating (US Patents 5,455,106 and 5,480,707) Hyper-Therm High-Temperature Composites, Inc.

Tensile Properties of Nuclear Grade SiC/SiC Tensile strength dependent on architecture with consistent 0.5% failure strain SiC/SiC FCI needs to stay below the proportional limit strength to maintain Pb-Li impermeability High ultimate strength and failure strain provide fail-safe behavior 5 HS construction considered primary candidate with unidirectional used for joining Architecture Fiber Volume σf εf Unidirectional 47% 720 MPa 0.47% 5 Harness Satin 36% 400 MPa 0.54% Plain Weave 30% 350 MPa 0.45% Triaxial Braid 23% 190 MPa 0.50% Hyper-Therm High-Temperature Composites, Inc.

Tensile Properties of Nuclear Grade SiC/SiC Tensile properties are insensitive to temperature up to 1200ºC (data still being accumulated) Room temperature tensile testing of 4 batches of nuclear grade SiC/SiC; 16 total samples were tested Statistical allowable calculated Acceptable stress levels should be less than 157 MPa if no matrix cracking is to occur Mechanical Properties of Nuclear Grade SiC/SiC (5HS) E σf εf σPL σILSS(RT)/σILSS(800C) Mean 270 400 MPa 0.54% 180 MPa 42.3/37.9 MPa B-basis Allowable* --- 344 MPa 0.39% 157 MPa 23.9/27.0 MPa * 95% confidence that 90% of the material will be greater than the allowable Hyper-Therm High-Temperature Composites, Inc.

Irradiation Dimensional Stability Dimensional change under neutron irradiation is fairly small Depends on irradiation temperature and is independent of irradiation fluence after reaching saturation Greater swelling at lower temperatures 500ºC temperature difference will induce ~0.21% strain Strain of ~0.13% for 500ºC-300ºC Irradiation induced strains opposite thermal expansion strains Newsome et al., J. Nuclear Mat’ls, 371, 2007, pp 76-89 Hyper-Therm High-Temperature Composites, Inc.

Thermal Expansion Thermal expansion of nuclear grade SiC/SiC is essentially isotropic Mean CTE (RT-300ºC)=3.51 ppm/ºC (RT-500ºC)=3.92 ppm/ºC (RT-800ºC)=4.38 ppm/ºC Approximate strain for 500ºC-300ºC thermal gradient is 0.09% Greater temperature difference through wall of the FCI will result in an increasing effect of thermal expansion as compared to irradiation dimensional change, but becomes more unstable due to balance of larger dimensional changes Balance of dimensional changes (thermal and irradiation) within nuclear grade SiC/SiC is within the allowable strain even if restrained Hyper-Therm High-Temperature Composites, Inc.

Pb-Li Compatibility Nuclear grade SiC/SiC has been exposed to Pb-Li at up to 360ºC White residue remaining on surface identified to be LiOH No LM penetration or degradation observed within composite Planned testing includes higher temperatures; overpressure and as a function of pre-stress level Hyper-Therm High-Temperature Composites, Inc.

Electrical Properties Electrical conductivity measured in the through-thickness and in-plane directions Through-thickness electrical conductivity is ~3 orders of magnitude lower than in-plane conductivity In-plane conductivity dominated by small amount of carbon in fiber coating Meets low through-thickness electrical conductivity requirement to minimize magnetohydrodynamic pressure drop Hyper-Therm High-Temperature Composites, Inc.

Thermal Conductivity Through-thickness thermal conductivity of nuclear grade SiC/SiC is too high to be a sufficient thermal insulator Additions of N were examined SiCxNy composition reduced thermal conductivity to 2.5 W/m/K However composition not neutronically favorable and this matrix material will likely also have poor irradiation stability Low N additions to CVI SiC had minimal effect on t-c Through-thickness thermal conductivity of nuclear grade SiC/SiC does not meet the 1-2 W/m/K targeted requirement for FCI Hyper-Therm High-Temperature Composites, Inc.

Architectural Construction of FCI Add thermal conductivities as thermal resistances in series with flutes added in parallel to calculate equivalent “bulk” through thickness thermal conductivity Examined strut angle and frequency For lower thermal and electrical conductivity, minimize strut cross-section and number of struts/unit length For lower thermal conductivity and a higher electrical conductivity, maximize the core thickness and minimize the face sheet thickness Hyper-Therm High-Temperature Composites, Inc.

Low Thermal Conductivity Construction Equivalent thermal conductivity of 1.4 W/m/K is predicted 1.0 mm face sheets with 0.5 mm struts Possibility of engineered high compliance in core to mitigate deformation in the composite Plan to fabricate and measure equivalent t-c 5 mm 18 mm Hyper-Therm High-Temperature Composites, Inc.

End Close-Out Address need to close-out ends to prevent LM penetration Working with PNNL to adapt Ti3SiC2 joining technology to nuclear grade SiC/SiC and to pressureless fabrication Current concept is to use unidirectional nuclear grade SiC/SiC pins to provide mechanical restraint to bond and Ti3SiC2 is for sealing only Plan to evaluate for LM penetration and stress/strain limit for joint region Ti3SiC2 SiC Hyper-Therm High-Temperature Composites, Inc.

Anticipated Loading of FCI Dimensional change resulting from through-thickness thermal gradient dominates loading Results in differential expansion of inside and outside of FCI Irradiation induced swelling is greater than thermal expansion difference Slot would allow free expansion and minimal stresses if unrestrained End close-out and edges create localized restraints, which result in interlaminar stresses Deformation will be asymmetric Can be modeled as combined effect of irradiation and thermal expansion Irradiation Induced Swelling Thermal Expansion Hyper-Therm High-Temperature Composites, Inc.

Alternate FCI Design Closed box section provides greater geometric stability and symmetric deformations Assumes only purpose of slot is pressure equalization Restraint in closed section increases in-plane stresses, but reduces interlaminar stresses Very important for fluted core construction FEM planned to determine which approach is minimal stress Hyper-Therm High-Temperature Composites, Inc.

Summary Developing database of mechanical, electrical and thermal properties for Nuclear Grade SiC/SiC with respect to the flow channel insert Current data indicates that Nuclear Grade SiC/SiC meets all requirements of FCI with the notable exception of through-thickness thermal conductivity Design of composite as a structure itself allows thermal conductivity to be engineered to meet FCI requirements within known manufacturing capabilities Measure elevated temperature mechanical properties Directly measure effective through-thickness thermal conductivity of SiC/SiC engineered fluted core structure Develop end close-out method to seal core of FCI Finite element modeling of SiC/SiC structure to address end effects and demonstrate that SiC/SiC structure meets mechanical requirements Produce sub-scale FCI and subject to thermal difference that simulates anticipated strain from combined irradiation and thermal loading Planned Work Hyper-Therm High-Temperature Composites, Inc.

Acknowledgment We would like to acknowledge Department of Energy – (National Nuclear Security Administration) SBIR Funding under Award Number DE-FG02-07ER84717 This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Hyper-Therm High-Temperature Composites, Inc.