1. Specialized SiC Components for Flow Channel Insert Applications
R.J. Shinavski
Hyper-Therm HTC
Huntington Beach, CA 
FNST Meeting
UCLA, Los Angeles, CA August 12, 2008
Hyper-Therm High-Temperature Composites, Inc.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.