Scoping the Potential of Mobile Tiles for and IFE Power Plant

1. Scoping the Potential of Mobile Tiles for and IFE Power Plant Lance Snead, Hsin Wang, Jim Kiggans Oak Ridge National Laboratory Igor Sviatoslavsky, Mohamed Sawan,Carol Alpine, Greg Sviatoslavsky University of Wisconsin Presented at the HAPL Meeting PPPL, Princeton, NJ December 12-13, 2006

2. Basic Idea • Fundamental problem with graphite is solved by limiting residence time of graphite tile in chamber and post-processing tile in vacuum furnace. -- Post processing restores graphite properties -- Post processing removes tritium -- Erosion mitigated by limiting time in chamber ----> let’s consider recycling the tiles…. • Material and Design: Intermediate quality graphite tile similar to matrix nuclear graphite (good thermal conductivity, very high fracture toughness.) • Tile rides on rail from top of reactor to bottom, through furnace, inspect, back to the top of the reactor.

3. The original twisted biscuit LAF Rail Na Coolant Side View: View Face Changes With Twist of Oval Rail Chamber Support Composed Of Twisting Metallic Oval Rail Top View With Oval Rail Graphite + Breeder + Multiplier

4. H451 Graphite 900°C 2 dpa 600°C

5. Tritium Outgas & Property Recovery (1200-1300°C, hours) 3T remove

6. replace Recycle & Re-fabricate (1100-1200°C) Fresh C + Be + Li2O bad Inspection & Storage good Tritium Outgas & Property Recovery (1100-1200°C, hours) 3T remove

7. Geometric Description • The chamber is 10 m in radius. • Tiles are 15 cm X 15 cm X 6 cm • Tiles are supported on oval cooling channels. As the tiles move down on the cooling channels, they twist such that at mid-plane they face the target 15 cm X 15 cm. • The tiles are inserted at the top, 105 tiles at 1 m radius, 105 tiles at 2.5 m radius and 210 tiles at 5 m radius. • There are 25,200 identical tiles in the chamber at any one time. • At replacement time, the tiles slide down on the cooling channels and are removed at the bottom. • Blankets are located behind the tiles as shown in the figure.

8. Neutronics Assessment and Assumptions • Neutronics calculations assess breeding potential as a function of ceramic breeder content and lithium enrichment • Used HAPL target spectrum in 175 neutron, 42 gamma groups • 4 cm graphite tiles with coolant rails (75% C, 10% FS, 15% Na) • 1 m blanket made of 80% C/breeder mixture, 10% FS, 10% Na • Assessed replacing FS/Na in tiles and blanket by SiC/He • A zone consisting of 85% FS, 15% He used behind blanket to represent reflection from shield/VV • Lithium silicate (Li4SiO4) used as ceramic breeder (breeder potential nearly the same for different ceramic breeders) • Required local (1-D) TBR>1.15 for tritium self-sufficiency

9. Local TBR : No Breeding Materials FW tiles 75% graphite 10% structure 15% coolant Blanket X% Li4SiO4 (80-X)% graphite 10% structure 10% coolant • Largest TBR achieved with high breeder content / low Li enrichment • Achievable TBR not adequate • Replacing FS/Na by SiC/He is not Helpful

10. Local TBR : Addition of Be2C FW tiles 55% graphite, 20% Be2C, 10% FS, 15% Na Blanket X% Li4SiO4, Y% Be2C, (80-X-Y)% graphite, 10% FS, 10% Na Be2C content was limited to 20% • Adding 20% Be2C in FW tiles and blanket results in ~15% increase in TBR • Largest achievable local TBR is 1.108 with 50% Li4SiO4, 20% Be2C, 10% C, 10% FS, 10% Na in blanket and 30% lithium enrichment. This value is getting close to the goal value of 1.15

11. Further TBR Enhancement • Increasing Be2C content in blanket • Tiles: 55% graphite, 20% Be2C, 10% FS, 15% Na • Blanket: 10% graphite, 30% Be2C, 10% FS, 10% Na, 40% ceramic breeder • Local TBR = 1.16 • Increasing Be2C content in FW tiles • Tiles: 45% graphite, 30% Be2C, 10% FS, 15% Na • Blanket: 10% graphite, 30% Be2C, 10% FS, 10% Na, 40% ceramic breeder • Local TBR = 1.18 • Adding ceramic breeder in FW tiles • Tiles: 35% graphite, 30% Be2C, 10% FS, 15% Na, 10% ceramic breeder • Blanket: 10% graphite, 30% Be2C, 10% FS, 10% Na, 40% ceramic breeder • Local TBR = 1.19 • It is possible to achieve adequate tritium breeding with the mobile tiles design with proper composition optimization keeping in mind constraints on material content

12. Cooling the Tiles is of Paramount Importance • The target energy in the form of ions, neutrons and x-rays impacts the tiles and is conducted to the back through the graphite. • At this point the energy has to be transferred to the coolant channel. • Radiating the energy is not adequate since it causes the front temperature to be excessive (2700 oC). • A scheme for conducting the energy is shown in the figure. Graphite felt lines the inner channel of the tile facing the target. • A linkage built into the cooling channels, when engaged applies forces on the the tile and compresses the graphite felt allowing the energy to be conducted to the cooling channel.

13. x-rays Burn ions Debris ions Type of deposit. Exponential Uniform Uniform Deposit. Depth(m) 2 10 1 Deposit. Time (ns) 1.4 1100 3800 Fluence (J/cm2) 0.39 2.98 4.0 Max. Temp. (oC) 1330 1400 1600 Estimating the Tile Surface Temperature • Assuming radiation, to transfer the energy from the tile to the coolant channel exceeds desirable temperatures at the surface. For example, assuming the coolant channel is at 200 oC, the maximum tile surface temperature is ~ 2700 oC. Conduction is needed to transfer heat from tile to the coolant rails. • Tile temperature is critically dependent on the contact conductance between tile and coolant rails. This tile thermal conductivity and contact conductance can only be guessed at currently, though there are potential engineering improvements which can be considered. • Graphite felt in the space between the tile and the coolant channel, when compressed, provides a conduction path. Assuming a coolant temperature at 400oC, a conductivity for the graphite felt of 1 W/mK, a thickness of 0.1 cm, resulting temperatures are shown below.

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15. Progress Toward Measuring Thermal Contact Conductance • Estimation of the thermal contact resistance is a critical path issue for the mobile tiles and will be a strong function of temperature and interfacial pressure 15°C 30°C IR Thermal Camera • 256 x 256 InSb focal plane • detector array • Temp. resolution: 0.015°C • Spatial resolution: 7.5 mm • Frame speed: 130 frames/sec

16. Progress Toward Measuring Thermal Contact Conductance Flux via electrical resistance delivered to mandrel of load frame Top Specimen 25mm x 5mm diameter TC for measure of ambient Temperature Bottom Specimen 5mm x 5mm diameter Sample Pair sat atop hemispherical tungsten carbide support • Estimation of the thermal contact resistance is a critical path issue for the mobile tiles will be a strong function of temperature and interfacial pressure

17. Progress Toward Measuring Thermal Contact Conductance The cover is place on the furnace and the heating elements placed in the top 4 slots to help induce heat flow. A sapphire widow is placed over the opening to retain heat while allowing for the capture of the thermal image

18. Test Set Up w/IR Camera

19. System tested at RT, 50, 100, 150, 200 C and 1, 25, 50, 75 MPa For these initial runs the top “specimen” was aluminum alloy and the bottom specimen a lower conductivity steel Data was taken at RT with and without the sapphire window There was a 20 minute interval between heat cycles For Example, lets say data was collected at 50 C for the 4 pressures. The furnace would cycle on, and allowed to heat to 100 C (10C/min). Once at 100 C, it was left there for 8 minutes, at which time the flux source was turned on. After 4 minutes, data collection began for the 4 pressures, after which time, the furnace began to heat up to 150 C. This continued up to 200 C, till all the data was collected. Test Schedule / Process

20. Test System Data Raw Data w/Curve Fit The linear approximations were obtained using the 5 mm closet to the interface for the top specimen and 3mm for the bottom specimen. The true location of the interface itself (the vertical line) is very subjective and sensitive to the calculation of the interface TCC. Flux was determined for both top and bottom specimens using (dT/dx)Kmat’l, then averaged to get Qave. Qave/dTinterface = TCC (mW/mm2-C)

21. Thermal Contact Conductance of Model Al/Steel System 200°C 150°C 100°C 50°C 20°C • Additional optimization to the analysis program would be helpful, but system appears ready for application to a mobile tile graphite/metallic interface.

22. Fabricating Test Tiles - A Next Step ? • Properties and property evolution in-reactor can only be estimated for this graphite-ceramic. Assuming preliminary designs suggest promise, property measurement on prototypic materials will be required. • Mix Be2C powder ( ESPI Metals) • Graphite powder, and carbon binder • Dry mixture • Hot press powder to ~ 1100 °C (low • temperature to avoid vapor hazard) • Carbon binder will bond powders • together under pressure Interior of ORNL Brew hot press showing graphite die

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24. Sawicki JNM 1989

25. Saeki JNM 81 Neutron irradiation to ~3E19 n/cm2, 250-400

26. JT-60 tile

28. Local TBR : No Breeding Materials FW tiles : 75% graphite, 10% structure, 15% coolant Blanket : X% Li4SiO4, (80-X)% graphite, 10% structure, 10% coolant FS structure, Na coolant SiC structure, He coolant • Largest TBR achieved with high breeder content / low Li enrichment • Achievable TBR not adequate • Replacing FS/Na by SiC/He is not Helpful

29. H451 Graphite 900°C 600°C Degradation in Thermal Conductivity • For graphite held in the 600-1000°C range, thermal conductivity will slightly degrade and density somewhat. Some recovery during furnace anneal will occur. Burchell data 2 dpa