Sudarshan K. Loyalka Nuclear Science and Engineering Institute

1. Recent Graphite Research at the Nuclear Science and Engineering Institute – University of Missouri Sudarshan K. Loyalka Nuclear Science and Engineering Institute Particulate Systems Research Center University of Missouri, Columbia, USA September 16, 2014

2. Hd We G ORConsolidated Codes Tier 1: MELC Timeline of Nuclear Safety Technology Evolution Integrated Code Tier 2: Mechanistic CodesSCDAP, CONTAIN, VICTORIA Phenomenological Experiments(PBF, ACRR, FLHT, HI/VI, HEVA) Phebus FP, VERCORSEuropean Codes Deterministic Bounding Analysis Chicago Critical Pile Probabilistic Risk Informed Analysis Risk Informed Regulation Atomic Energy Act of 1946 (AEC) Atomic Energy Act of 1954 USS NautilusShippingport 9-11-2001 TMI-2 Chernobyl AEC NUREG-1150 MOX LTA revised 1465 1940 1980 Windscale NRC NPP Siting Study NUREG 1465 alternate source term TID 14844 source term NUREG 0772 Nuclear Technology Outlook Optimistic Guarded Pessimistic NP-2010 and Gen-IV WASH 1400 Emerging Issues MOX, High Burnup, Life Exension Environmental ConcernsGlobal Warming and Where are we going ? Vulnerability to Terrorism 1950 1960 1970 1990 2000 2010

3. The First “Source Term”

4. HTGRs: Source Term • Need to understand and predict: • FP diffusion through the particles and graphite • FP release into and plateout from the coolant • Moisture and dust interactions

5. Introduction • Renewed interest in graphite-fueled reactors • Need for measurement of modern nuclear graphite properties and interactions • Research areas: • HTGR source term issues • Graphite dust particle generation • Graphite oxidation • Adsorption of fission products on graphite • Fission products diffusion in graphite • Fission products transport to aerosols • Dust adhesion to surfaces • Dust re-suspension • Coagulation • Emissivity

6. Graphite Dust Particle Generation • Graphite dust is produced during PBR operation • Sources of generation • Fuel handling system • Pebble on pebble abrasion • Pebble on reactor components

7. Graphite Dust Particle Generation Experimental setup

8. Graphite Dust Particle Generation Surface area and Sliding Distance Surface properties of graphite samples * Data from nitrogen adsorption at 77 K. The BET surface area is calculated using the Brunauer - Emmett - Taylor equation. The total pore volume is measured at maximum nitrogen pressure. Fig. 13. Multi-Point BET measurements for abraded graphite at 53.9 Kg and 450 RPM.

9. Pore Size distribution in abrated graphite powder

10. Publications R. Troy, R. Tompson, T. Ghosh, and S. Loyalka, "Generation of graphite particles by rotational/spinning abrasion and their characterization," Nuclear Technology, vol. 178, (2012) 241-257 . A Paper on sliding friction is to appear in Nuclear Technology (2014-15). Others on nuclear graphites in preparation.

11. Oxidation of nuclear graphite Objectives To predict the oxidation rate of nuclear-grade and matrix-grade graphite under various air ingress accident conditions for VHTR Study the oxidation attack mechanism Characterize the surface and microstructural changes Model the oxidation rate in air using the Arrhenius equation

12. Arrhenius equation and Ea In the chemically-controlled Regime I of graphite oxidation, pre-exponential factor apparent activation energy reaction rate reaction velocity constant reaction order Ea , A and n are determined experimentally. The slope of the mass loss plot = - Ea/R, where R is the ideal gas constant. From collision theory, before a reaction can occur the molecules of reactants must have an energy of activation Ea above their normal, or average energy.

13. Oxidation rate of IG-110 and NBG-18 from TGA data (600 to 1600°C)

14. Distribution of Oxidized Layer There is a strong correlation between density of nuclear graphite and its physical and mechanical properties. rod orientation Surface of rod

15. IG-110 and NBG-18, pure and oxidized in 100% air at 1023 K Pure IG-110 Pure NBG-18 Oxidized NBG-18 Oxidized IG-110

16. Conclusions on IG-110 and NBG-18 • IG-110 oxidized more rapidly and more uniformly in the same experimental conditions as NBG-18 • IG-110 is more porous and therefore experiences larger increases in surface area in the kinetic regime

17. Matrix-grade graphite oxidation • ORNL manufactured GKrS by using the German A3 recipe but with modern materials and hot pressing method (we thank ORNL for providing us with this material). • While nuclear-grade graphite is almost fully graphitized at temperatures around 2800°C, matrix-grade graphite is only “partially graphitized” <2000°C fuel fabrication temperature • Air ingress into matrix graphite can affect retention properties of the fuel and we have shown the oxidation rate is high in the kinetic regime

18. Nuclear- vs matrix-grade graphite oxidation studies

19. Publications • Jo Jo Lee, Tushar K. Ghosh, Sudarshan K. Loyalka, “Oxidation rate of graphitic matrix material GKrS in the kinetic regime for VHTR air ingress accident scenarios,” Journal of Nuclear Materials, 451 (2014) 48-54. • Jo Jo Lee, Tushar K. Ghosh, Sudarshan K. Loyalka, “Oxidation rate of nuclear-grade graphite IG-110 in the kinetic regime for VHTR air ingress accident scenarios,” Journal of Nuclear Materials, 446 (2014) 38-48. • Jo Jo Lee, Tushar K. Ghosh, Sudarshan K. Loyalka, “Oxidation rate of nuclear-grade graphite NBG-18 in the kinetic regime for VHTR air ingress accident scenarios,” Journal of Nuclear Materials, 438 (2013) 77-87.

20. Particle Deposition Experiments

21. TEM Images of Carbon, Silver, Palladium and Gold Nanoparticles Silver Carbon Gold Palladium

22. J Nanopart Res (2011) 13:6591–6601 6599 1 3 Fig. 8 SEM images of some larger nanoparticles a1 gold, b1 silver, and c1 palladium. Energy dispersive X-ray spectra (EDS) of the particles a2 gold, b2 silver, and c2 palladium, confirms the particles for gold, silver, and palladium, respectively

23. Thermophoretic Nanoparticle Deposition Cell Inlet: Argon gas with particles at higher temperature Outlet Small notch to position TEM copper grid Aluminum rod in ice cold water

24. CFD computations for thermophoretic deposition • Simulated experiment of Romay et. al. – NaCl particles in dry air • Case 1: 100 nm particles • Case 2: 482 nm particles 96.5 cm 0.49 cm mesh with 2,066,400 volume cells mesh with 7,029,360 volume cells Cross-sections of two different meshes used in this computation Boundary conditions Totally four meshes (465,520, 899,160, 2,066,400, 7,029,360 volumes) were used in this study.

25. Adsorption of fission products on graphite Objectives • Review pervious works on the adsorption of iodine to graphite. • Examine experimental methods used in the past. • Determine the usability of data with adsorption isotherm equations and Polanyi's Potential. • Design and build experiments for iodine adsorption with more accurate means for generating and measuring iodine vapor • Obtain adsorption isotherms of IG-110: • For a single particle (up to 300 °C) • For bulk powder (up to 1000 °C) • Model data with for newly acquired data: isotherm models, kinetics, …

26. Adsorption of fission products on graphite • Review of iodine literature published in Progress of Nuclear Energy (Volume 73, May 2013, Pages 21-50) • Summary of the graphites reviewed in the paper:

27. Adsorption of fission products on graphite • Some data for high temperature adsorption on graphite from the review. 1273 K Isotherm 1073 K Isotherm

28. Iodine Adsorption • Obtain adsorption isotherms of IG-110: • Both single particle and bulk powder forms • Temperature range: Low (room to 200 °C) and high (300 to 1000 °C) • Develop models from newly acquired data: • Isotherm Models • Polanyi Potential • Adsorption Kinetics (if possible) Electrodynamic balance (EDB) for single particle adsorption experiments. Packed bed-tube furnace experiment for bulk adsorption measurements.

29. Diffusion of Fission Products in Graphite Objectives • Characterize physical properties of the nuclear grade graphite (i.e. density and porosity). • Determine the diffusion coefficient of silver through nuclear grade graphite. • Model the diffusion of silver through nuclear grade graphite.

30. Experimental Setup

31. Results Samples were annealed for 4 days at 1150o C.

32. Publications • Thomas R. Boyle, et al., "Measurement of Silver Diffusion in VHTR Graphitic Materials." Nuc .Tech.183(2) (2013) 149-159.

33. Fission Product Condensation on Graphite Problem: • VHTR aerosols are not nicely shaped for computations • Jagged shapes • Agglomerations • Porous Materials R. Troy, R. Tompson, T. Ghosh, and S. Loyalka, "Generation of graphite particles by rotational/spinning abrasion and their characterization," Nuclear Technology, vol. 178, pp. 241-257, 2012. .

34. Problem—Approximations R. Troy, R. Tompson, T. Ghosh, and S. Loyalka, "Generation of graphite particles by rotational/spinning abrasion and their characterization," Nuclear Technology, vol. 178, pp. 241-257, 2012. Z. Smith and S. Loyalka, "Numerical Solutions of the Poisson Equation: Condensation/Evaporation on Arbitrarily Shaped Aerosols," NUCLEAR SCIENCE AND ENGINEERING, vol. 176, pp. 154-166, 2014.

35. Adhesion Force- AFM Objective: • To understand the adhesion of graphite particles and fission products (with and without the influence of surface roughness) to reactor materials of interest - Hastelloy X, Haynes 230, and Alloy 617 • Oxidation Important for reactor design, safety and system analysis because it increases surface roughness which affects emissivity and decay heat removal Plays important role on sustainability of structural integrity of materials over long period. • Adhesion roughness may affect adhesion of particles to surfaces due to reduced contact area Adhesion force is critical in understanding re-suspension of particles under LOCA

36. Experimental Matrix • Hastelloy X material surface conditions: • Oxidized for 5, 10, and 15 min @ ~800 0C and 10 -6torr • As receive and polished surfaces • Mica as a benchmark (standard) • Particle of interaction: • Graphite cluster as a particle (size ~ 6 µm ) produced in VHTR among fission products aerosols • Conditions and parameters of interest: • Environment – Air; Approach rate -1.7848 µm/s;

37. Surface Characterization Figure 1.: AFM images, (a)Mica as benchmark, (b) Hastelloy X polished, (c) Hstelloy X 5 min oxidation, (d) 10 min oxidation, (e) 15 min oxidation.

38. Adhesion Force Measurements Measurements of Adhesion Force (nN) and work of energy(mJ/m2) obtained when a 6 μm diameter Irregular Graphite Particle Probe (Approximated as a Sphere) Interacts with Graphite Sprinkled Hastelloy X Surfaces of Different Conditions. Approach-Retract Rate is 1.7848 μms−1. Adhesion Force (nN) and work of energy(mJ/m2)calculatedusing the JKR Theory and Assuming a Spherical Graphite Particle witha 6 μm Diameter Estimated using Optical Microscope.

39. Adhesion Force Measurements Conclusion • The adhesion force was relatively small in all cases, especially, when when compared to the theoretical values. • Graphite particle was a cluster and not well characterize and surface asperities of the particle where not included. • Large difference between calculated values from JKR theory and measured values may be due to assuming the particle to be spherical in shape. • The pikes seen during measurement may be caused by many factors • large loading force applied on sample by the probe. • Interaction of particle with graphite first then with Hastelloy X or other nano- graphite particles on the surface.

40. Publications • Mokgalapa, N. M., Ghosh, T. K., & Loyalka, S. K, “Graphite Particle Adhesion to Hastelloy X: Measurements of the Adhesive Force with an Atomic Force Microscope,”Nuc.Tech.,186(1) (2014) 45-59.

41. Dust Transport: Role of Charge • VHTRs generate charged aerosols during normal operation. • All reactors can release charged aerosols during severe accidents. • Charged aerosol behavior is complex. • Charge effects on coagulation • Electrostatic forces • Current codes and models are inadequate, relying on numerical techniques which do not account for charge effects.

42. Effects of Charge on Kernel Kernel Repulsion Attraction .

43. Test Problem 2 Size and Charge

44. Measurements Sampling probe Spark generator Transfer function Unknown distribution

45. Emissivity (role of graphite dust)

46. Hastelloy X and N and Nickel

47. Acknowledgements • Grad Students/Post Docs F. De-La-Torre Aguillar SunitaBoddu Matthew A. Boraas Tom Boyle Sean Branney Shawn Campbell Sergio Correra Andrew Gordon Rajesh Gutti Paul Harden Jo Jo Lee Leroy Lee Ray Maynard Ryan Meyer Naphtali Mokgalapa Shawn Nelson Giang Nam Nguyen John Palsmeier Michael Reinig Matthew Simones John-David Seelig Zeb Smith Lynn Tipton Raymond Troy Kyle Walton Nathan White Jason Wilson

48. Faculty • Tushar Ghosh – Professor & Director of Graduate Studies • Sudarshan Loyalka - Curators’ Professor Fellow: ANS, APS; PE Mark Prelas - Professor & Director of Research Fellow: ANS; PE Robert Tompson - Professor • Dabir Viswanath - Emeritus Professor & Chair of ChE Fellow: AIChE; PE

49. Funding by U.S. Department of Energy U. S. Nuclear Regulatory Commission andU.S. Department of Education • NERI-C , VHTR Consortium, NSEI lead (with NCSU and MST) , 2007-2013, NERIC-08-043 • Infrastructure for FP/Aerosol Transport, 2010- • Computations for Aerosol and FP transport, 2011-2014, NEUP -964 • Adsorption/Diffusion of FP in Graphite, 2011-2015, NEUP-2982 • Measurements and Modeling of Emissivity (2014-2017), NEUP-6282 • Graduate Fellowships (NRC), GAANN (DOEd)