Generation of Graphite Particles by Rotational/Spinning Abrasion and Their Characterization

1. Raymond S. Troy, Robert V. Tompson, Jr., Tushar K. Ghosh and Sudarshan K.Loylalka Particulate Systems Research Center & Nuclear Science and Engineering Institute, University of Missouri, Columbia, MO 65211 Generation of Graphite Particles by Rotational/Spinning Abrasion and Their Characterization

2. Pebble Bed Reactor • Made up of about 400,000 pebbles • Online refueling • Fission potential measured when removed from reactor • Remain in cycle up to 6 years (~10 trips)

3. Pebbles (Fuel)1 1. PBMR Ltd. http://www.pbmr.co.za/index.asp?Content=213&GState=Image&CatId=-1&Image=44&Page=1

4. The Problem • As the reactor operates, the pebbles are in contact with each other, the fuel handling system, and reactor components (pressure vessel, etc.) and graphite dust is produced. For many reasons, information about this dust must be collected. • Our goal is to characterize graphite particles generated by fuel pebble abrasion

5. Motivation • Safety • Modeling • “the production of dust by fuel element abrasion and its effect on fission product transport…would complete a comprehensive model for the core release behavior under normal operating conditions1.” • Data provided to codes • Accident mitigation • Amount of dust • Inhalation • Cancer/dose calculations 1. http://www.iaea.org/inisnkm/nkm/aws/htgr/fulltext/29009817.pdf

6. Motivation • Operation • Radioactivity levels • Estimate radioactivity levels in the loop • Mechanical • Clogs • Length of pebble life • Re-suspension • Depressurization of the loop may cause re-suspension of dust • Modeling of thermophoresis • Uneven distribution of particles along loop 1. http://www.iaea.org/inisnkm/nkm/aws/htgr/fulltext/29009817.pdf

7. Data Collected • Size Distribution • Mean, Standard Deviation and Median calculated • Loading and rotational speed measured • SEM images of sample and abraded particles • Unbraded Surface roughness • BET surface area, pore analysis • Humidity and temperature in room

8. Experimental Design • Our experimental apparatus allows us to control loading, atmosphere, rotation speed, graphite type and the shape of the graphite interface.

9. SMPS System • Measures particle size distributions in the diameter range 2.5 nm to 1000 nm • Pulls vacuum of 2.4 L/min and particles are drawn into the machine http://www.tsi.com/Scanning-Mobility-Particle-Sizer-Spectrometer-3936/

10. APS System • Measures particle size distributions in the diameter range 500 nm to 20,000 nm • Pulls vacuum of 5 L/min and particles are drawn into the machine http://www.tsi.com/Aerodynamic-Particle-Sizer-Spectrometer-3321/

11. Apparatus • The loading between the two hemispheres is measured by a Mettler Toledo Scale, model number PBA 430, with an IND 560 readout having accuracy to 0.001 kg. • The rotational speed is determined by the machine’s preset speeds.

12. Apparatus

13. Material Used

14. Samples

15. Test Procedure • Samples were prepared • Machined inserts • The assembly was dismantled and cleaned • Background samples were taken • With graphite samples in cylinder • Machine was started • Loading set • Collection of data

16. Test Matrix

17. Data 10 Kg and 1500 RPM

18. Data 31 Kg 1500 RPM

19. Data 56 Kg 1500 RPM

20. Data 22 Kg 310 RPM

21. SEM Images A) Before the test B) After the test

22. SEM Images

23. Samples (after test)

24. Surface Area Analysis • 626 m2 gm−1 • This is very high

25. Surface Area Analysis • Diameter of most of the pores is in the range of 10 to 60 Å.

26. Surface Area Analysis • Total cumulative pore volume was found to be 1.213 cm3 gm−1. • Porosity of the generated particle is about 68%.

27. Surface Roughness • Measured with a atomic force microscope (AFM) • Average Ra of pre abraded samples was 0.96 µm • The AFM did not have capability to measure post abrasion surface roughness (too rough) • We have a new method to measure surface roughness and this will not be an issue for future tests

28. Analysis • The size distribution and the concentrations change with time • wear has a strong effect on particle generation rate as well as size, and physical/mathematical models for particle generation should account for the aging of the pebbles. • Time changes at what size particles are generated

29. Analysis • Certainly, with different loadings, graphites, atmospheres, and rotational speeds the particle size distributions will change • models for particle generation will need to account for abrasive effects

30. Ongoing/Future Work • Dry Air (to produce dusting effect) • Reactor grade graphite • Shape of interface (disk, point) • Sliding abrasion • Wear Rate • Statistical Fit of size distribution • Temperature and Humidity measurements inside chamber • Surface roughness before and after test

31. Conclusion • Questions?