Generation of Graphite Particles by
Rotational/Spinning Abrasion and Their Characterization
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
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)
Pebbles (Fuel)1
1. PBMR Ltd. http://www.pbmr.co.za/index.asp?Content=213&GState=Image&CatId=-1&Image=44&Page=1
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
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
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
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
Experimental Design
• Our experimental apparatus allows us to control loading, atmosphere, rotation speed, graphite type and the shape of the graphite interface.
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/
APS System
http://www.tsi.com/Aerodynamic-Particle-Sizer-Spectrometer-3321/
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
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.
Apparatus
Material UsedManufacturer: Graphtek LLC
Method of Manufacturing: Isostatically PressedDescription: Isomolded, very fine grain, high strength,
low ash graphite with superior oxidation resistance.
PROPERTY US VALUE METRIC VALUEDensity 0.063 lb/in3 1.75 gr/cm3
Particle Size 0.0015 in
0.00381 cm
Flexural Strength 8570 psi 59.1 mpaCompressive Strength 14280 psi 98.5 mpaResistivity 5.5 ohm/in*10-4 Hardness 50 scleroscope 50
CTE 2.1 in/in °F*10-6 3.8Microns/m °C
Porosity 13.2 % 13.2 %
Thermal Conductivity 52BTU/(h.ft2 °F/ft) 90
W/(m2 . K/m)
Ash 0.01 % 0.01 %
Samples
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
Test Matrix
Test No.
Loading (kg)
Rotational Speed (RPM)
1 60 15002 31 15003 10 15004 22 310
Data
10 Kg and 1500 RPM
Data
31 Kg 1500 RPM
Data
56 Kg 1500 RPM
Data
22 Kg 310 RPM
SEM Images
A) Before the test B) After the test
SEM Images
Samples (after test)
Surface Area Analysis
•626 m2 gm−1
•This is very high
Surface Area Analysis
•Diameter of most of the pores is in the range of 10 to 60 Å.
Surface Area Analysis
•Total cumulative pore volume was found to be 1.213 cm3 gm−1. •Porosity of the generated particle is about 68%.
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
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
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
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
Conclusion
Questions?
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