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?