Particle Production by Rotational Abrasion Between Graphite Spheres

Characterization of graphite particles (dust) produced by the rotational abrasion that would occur in a shifting pebble bed reactor is of interest for purposes of maintenance, safety, and operation. To better understand this type of particle generation, we have modified and used our existing test apparatus to achieve rotational abrasion in a 1% to 5% relative humidity air environment. We have used both a commercial, nonnuclear-grade graphite (GM-101 from Graphtek, LLC) and a nuclear-grade graphite (MLRF1 from SGL Carbon, Ltd.). In both cases, we used two spheres with one being held stationary and with the other being rotated while under load and in contact with the first. We have obtained size distributions for the abraded particles. We have also fit lognormal functions to those size distributions (for use in nuclear computer codes); determined particle shapes; measured chamber temperature and humidity during the tests; measured and calculated wear rates of the spheres; measured the surface roughness of both pretest and posttest samples; and measured particle surface areas, pore volumes, and pore volume distributions of the particles produced during the abrasion of the graphite surfaces under different loadings and with different rotating speeds. We also carried out additional tests to measure the surface temperature near the contact point. The experiments showed that as loading (analogous to pebble depth in the reactor) and rotation speeds increase, so do wear rates, concentrations of particles, and particle surface area. The shape of the dust particles was in every case nonspherical, as one would expect. The surface area of bulk GM-101 graphite is ∼0.58 m²·g⁻¹, and the surface area of bulk MLRF1 is ∼2.78 m²·g⁻¹. After testing, abraded particle surface areas were observed to increase to 493 m²·g⁻¹ for GM-101 and to 545 m²·g⁻¹ for MLRF1. Wear rates of the spheres during testing were observed to range from 0.003 to 0.07 g min⁻¹ per contact site. The upper limit on the size of the abraded particles that was observed was less than ∼4000 nm.