Thermogravimetric Oxidation Analyses of Carbon Tokamak Codeposits and Flakes

The erosion and redeposition of first-wall armor materials is a problem in nuclear fusion devices with carbon walls, where deuterium, tritium, and (eroded) carbon present in the plasma are deposited on the walls of the device, trapping the expensive and radiologically hazardous tritium. Thermo-oxidation, in which vessel surfaces are heated and oxygen containing gas is injected, is a possible solution. It results in the production of carbon oxides and tritiated water vapor, which can be pumped out by the vacuum pumps and recycled in a tritium recycling facility. In the present study, thermogravimetric analysis was used to measure the mass loss (or gain) of codeposited specimens from the General Atomics DIII-D National Fusion Facility under thermo-oxidation, in addition to laser thermal desorption spectroscopy. X-ray photo-electron spectroscopy was also used in this work to examine the tile’s surface composition pre and post oxidation. Dust scraped from the specimen was also studied, as this is a surrogate for dust that naturally falls from the tile codeposits and builds up in the tile gaps. One key conclusion is that boron oxides form where boron is present in the codeposit as an impurity, and these oxides dominate the weight-change behavior of the codeposit specimens for long exposures.