Carbon coating of simulated nuclear-waste material

The development of low-temperature pyrolytic carbon (LT-PyC) coatings as described in this report was initiated to reduce the release of volatile waste form components and to permit the coating of larger glass marbles that have low temperature softening points (550 to 6000C). Fluidized bed coaters for smaller particles (<2mm) and newly developed screw-agitated coaters for larger particles (>2mm) were used. Coating temperatures were reduced from >10000C for conventional CVD high temperature PyC to approx. 5000C by using a catalyst. The coating gas combination that produced the highest quality coatings was found to be Ni(CO)4 as the catalyst, C2H2 as the carbon source gas, and H2 as a diluent. Carbon deposition was found to be temperature dependent with a maximum rate observed at 5300C. Coating rates were typically 6 to 7 μm/hour. The screw-agitated coater approach to coating large-diameter particles was demonstrated to be feasible. Clearances are important between the auger walls and coater to eliminate binding and attrition. Coatings prepared in fluidized bed coaters using similar parameters are better in quality and are deposited at two to three times the rate as in screw-agitated coaters