Abstract: Employee #1 was adding cement to a waste ceramic shell containing thorium oxide, resulting in a caustic sulfuric acid mixture that began to heat up and fume. When he added water, the mixture began to smoke and fume at a greater rate. The fumes spread to other areas of the plant, and Employees #2 through #9, who were working in the area of the plant nearest to the cleaning department, were evacuated first, followed by the remainder of the facility. Exposure to sulfuric acid and possibly oxides of sulfur was suspected, although the actual chemical products to which the workers were exposed are still under investigation. Employees #1 through #9 suffered upper respiratory tract and lung irritation, but none were hospitalized. They were wearing respirators at the time of the accident. This incident may be related to a similar occurrence on March 9, 1990, although no workers were sent to the hospital at that time. The fumes may have spread because a bay door to the outside was partially open, defeating the negative pressure in the cleaning area.