Abstract: At 10:00 p.m. on April 1, 1990, Employee #1 returned from lunch to his welding operation at tub #1 in the clean room (aisle 17). He was welding into a zirconium metal tub that had a 6-in.-high rim. He was using a zirconium welding rod along with argon gas (at an unknown flow rate) as a protective atmosphere. Employee #1 apparently tilted the tub at a 45-degree angle to weld a nozzle. He was found unconscious in the angled tub at 12:15 a.m. He died of asphyxia. At a 45-degree angle, the tub was considered a confined space. An oxygen-deficient atmosphere had been created by the argon accumulating in the lower portion of the tub. No ventilation or supplied-air respirator had been used. Employee #1 was working alone at the time of the accident; other employees, including the superintendent, were in aisles 21 to 25.