Abstract: On June 5, 1992, a leak was detected on the top of the high vacuum valve of a diffusion pump. The furnace system became back-filled with argon to atmospheric conditions. On June 8, 1992, Employees #1 and #2, maintenance personnel, were repairing the leak. Employee #1 climbed on the diffusion pump to clean the vacuum valve seat, which is about 32 in. below the flange opening. (The diffusion pump is 35 in. in diameter and 9 ft tall.) Employee #2 was on the ground cleaning parts. After approximately 10 minutes, Employee #2 climbed to the platform to get a bottle of isopropyl alcohol. He noticed that Employee #1 was inside the diffusion pump, overcome. Employee #2 called for help and then climbed inside the diffusion pump without a respirator or rescue equipment. He was also overcome. Employees #1 and #2 were asphyxiated and required hospitalization. Employee #1 died on June 10, 1992. The bottom lid of the furnace had not been dropped to dissipate the argon.