Abstract: Employees were removing a hot buildup of smelted concentrate inside the waste heat boiler of an Outokumpu flash furnace at a copper smelter. The partly smelted concentrate was composed of extremely fine particles that had passed through the flash furnace at temperatures of up to 2,300 degrees F. As the partly smelted concentrate passed into the waste heat boiler, it stuck on the sides of the boiler, eventually forming a mass that bridged across the walls of the boiler from the throat to a water-cooled 16 ft high baffle wall. When the boiler was taken down for repairs, the mass was about 800 to 1,000 tons and had retained much of the heat. Blasting with explosives was used to break up the face of the mass; holes were drilled or burned with oxygen lances to accept the explosive charges for blasting. Some pockets of molten material were found near the furnace end of the mass when the oxygen lances burned into it. As the blasting process continued, cracks developed in portions of the mass and the face of the mass fell to the bottom of the boiler. A water lance with a valve was used for cooling the holes. The valve handle for the water lance was broken. Rather than fix the valve, or go outside the boiler to turn off another valve, employees at times allowed water to flow onto the top of the mass. Also, some boiler tube leaks had been spraying onto the bottom of the mass. Apparently, some of the water entered a crack and circulated into the hot mass, probably to a pocket of molten material. A steam explosion occurred within the surrounding mass, breaking a large section (estimated to be at least 2 tons) off the left part of the face. The blast struck Employee #1, who was standing on the upper platform in line with the explosion, causing him to fall about 4 1/2 ft to the lower platform, where he apparently struck and broke a 2 by 4 brace below the upper platform. Employee #2, who was sitting closest to the explosion, was severely burned and sustained a depressed skull fracture and puncture wounds from explosion fragments. Employee #3, who was standing behind Employee #2, sustained minor burns. Employee #4 was on the lower level platform and sustained a temporary hearing loss. Employee #1, who was unconscious, died about an hour after the explosion, at about the same time that rescuers were able to cut a large enough opening in the boiler to remove him on a stretcher. The entry door was 18 in. by 18 in. before the accident.