Four employees were working on the melt floor when Employee 4 noted a change in sound within a furnace. The sound indicated that water was leaking into the furnace, and water was also visibly coming out of an observation port at the bottom of the furnace. Employee 1 went to investigate and determined that the roof (lid) of the furnace could be opened to view the extent of the leak. Employee 1 gave Employee 4 the okay to open the roof of the furnace via the radio. When the roof was opened, a large piece of slag dropped from the bottom of the roof down into the shell of the furnace. This slag dropped into the molten metal and water below, causing a rapid expansion event that spewed slag and molten metal out of the furnace into the surrounding area. The slag and molten metal struck Employee 1 and Employee 2. Employee 1 was struck on the left side of the face and neck, causing him to be thrown to the ground. He suffered burns and abrasions to the left side of his face and neck. Employee 2 was struck on the back of the helmet, knocking him to the ground, bruising the back of his head, and causing a concussion. Employee 2 also received several contusions and a large hematoma to his elbow due to being thrown back from the blast.