Abstract: Employee #1 was watching a Crysen employee retrieve a catalyst sample from the reactor of a MDDW unit that had been purged with nitrogen. Employee #1 had designed and made the sample tool the Crysen employee was using and was there to make sure that it worked as it should. As the Crysen employee came out of the reactor so that workers could change a breathing air cylinder, he either accidentally disconnected the air line to his breathing apparatus or ran out of air. He stood up to grab the chain ladder, then fell over and appeared to be unconscious. The Crysen maintenance supervisor ran to an air bottle that was about 15 feet away to check the air, which was already dangerously low. When the supervisor got back to the vessel opening, Employee #1, who was not wearing a breathing apparatus, had already jumped into the vessel. The maintenance supervisor and the engineer consultant, who was also at the scene watching the sampling, went to the opening, saw Employee #1 attempting to lift the Crysen employee to the opening, and were able to grab the Crysen employee's arm. At that time, the engineer consultant saw Employee #1 go down. They attempted to pull the Crysen employee out, but he seemed to be hung up on something. The nitrogen coming out of the hole was so concentrated that the supervisor and consultant started to pass out, and could not hold on any longer, and let go of the Crysen employee. Once they were away from the opening, the maintenance supervisor radioed for help and three employees from the control room came to assist. One of them attempted to enter the vessel with a SCBA, but the opening was only 18 in. wide, and he could not get through. Rescue attempts were stopped until the fire department arrived. However, the ambulance report stated that Employee #1, and the Crysen employee, both of whom had died, were out of the vessel when it arrived.