Abstract: At 6:00 a.m. on August 31, 1986, Employee #1, the chief operator of unit #1944, arrived at work. At the start of the shift, all employees were informed that the day would be spent sweeping the reactor (circulating hot hydrogen through the system). They were instructed to maintain the sweeping process and to keep the hydrogen temperature near 400 degrees F. At approximately 12:00 noon the pressure began to build up in the system, and at about 1:00 p.m. the control board operator received an alarm that the pressure indicating control for unit #139 was high. The control was set for 50 lb, but was showing 80 lb. The controller informed the second operator about the condition. The second operator went over to D-11, reset the pressure, and looked inside the heater before calling the controller to inform her that the fire in the heater looked fine. About 45 minutes later, the temperature indicating control for unit #168 showed a drop of approximately 30 degrees F. The controller alerted the second operator that the temperature was dropping and that the gas valves were opening in an attempt to bring the temperature back up. By this time the valves on the temperature indicating control for units #168 and #147 were about 75 to 85 percent open, but their temperature readings were still dropping. The controller also told the second operator that two coworkers had reported smoke coming out of the stack. The control operator then began to cut back on the temperature, 10 degrees at a time, trying to close the valves and put less gas into the heater. Before the valves could be closed, however, a heater exploded. Employee #1, who was at the heater checking out the problem, received second- and third-degree burns over 50 percent of his body. He died on December 7, 1986.