At 36 seconds after 4:00 a.m. on Wednesday, March 28, 1979, a series of pumps stopped in Unit 2 of the nuclear plant at Three Mile Island, 10 miles southeast of Harrisburg, Pennsylvania. The plant was running at 97 percent power. Its safety systems shut down the turbine and the reactor, as they were built to.
A relief valve on top of the reactor’s cooling system opened to let off pressure. Thirteen seconds into the accident, it should have closed again. It didn’t. On the control panel, a light showed that the electric power that opened the valve had gone off. The operators took that to mean the valve had shut.
Two operators were in the control room when the first alarm sounded. A cascade followed. By the account of the presidential commission that investigated, it numbered 100 within minutes. Even on a normal day, the room had “panel upon panel of red, green, amber, and white lights; and alarms that sound or flash warnings many times each hour.” One of them later told the commission: “I would have liked to have thrown away the alarm panel. It wasn’t giving us any useful information.”
The evidence that the valve was open was in the room. One emergency procedure said a temperature of 200°F on the drain pipe below it meant the valve was open. The recorded data later showed it reached 285°F. But the pipe often ran hot because of a slight leak, and the crew dismissed the readings. At 4:11 a.m., an alarm signaled high water in the containment building’s sump.
Meanwhile, the water level in a tank called the pressurizer kept rising. The operators had been trained to keep the system from filling completely with water, and they read the rising level as a sign of plenty. An operator throttled back the emergency cooling pumps. The commission wrote that the crew stayed “oblivious for over 4 hours” to the far greater danger: that the reactor’s core could be uncovered.
The valve stayed open for 2 hours and 22 minutes. In the first 100 minutes, some 32,000 gallons, over a third of the reactor coolant system’s capacity, escaped. A backup valve was shut at 6:22 a.m. By then the top of the core had been uncovered and had heated to the point where the cladding of the fuel rods reacted with steam to make hydrogen.
The commission, chaired by Dartmouth’s president, John G. Kemeny, put the main weight on what the operators did, and then on what led them to it: training it called greatly deficient, confusing procedures, lessons from earlier accidents that never became clear instructions for them, and the control room. On the last point it was specific. “During the first few minutes of the accident, more than 100 alarms went off, and there was no system for suppressing the unimportant signals so that operators could concentrate on the significant alarms.”
Some commissioners favored a complete modernization of control rooms. All of them agreed that “a relatively few and not very expensive improvements in the control room could have significantly facilitated the management of the accident.”
The light on the panel had been right all along. The power to the valve was off. It just answered a different question from the one the crew needed answered.