- Last Updated: 29 January 2016 29 January 2016
The Initiating Event
On March 27, 1979, Three Mile Island unit 2 was operating normally and under fully automatic control. As the overnight shift took over at 11 PM, the plant was on the grid and generating power at 97% of its rated 1,000 MegaWatt capacity. Operators Craig Faust and Ed Frederick, under the supervision of shift supervisor Bill Zewe, settled in for a long night in the control room. Faust and Frederick, like Zewe, were former Navy reactor operators who had decided to make nuclear power their careers in civilian life as well. All were licensed, experienced men, and all had scored well above average on the tests that culminated their training.
The remainder of Zewe's 16-man crew, who were trained but unlicensed "auxiliary" operators, dispersed to various locations throughout the plant to perform regular maintenance duties. These men do the hard work in a nuclear power plant -- maintaining valves, pumps, and other machinery on a hands-on level.
As the hour of 4:00 AM approached, two such men were becoming quite frustrated. In the bowels of the plant, the men were working on a feedwater polisher. Sometimes called ion-exchange tanks or demineralizers, these large tanks are filled with resin beads which remove contaminants from the feedwater. This tank, one of eight, had been isolated from the rest of the feedwater system so that its spent beads could be flushed out and replaced. Pains were taken to see that feedwater continued to flow through the other tanks unimpeded, and it was hoped that this operation could be completed quickly. However, as the technicians began to wash out the old beads, the beads formed a "clot", and became stuck. Several attempts to dislodge the beads with water, air, and steam had failed.
A tank clogged with beads was not an unusual occurrence at TMI. In fact, it happened so often that compressed air lines from the plant's general-purpose air system had been installed on each scrubber. The beads could be usually be dislodged with quick bursts of air from these lines, and this among other things is what the technicians were trying. The job was taking forever, and there was a schedule to keep; concerned about the delay, Zewe dispatched shift foreman Fred Scheimann to lend a hand.
In addition to the general-purpose air system, TMI-2 had another compressed air supply. This one, called the instrument air system, was used to control pneumatically operated valves and controls in the plant, and was critical to safety. Unknown to the operators on duty, sometime during the night, someone had connected a rubber air hose between this instrument air system and a water line. Perhaps he was trying to pressurize the water line, perhaps he was trying to connect the two air systems together. The instrument air, general purpose air, and plant water systems all used the same, familiar Chicago Pneumatic fittings, the area was nearly dark, and the fittings were not labeled well. The water line carried more pressure than the instrument air system, so the water began to work its way into the air lines.
At 3:57 AM, as Scheimann stood atop an 18" feedwater pipe peering through a sight glass at the beads in the tank, the leaking water finally reached the valve control piping. Swept along by air pressure, it raced through the air lines, finally entering valve controls.
Ironically, soon after the plant was put online, years ago, someone became concerned about what would happen if water got into the instrument air system. It would almost certainly cause valves to close. A plan was devised to modify the valve control system, so that the valves would fail "as is", in their current positions, should the air supply fail. No one knows why, but the wiring for this modification was never connected.
Instantly, nearly every valve in the feedwater system slammed shut. Water hammer occurred, precipitated by the sudden stoppage of the feedwater flow. Faust and Frederick felt the control room floor shudder as the violent shock tore out valve controls, ruptured the casing of a feedwater pump, and shattered pipes. Scheimann jumped clear just as the pipe on which he was standing heaved violently, lifting completely out of its mounts. Another operator, on his way to the control room, wisely chose a different route as another feedwater pipe jumped three feet into the air within inches of him. Within seconds, the entire auxiliary building was filled with water vapor, and awash in scalding water. Unit 2 had just gone out of business.