A Pump Failure and Claxon Alert
The first warning that something was wrong came when a double-tone claxon blast went off in the control room.
Until then, the only sound intruding on the Susquehanna River in the early morning hours was the deep hum of the big bank of turbines hooked up on the south end of Three Mile Island Nuclear Reactor No. 2.
After three months on the line, No. 2 seemed at last to have shaken out its kinks.
Since it began operations just one day before the end of 1978, No. 2 had proved to be a continuing source of frustration. By starting up before the yearend, the plant qualified its owners for as much as $40 million in federal tax credits and write-offs. In the ensuing days of January, it had been shut down for a two-week interval while engineers from the Metropolitan Edison Co., operators of the reactor down-river from Harrisburg, Pa., traced sources of leaks in the piping and pump system.
But on the morning of March 28, the 880-megawatt plant was going full blast. Large plumes of water vapor drifted from the lips of its two 370-foot cooling towers into the chilly air.
Encased in thick concrete walls and behind bullet-proof windows and reinforced steel doors, the regular four-man crew of control room operators was sitting the watch.
The control room is a vision from science fiction. It sits under the shadow of the 190-foot-high domed reactor containment building. Inside, a horseshoe-shaped panel stretches 40 feet along three walls lined with dials, gauges and 1,200 warning lights color-coded red and green.
This was the scene inside the control room when, shortly before 4 a.m., something went wrong.
Across the Susquehanna, the nearest house to the reactor belongs to Holly and John Garnish. Their red brick ranch house sits just across Route 441 from the plant on the corner of Meadow Lane. Like most of their neighbors, the Garnishes slept in the shadow of the giant cooling towers across the river.
But that morning, Holly Garnish awoke with a start. Outside, in the direction of the plant, a loud roar came from Three Mile Island.
"Picture the biggest jet at an airport and the noise it makes," she recalled. "That's what I heard. It shook the windows, the whole house."
Her husband did not awaken. She looked at the alarm clock on the night table. It read 3:53 a.m. "I remember because I got up and put the dog outside," she said.
Over on the island, America's worst nuclear accident was taking form in quick, inexorable steps.
A pump that sends hot water to the steam generator failed for reasons still unexplained. Instantaneously, a second pump feeding cooling water to the reactor shut down. It had been fed water from the first pump.
A sensor " realizing" that the steam generator no longer was receiving water immediately shut down the plant's giant turbine. With electronic prescience the reactor sensed that the turbine did not want any more steam. A switch was automatically thrown, and a powerful jet of steam shot up from the plant's turbine building at a pressure of 1,000 pounds per square inch.
That was the noise that awakened Holly Garnish.
Only three to six seconds had passed from the start of the incident when yet another event occurred: a relief valve automatically opened to blow off superheated, radioactive water within the containment structure.
No more than six seconds later the reactor "scrammed;" the control rods that stop the chain reaction inside the reactor vessel automatically dropped into place among the fuel rods. In effect, the reactor was shut down. Fissioning inside the uranium fuel rods immediately began to slow down.
The pressure inside the reactor vessel than began to fall. This should have been the signal for the open relief valve to close. Instead, it stayed open, apparently stuck. Pressurized steam went right on pouring out of the reactor.
The instant the main pumps failed, three auxiliary coolant pumps kicked on. Unhappily, valves that should have been open in the auxiliary feed-water system were closed, locking out the water the pumps were trying to drive.
As water was lost, the temperatures inside the reactor began to soar. Readings climbed 30 degrees in less than three seconds.
In the control room, "bells were ringing, lights were flashing, and every body was grabbing and scratching," said one Nuclear Regulatory Commission source.
The shift supervisor, sitting in a glass-walled office facing the console, bolted out onto the main floor and took charge.
The pressure, meanwhile, continued to plunge, causing more water in the reactor vessel to flash into steam and escape through the open relief valve.
At that point if all systems had been working properly what had been an unusual but not yet serious occurrence automatically would have been brought under control.
Afterward Nuclear Regulatory Commission officials singled out the valve problem as a key one during daily briefings later with the press gathered in the nearby Middletown Borough Hall. "There would have been an entirely different outcome if they (the pumps) had been operational, as they should have been," said Harold R. Denton, NRC's chief of reactor operations.
Apparently the valves were closed for routine maintenance, in violation of one of the most stringent rules that the Nuclear Regulatory Commission has. The rule states simply that auxiliary feed pumps can never all be down for maintenance while the reactor is running.
"If you take all of these pumps out at once, even for a limited time," said an NRC source, "you're supposed to hit the down button and shut the reactor down in a hell of a hurry."
With no fresh, cold water reaching the steam generator and the reactor, the operators on the control room whether they realized it or not were in real trouble.
The steam generator had begun to boil dry, taking even more water out of the cooling system. In the reactor vessel, even though the chain reaction had been essentially halted, heat was still being generated as fission wound down. Temperatures in the reactor continued to climb.
Pressures continued to fall in the reactor because the relief valve was still stuck open. "The flow through that valve could have been terminated by pressing the right button in the control room," said a source at Babcock & Wilcox Co., builder of the reactor. "That was ultimately done but it was done 32 minutes later."
Two minutes into the accident, the pressure fell to 1,600 pounds per square inch, automatically turning on the plant's emergency core cooling system. There was still time to prevent these mishaps from mushrooming into a major accident.
For still unexplained reasons, an operator in the control room turned off the two pumps that drive the emergency cooling system. He shut down one pump 4 minutes 30 seconds into the accident and the second pump six minutes later.
The prevailing theory at the NRC and Babcock & Wilcox is that he was looking at only one of two gauges he should have checked.
"He thought he saw fluid rising in the pressurizer, suggesting that the reactor vessel was still filling with the water.
"So he thinks, 'Ah, ha. I've got the system full of water; any more I pump in there is just going to spill on the floor,'" an NRC source said. "Big mistake!"
What was really happening was that pressure was still plunging, water was still flashing into steam, and water levels inside the reactor vessel were in fact dropping.
Much of that water was still spurting out of the reactor vessel through the open pressure valve into the containment.
Only 7 1/2 minutes after the start of the accident, the radioactive water on the floor of the containment building was two feet deep. The building's sump pumps at this point automatically comes on, rushing the water out of the containment structure into tanks in the auxiliary building.
This development would come back to haunt the operators in the days ahead.
"It would have been a help if they had recognized that they ought to cut off that containment sump pump," an NRC source said. "It should have occurred to them: 'Let's not pump it out to the auxiliary building. Let's just leave it in the containment until we know what's going on."'
Eight minutes after the start, an operator in the control room must have realized the auxiliary cooling system hadn't worked because he threw the switch he should have thrown eight minutes earlier to unlock the closed valves in the feedwater line, turning on the system.
In three more minutes, an operator restarted the emergency core cooling system that had been mistakenly turned off. For the next 50 minutes, the accident appeared to have diminished in size. Reactor pressure stopped falling. The water level inside the reactor vessel was still sufficient to cover the tops of the 36,000 fuel rods. Though some fuel rods were probably perforated by thermal shock at the start of the accident, they still had not suffered any heavy damage.
"The core had pretty much been covered up to that point," a Babcock & Wilcox source said. "While things weren't real good, things were correctable."
Then, the inexplicable happened again.
Though the cooling pumps had come back on, they were not running smoothly. In fact, they had begun to vibrate as they strained to drive cooling water to a reactor whose pressure had fallen so dramatically.
An operator turned off four cooling pumps, two at 1 hour 15 minutes and two more at 1 hour 40 minutes into the accident. The NRC still has no explanation for these moves, though one explanation could have been concern that the pumps were straining so hard they were about to fall.
"The operators obviously were worried about the pumps damaging themselves," a Babcock & Wilcox source said, "but turning them off was a bad idea."
When the final two pumps were stopped, the water level in the reactor vessel plunged again, uncovering the core and fuel rods for the first time. Heat in the reactor began building up rapidly. Within 14 minutes the temperature at the top of the reactor had climbed right off the scale.
In the control room, the computer monitoring the temperatures in the dome of the reactor printed readings up to 750 degrees, then began printing question marks for much of the next eleven hours.
Nobody in the plant had any idea that the reactor core had become uncovered but uncovered it was. The water level had dropped at least four feet below the top of the core, uncovering one-third of the fuel rods. The stainless steel cladding (coating) on the rods had begun to crumble, creating rubble at the top of the core.
Highly radioactive fission products now began to pour from fuel rods that were rupturing in the matter of minutes. The cladding on no fewer than 20,000 of the 36,000 rods is believed to have been oxidized, plunging radioactivity into the reactor coolant. The only thing providing cooling to the fuel rods for the next 11 hours was the steam flashing out of what little water stayed in the bottom of the reactor vessel.
Meanwhile, the number of persons in the control room continued to grow. Executives began arriving in a steady stream in the chilly predawn to join the superintendent of operations, who arrived 20 minutes after the incident started.
"An hour or two after it happened, the place was swarming with white hats," said a control room operator who had been on duty at neighboring Three Mile Island Plant No. 1. "They were looking the thing over, and trying to figure out what to do."
As first light began to break over Three Mile Island, it was becoming increasingly clear to the worried officials that they had a serious threat of radiation leakage.
Shortly before 7 a.m., an emergency siren began to wail the signal that workers at the Three Mile Island plant should evacuate certain critical areas.
A number of workers dashed for their cars, hoping to get across the bridge to the mainland before they would be confined to the island as a precautionary measure. Two cars made it before the gate slammed.
At 7:02 company officials notified the Dauphin County civil defense office that they had declared a site emergency.
The situation, however, was even worse than they thought.
Back in the control room, less than 20 minutes later, an alarm sounded. An automatic detector in the containment, set to activate when the radiation level reaches 8 rems, had gone off.
Three Mile Island now had become a general emergency.
© Copyright 1979 The Washington Post Company