Inside a nuclear power plant 10 miles southeast of Harrisburg, Pennsylvania's capital, the first of a series of pumps supplying cooling water to a steam generator inside the plant's containment structure -- which includes the reactor vessel and the generators -- unaccountably "tripped," or shut down, at 36 seconds after 4 a.m. on March 28, 1979.
The tense, sometimes terrifying week that followed, marked by official confusion and "surreal" misstatements about the crisis's severity, became known forever as the Three Mile Island accident, named after the reactor site on the Susquehanna River.
When the accident occurred, movie theaters nationwide were showing the movie "China Syndrome" about a nuclear plant meltdown. After engineers finally got inside the stricken Three Mile Island Unit 2 after the accident, they learned how closely reality had closed in on fiction.
With the initial loss of water to the steam generator, water inside the reactor flashed to steam, uncovering part of the fuel core and causing portions of the 100 tons of radioactive uranium fuel to heat up. A chain reaction of multiple equipment failures and control room operators' mistakes followed. Before the damage was brought under control, nearly half the reactor core with its fuel had melted down. A bubble of hydrogen gas exploded inside the containment structure, and fears of another explosion gripped the Harrisburg area for several days.
The accident stopped the U.S. nuclear power industry in its tracks.
No more nuclear plants were ordered in the United States following the accident and none started after 1974 were completed, former nuclear regulator Peter Bradford notes.
"The credibility of an industry was lost," Bruce Williams, a vice president of Exelon Nuclear, which now owns the Three Mile Island station, told a Pennsylvania newspaper in 2004.
Thirty years later, the U.S. nuclear power industry is attempting a revival, citing reactors' ability to generate electricity without the climate-threatening carbon emissions that spew from coal-fired generators.
The Nuclear Regulatory Commission, overseer of the nation's 104 civilian nuclear power plants, is reviewing industry proposals to build a new generation of reactors. The industry is asking the Obama administration and Congress to guarantee loans to pay a majority of construction costs of the first round of new plants, whose price tags today are estimated at $5 billion or more for each 1,000-megawatt reactor.
With nuclear power on the threshold of a possible revival, the industry, its regulators and its critics draw markedly different conclusions from the Three Mile Island accident.
In Senate testimony this week, NRC Chairman Dale Klein stressed his agency's actions since the accident to tighten safety regulation across the board.
Industry leaders note that nuclear plants have logged more than 20 million hours of operations since the 1979 accident without an emergency of that magnitude. With today's higher electricity prices, the existing nuclear plants are big moneymakers, and the last thing their operators want is a prolonged plant shutdown because of safety issues, the Nuclear Energy Institute says.
Nuclear plant design requirements have been expanded and strengthened, Klein said. Control room monitors and controls have been improved. Simulators give control room operators "what if" training in emergencies. The commission has two of its inspectors working full-time at each nuclear plant. The barrage of misinformation about the Three Mile Island plant's condition, which fed public panic and compromised evacuation planning, led to the creation of the Federal Emergency Management Agency.
Other actions to protect plants followed the Sept. 11, 2001, terrorist attacks.
"Changes resulting from the accident have significantly reduced the overall risks of a future serious accident. Today, reactors are operating far more safely and reliably than ever," said Harold Denton, the retired NRC official who commanded commission operations at Three Mile Island at the peak of the crisis.
But some leading nuclear-power critics say the industry still does not go far enough to insure safe reactor operations, or troubleshoot for possible breakdowns in materials in today's aging nuclear plants.
"We think there is overconfidence on the part of the industry and NRC that has led to complacency," said Ed Lyman, senior staff scientist at the Union of Concerned Scientists. "The absence of a severe accident doesn't tell you how likely it is that one could occur tomorrow."
"There are still surprises that are being encountered in operating reactors," he said, citing new evidence about the vulnerability of critical control wiring in the case of reactor fires. "The approach to solving this problem is creeping along at a very slow pace."
NRC and the nuclear industry "continue to make decisions based on risk assessments with incomplete knowledge," Lyman said.
Despite improved safety regulations, critics contend that there are troubling parallels between today's environment and the complacency about safety that preceded the Three Mile Island accident.
Investigations of the 1979 accident put the initial blame on the plant's four control room operators, whose frantic struggles to understand the fast-moving pre-dawn calamity still make chilling reading.
The initial "trip" of the water supply pump to the reactor was probably related to a faulty valve -- a problem that had happened at least twice prior to the accident. It was known but was not remedied, according to the Carter administration's Three Mile Island investigation headed by then-Dartmouth College President John Kemeny.
Two emergency water pumps automatically started to put more water into the reactor core, and 14 seconds into the accident, an operator noticed the pumps were running. But he did not see the control panel lights that indicated another set of valves were closed, preventing that water from flowing to the reactor. One light was covered by a maintenance tag. The other was simply missed.
As water surrounding the reactor fuel rods became superheated and steam built up, a pressure relief valve on top of the reactor (and inside the reactor's surrounding containment structure) opened as it was supposed to. But instead of closing automatically as pressure fell, it was stuck open and remained so for 2 hours and 22 minutes, draining vital cooling water inside the reactor.
Although the reactor shut down, the heat buildup was enough to melt the top of the fuel assembly. The operators did not detect that the pressure valve had failed and made no corrections. If any of these failures had been averted, the accident "would have remained little more than a minor inconvenience" for the plant owners, the Kemeny investigation concluded.
But the Kemeny panel said stopping the critique with the operators' failures would miss the larger, more systemic problem involving the industry and NRC, its regulator. The investigation said that an overmatched NRC staff could not keep up with the pace of nuclear plant construction in the 1970s prior to the accident and was critically dependent upon the nuclear power companies to monitor their own compliance with safety standards during construction.
The panel cited the case of an NRC regional inspector named James Creswell, who learned of water pump problems at the Davis-Besse reactor in Ohio in 1977. He believed it signaled a potentially serious design safety flaw in nine similar plants -- including Three Mile Island.
Creswell could not get the company or his own superiors to respond to his warnings. Finally, he took his concerns privately to Bradford and a second NRC commissioner. They met on March 22, 1979. "The Three Mile Island accident was six days away," Bradford said.
Creswell told the Kemeny investigators after the accident, "within the decision-making structure of the NRC [there is] a reluctance to come to grips with very serious safety issues." The commission was more interested in promoting nuclear power than regulating it, the panel concluded.
2002 breach at Ohio reactor
Hopes that the lessons of Three Mile Island had been learned throughout the nuclear power industry were floored in 2002 by a potentially devastating breach of a reactor vessel at Ohio's Davis-Besse plant, the same one that had prompted Creswell's unheeded warnings three decades earlier.
This emergency was caused by extended leakage of acid-laden cooling water through cracks in a sleeve the top of the steel reactor vessel, which ate away a football-sized cavity in the vessel. It threatened the same emergency loss of cooling water that doomed Unit 2 at Three Mile Island.
The NRC staff had previously been alerted to possible boric acid corrosion issues at Davis-Besse and plants of a similar design. It notified plant operators that they would have to shut down for a safety inspection of the issue by Dec. 31, 2001, unless they had already done so.
Dominion, the Richmond-based power company, voluntarily idled two nuclear units to make the inspection, winning NRC's praise. But FirstEnergy Corp., owner of the Davis-Besse plant, "fought and clawed every inch of the way," to extend the December deadline, according to an NRC investigator's interview with a NRC inspector. The early shutdown would cause unacceptable costs, FirstEnergy said.
A NRC review body voted to overrule the inspectors, and Davis-Besse was given until mid-February 2002 to do the shutdown and inspection. "At a meeting like that, with your boss and your boss's boss presiding, it takes something to raise your hand and say, 'I think, you know, we should shut them down,'" the unnamed NRC inspector later said.
The football-sized cavity was discovered in March 2002 when the plant was finally closed for inspection, six months after the NRC staff's initial alert. A possible reactor vessel rupture could have been weeks away.
Following investigations, FirstEnergy paid a record $33.5 million in fines to settle civil and criminal complaints. A Davis-Besse engineer and supervisor were convicted of felony charges of willfully giving NRC false information about safety inspections, receiving fines and probation.
"The real lesson of Davis-Besse or even [Three Mile Island] is that we must never get complacent," NRC Commissioner Gregory Jaczko said this month. "Neither event was thought to be probable, or significant, until the very moment when they happened."
A March 27 story about the 1979 accident at the Three Mile Island nuclear plant should have said the accident began with the failure of pumps providing water to a steam generator inside the plant's containment structure that contained the reactor vessel. That led to a loss of water covering the reactor fuel, ending in a partial meltdown. A failure of a valve on the unit's pressurizer tank released radioactive steam into the containment structure. An explosion of hydrogen gas during the accident occurred inside the pressurizer tank. The article described the sequence incorrectly.