To come as close as possible to the site of the accident, we leave the control room, descend two flights of metal stairs, and enter the basement. The place is a forest of pipes, with bare concrete floor and walls.
''There was light contamination in this whole area,'' says my guide, Douglas Bedell, of General Public Utilities.
A yellow tarp is jammed into one corner, as if to stop a leak. The tarp is cordoned off by a low dike of sand and by a rope hung with ''Danger'' signs.
''Anytime you come across those yellow-and-magenta ropes, you don't cross them,'' Mr. Bedell says. ''It means that's a radiological work area.''
Welcome to Three Mile Island. Behind that tarp, on the other side of a concrete wall, is Unit 2 - the nuclear reactor that five years ago suffered the worst mishap in the history of United States atomic energy.
Today, as men in moon suits slowly clean up the damaged reactor, controversy still swirls around Three Mile Island (TMI). At issue now is whether the undamaged Unit 1, which has lain fallow since the accident to its brother Unit 2 , should be restarted.
The answer to this question could greatly affect the future of atomic energy in the United States, since TMI and its memorable cooling towers have come to symbolize the problems of the industry as a whole.
''All (US) nuclear reactors are hostage, in a sense, to the poorest performing units,'' a recent congressional staff report concludes.
For motorists meandering down Pennsylvania Route 441, 10 miles south of Harrisburg, the Three Mile Island plant is hard to miss. It appears suddenly by the side of the road, its cooling towers rising off a Susquehanna River island like 40-story flower vases.
To the workers of TMI's day shift, rattling across a private bridge in pickups and sports cars, those towers mean jobs. To some other area residents, they are a disturbing reminder of the accident that was, and the restart that might yet be.
''That plant presents a daily danger,'' says Larry Hochendoner, a commissioner of Dauphin County, where TMI is located. ''And I think that reflects the opinion of the majority of the county.''
Five years ago, in the early hours of March 28, 1979, a pump feeding cooling water into TMI's Unit 2 reactor suddenly stopped. Backup pumps kicked on - to no effect, because of two inexplicably closed valves. Human error and deficient equipment compounded the problem, and for dangerous minutes, part of the reactor core was not covered by coolant and overheated.
Inside the squat silo of its containment building, Unit 2 now lies quiet, its shattered fuel load bathed in a fission-stifling boron solution. Workers can stay in the same room as the reactor for only about three hours; after that, heat fatigue induced by their suits sets in. Radiation, in any case, would drive them out after six hours or so.
Cleanup is expected to take at least another four years. Since 1979, 1 million gallons of contaminated water from Unit 2 have been pumped out, mostly cleaned, and stored. Last month, workers removed the reactor's top in preparation for digging out the uranium within.
Several hundred yards away, in an identical building, is a reactor in a far different state - TMI-1.
Unit 1 was first started in 1974. It was down for refueling when Unit 2 was struck by the accident, and has remained shut since by order of the Nuclear Regulatory Commission (NRC), pending design improvements and an investigation into the competence of the GPU, which operates the reactors.
The NRC has been struggling with the restart issue for several years. Most of the agency's studies and reports on the matter are now finished, however, and commissioners could make a final decision before fall.
Three Mile Island's owners are outwardly confident they will be allowed to fire up Unit 1. GPU officials say the experiences of the last five years have annealed their company, forcing changes that have since made Three Mile Island a safer place.
''We have been humbled and humiliated,'' GPU's chairman, William Kuhns, admitted to the NRC last month. ''We have learned from (our) mistakes.''
There is, for instance, the matter of the training center. Everyone who investigated the Unit 2 mishap concluded that the reactor's operators were poorly taught. In response, GPU erected a new school for atomic employees, just behind the visitor center, and hired around 40 new instructors. The average operator's training time has been doubled since 1979, the company says.
GPU has spent $95 million refurbishing Unit 1 since the accident, company officials say. Dozens of safety changes have been made in the control room, they note: New alarms have been installed, and instruments have been regrouped so operators can better understand reactor status.
''The most basic changes have been in people,'' says Bedell. ''Now there's an awareness that something unexpected can happen.''
Nevertheless, GPU has been beset by events that sully its image:
* Six TMI operators, including a Unit 2 supervisor, have been caught cheating on licensing exams administered by the NRC.
* Three GPU employees complained publicly that a crane used in the Unit 2 cleanup was not properly tested. One of these whistleblowers was illegally fired by company officials, says the NRC.
* Last February, the GPU pleaded guilty to a federal charge of falsifying safety test data in 1978 and '79, before the accident.
Taking these events into account, the NRC staff says the GPU has made ''grave'' mistakes, but is now fit to operate Unit 1 safely.
Nuclear power critics disagree.
''GPU is just unfit to hold a license,'' says Ellyn Weiss, general counsel for the Union of Concerned Scientists (UCS).
Critics say TMI 1 still has physical flaws. Instruments that monitor the temperature of cooling water and its level in the reactor core either do not meet NRC requirements or have not been installed, UCS says. Unit 1's emergency feed-water system ''still fails to meet the most basic requirements,'' a UCS report charges.
In 1981, GPU discovered that sulfur had somehow gotten into the Unit 1 turbine system, and that 30,000 tubes in the steam generator were corroded.
The company says it has fixed the problem; some critics say the corrosion means the plant can't be run safely.
''No other nuclear plant has ever been shut down so long,'' says Joanne Doroshow, a lawyer for Three Mile Island Alert, a Harrisburg-area group.
Pennsylvania Gov. Richard Thornburgh (R) says TMI-1 should not be fired into operation - at least not yet. ''Adequate safety assurances have not yet been made,'' Governor Thornburgh told an August NRC hearing.
Some local citizens go even further. They would be not be disappointed to see TMI turned into an inactive relic.
''The corporation that has run that plant has proved itself incompetent and immoral,'' says Lauren Taylor, a Harrisburg-area mother and newsletter editor recently fined $10 for blocking traffic in front of the plant.
At bottom, say experts, the restart issue now is not so much new dials and gauges as the character of the company throwing the switch.
The NRC's task is made all the harder, say experts, because how they handle TMI will have lasting impact on US nuclear energy as a whole. Public confidence in atomic energy is quite low, a recent congressional report concludes, in large part because of the accident at Three Mile Island.
If the public perceives that the NRC hasn't done a good job evaluating Unit 1 , restart could very likely prove counterproductive.
''The confidence of the public, investors, rate and safety regulators, and the utilities themselves is too low to be restored easily,'' says the congressional Office of Technology Assessment study. ''Unless this trust is restored, nuclear power will not be a credible energy option for this country.''