The federal government said it found three violations at the Three Mile Island nuclear plant after investigating a small radiation leak that exposed 145 workers to radiation Nov. 21.
The Nuclear Regulatory Commission said the violations were of "very low safety significance" but "more than minor."
Because of the low safety significance and because Exelon is taking steps to make sure it doesn't happen again, the findings will be treated as "non-cited violations," the NRC said in its inspection report.
Still, the NRC criticized plant management, saying it didn't fully use engineering controls to limit the amount of radioactive materials in the air, didn't properly establish procedures that could have limited the release and didn't promptly assign "appropriate significance to the issue."
Unit 1 was not in operation and was undergoing a refueling and steam generator replacement at the time of the incident.
Airborne radioactivity monitors were set off, forcing the evacuation of 175 workers. Body scans determined that 145 of the workers received low-level doses of radiation either to the outside of their bodies or through inhalation. One worker breathed enough radiation to require reporting it to officials, but even that dose did not approach regulatory limits, the NRC said in its inspection report sent to Exelon on April 15.
The amount of radiation that escaped into the outside air was insignificant, the NRC said.
To remove the generator, a large hole had been cut into the containment building, the steel-reinforced concrete structure that surrounds the reactor.
Radiation was first released Nov. 12 from the opening liner. The problem was uncontrolled until alarms went off nine days later.
Exelon then realigned ventilation systems to restore negative pressure within the containment building and drew curtains over the openings to halt the release of radioactivity, the investigation found.
The NRC said it would monitor Exelon's corrective actions to prevent a recurrence of a radioactivity leak.