The problem -- uncovered during a maintenance inspection of the Unit 2 reactor at Indian Point in March -- showed that 227 of 832 stainless steel bolts in the reactor vessel were degraded, according to the Nuclear Regulatory Commission .
Environmentalists said the complications with the bolts, which hold together metal plates used in power production, could have catastrophic consequences.
"Failure of (the bolts) could cause coolant to leak through gaps between adjacent baffle plates, providing pathways for coolant to bypass the reactor core, potentially leading to a core meltdown," they said in their petition to the NRC.
Environmentalists said Unit 2 should be kept off line until regulators fully investigate the cause and remedies of the bolt problem, and that the sister Unit 3 should be shut until potential problems there can be assessed.
Nuclear foes have long sought to shut Indian Point , which is about 45 miles (72 kilometers) north of Times Square in Manhattan.
"In the case of Indian Point , located within 50 miles of tens of millions of people, there is no room for error."
Plant operator Entergy said it replaced degraded bolts in Unit 2 and installed additional bolts there. It also moved up the inspection of Unit 3 from 2019 to 2017 "out of an abundance of caution."
Entergy has said it plans to bring Unit 2 back online by the end of June in time for the peak summer cooling season.
"Rigorous technical analysis conducted by Entergy and outside engineering experts demonstrates Unit 2 and Unit 3 can continue to operate safely," it said.
"Entergy is proceeding according to NRC process and under the watchful eye of this regulator."
Neil Sheehan, a spokesman for the NRC, said the agency would not permit Unit 2 to return to service until its safety was assured.
The NRC is reviewing the latest submission from Entergy, and plans to set up a group to review the petition from environmentalists .
Sheehan said the findings were that Indian Point 's bolts were "degraded," meaning some sign of cracking.
"It doesn't mean failure," he said.
"The worst-case scenario is they get a significant failure of bolts and it could impact the flow of coolant through the core," Sheehan added.
"We've seen no indication that that's a likely scenario. But we will continue to look at the company's analysis and they're going to have to address this."
Sheehan declined to comment on a possible timetable for returning Unit 2 to service.