The NRDC disputes that position. "The Limerick nuclear power plant's safety analysis is decades out of date" and a new one is necessary, said Christopher Paine, director of the council's nuclear program.
"After Fukushima, it seems just plain nutty," he said. "Even if you were completely convinced of the safety of your unit . . . just as a concession to public concern, wouldn't you want to do that?"
Exelon spokeswoman April Schilpp said she was unable to discuss details because the company is reviewing the petition. However, she said, "as we always have, we'll honor the well-defined guidelines and the thorough Nuclear Regulatory Commission relicensing process. In the meantime, we're just going to operate Limerick safely and reliably."
Limerick's two reactor units, which are along the Schuylkill in Montgomery County, began operation in 1986 and 1990. Together, they constitute a large baseload source of power for the regional electrical grid, generating enough electricity to power two million homes.
Their operating licenses expire in 2024 and 2029. Exelon applied for a license renewal in June, seeking to extend the operation of the units to 2044 and 2049.
More than eight million people live within a 50-mile radius, the NRDC said.
Studies like the one in question - called a severe accident mitigation alternatives analysis - are intended to look not at the likelihood of a severe accident, but to assess the environmental, economic and other consequences should one occur.
Possible safety improvements would then be identified and evaluated on a cost-benefits basis. So, the higher the cost of a potential accident, the more rationale there would be for safety upgrades.
These studies are common in relicensing procedures, said NRC spokesman Neil Sheehan. "Essentially, plant owners, as part of their applications, are required to evaluate whether there are changes that can be made to the plant that would improve its safety, provided they are cost-effective," he said.
Sheehan said that he was unable to confirm Wednesday whether Exelon's position that no new study was needed was correct.
Studies on 18 other reactors that are similar to Limerick's have identified an average of four cost-beneficial safety upgrades per plant, and as many as 11 at one plant, the environmental group said.
The NRDC began looking more closely at nuclear power plants after the Fukushima disaster. Its attention soon focused on Limerick, partly because it is up for a license renewal and partly because it is in such a populous area.
Also, when the group looked at whether plants had completed the safety analysis, "Limerick stood out for not having done it," said Matthew McKinzie of the NRDC's nuclear program.
The earlier study relied on population data from 1980, the NRDC said. The NRDC contends that about one million additional people now live downwind.
Also, the group said, the study relied on outdated or insufficient meteorological data used to predict the extent of a plume of radiation; underestimated the time required for an evacuation; and did not include estimated economic impacts from an accident such as industrial income, personal income, and crop losses.
Since the earlier study, hundreds of potential safety improvements have been identified, and much more is now known about the safety of reactors like Limerick's, the group said.
"What was acceptable in 1989 is not good enough for the next 30 to 40 years," said NRDC scientist Jordan Weaver.
Although the Japanese plant was overwhelmed by a tsunami and then an earthquake, the root problem was that it lost power, the scientists said. A similar malfunction - the loss of both primary and backup power - could occur at a U.S. plant for any number of reasons, they said.
"If you look at the inspection records, some plants have gone pretty far down the road toward a serious event," Paine said.
In the past year, U.S. reactors have undergone emergency shutdowns as the result of a tornado, flood, earthquake and hurricane, they said.
Contact staff writer Sandy Bauers at 215-854-5147, email@example.com, or @sbauers on Twitter. Visit her blog at philly.com/greenspace