The owners of the Three Mile Island nuclear plant apparently years ago miscalculated the potential danger of flooding at the plant and now say the Susquehanna River could rise 4 feet higher than what they had originally projected.

Because of an updated analysis, Exelon in recent months upgraded its flood protection system, installing new flood doors, seals and hatches to protect safety-related equipment and buildings.

The recent flooding from Tropical Storm Lee, which created the fourth-highest recorded crest on the Susquehanna, did not come close to overflowing a protective dike that surrounds the northern tip of the island, according to Exelon officials.

According to the U.S. Nuclear Regulatory Commission, the river would have had to rise another 4.5 feet for an "unusual event" to have been declared at the plant and another 7.5 feet to force the shutdown of the plant.

Still, the NRC has notified Exelon that it is out of compliance with its safety analysis regulations "because of the insufficient flooding calculations."

"The NRC staff will have to determine the appropriate response to this noncompliance," agency spokesman Neil Sheehan said. "We will look at whether they should have identified this change earlier."

Exelon, on its own, undertook a new study last fall and used the latest technology and changing river features to re-examine the worst-case flooding scenario at TMI, Exelon spokesman Ralph DeSantis said. Independent engineering firms were hired to do the work, and the latest computer modeling was used.

The consultants found that the possibility of a worst-case flood was actually less likely to occur than originally projected, DeSantis said.

But, at the same time, it was projected that the river at TMI could rise 4 feet higher than what is listed in the nuclear plant's license.

A rock dike protects the northern end of the island in case of severe flooding. There are additional flood barriers closer to safety-related equipment and buildings, and there are watertight measures that can be taken to keep water from getting inside safety-related equipment and buildings, DeSantis said.

Together, the measures can protect against a flood 60 percent greater than what occurred after Tropical Storm Agnes in 1972, DeSantis said.

One factor that may have affected updated projections was the reconstruction of a down-river railroad bridge that has the potential to create a backflow condition.

The NRC currently is finalizing near-term safety measures that will be mandated for all nuclear plants. They stem from concerns raised by the earthquake and tsunami in March that crippled the Fukushima Daiichi nuclear plant in Japan.

Adequate protection against flooding and earthquakes are key items.

acrable@lnpnews.com