U.S. most vulnerable reactor: Indian Point 3, N.Y.

of how fast the risk estimates rose: The median value of all 104 reactors is now at a 1 in 74,176 chance as compared to the old estimate of 1 in 263,158.

This NRC process began in 2005 when its staff recommended taking a look at updated seismic hazards. It was late 2008 before NRC staff started working with a contractor, Electric Power Research Institute, on the design of a study. Overall, it took five years and three months from the staff recommendation until the seismic task force submitted its report in August 2010.

One problem is a lack of data about the nuclear reactors themselves. The NRC task force said the agency has detailed data on what it calls plant fragility — the probability that the expected earthquake would damage the reactor’s core — for only one-third of the nation’s nuclear plants. That is because only the plants that had been thought to be in areas of higher seismic risk had done detailed studies. For the rest, the scientists had to estimate from other information submitted by plant operators.

An NRC spokesman, Scott Burnell, said Tuesday that the NRC is preparing a letter to send to certain nuclear plants, asking them for more detailed data on equipment, soil conditions, and seismic preparedness. Then the plants and NRC staff will have an opportunity to analyze that data.

That process could stretch into 2012, Burnell said. Then, the NRC will have to decide “where the ability to respond to seismic events can be improved.”

In the middle of that process, perhaps late this year, a new round of geologic data will come out. That will be folded into new calculations.

The nuclear power industry is watching this process. A document distributed to the public by the industry’s Nuclear Energy Institute on Sunday, after the Japanese plant emergency began, referred to this NRC study and the possibility of changes by saying: “The industry is working with the NRC to develop a methodology for addressing that issue.”

The industry statement did not mention that the study increased the estimates of earthquake risk for nearly every nuclear power plant in the United States.

When the NRC saw that the new earthquake maps had pushed the level of risk into the range between 1 in 100,000 and the more likely 1 in 10,000, that change was enough to study the issue further, the task force said in its report. Since the