Federal regulators need to take a closer look at how nuclear plant operators assess rare but possibly catastrophic events, the Government Accountability Office said in a report released today.

The Nuclear Regulatory Commission should review how many reactor managers are using "probabilistic risk assessments," or PRAs, to gain insight into the types of accidents that could cause radiological release, according to GAO's April 26 report.

Although NRC has promoted the use of PRAs since 1986, the commission hasn't required operators to use such complex analyses.

Power plant operators currently use "historical experience, test results and expert judgment" to analyze a set of potential accident scenarios, according to the report. PRAs, on the other hand, calculate the frequency of events that could trigger reactor accidents and how the plant's complex systems would respond.

Rep. Ed Markey (D-Mass.), the ranking member of the House Natural Resources Committee, and Senate Environment and Public Works Chairwoman Barbara Boxer (D-Calif.), who requested the study, said GAO's findings highlight the commission's "failure" to use the most up-to-date assessments to protect the public against earthquakes and natural disasters that could cripple U.S. reactors.

"This report is yet another indication that while the NRC races ahead to issue or extend licenses for nuclear power plants, it has fallen behind inexcusably in addressing the safety of these very same facilities," Markey said in a statement. "We know what happened at Fukushima could happen here in the U.S., and we should utilize the best and latest information available to assess vulnerabilities so we can ensure the safety of our operating nuclear reactors."

More than half the 15 experts GAO interviewed -- professors, consultants, nuclear physicists and trade groups -- recommended expanding the use of PRAs but acknowledged that such tests can be expensive and that there is a limited number of specialists equipped to oversee the modeling.

NRC has agreed to assess whether plant operators should conduct probabilistic risk assessments and is already reviewing how well U.S. reactors can withstand tornadoes, hurricanes and flooding, NRC Operations Director William Borchardt said in a letter attached to the report.

NRC Commissioner George Apostolakis is also leading a task force to determine whether the commission should revamp its regulations in the wake of Japan's nuclear disaster last year, Borchardt said.

A magnitude-9 earthquake and tsunami struck the Fukushima Daiichi nuclear complex, which triggered core meltdown in three reactors, radioactive releases and evacuations. An internal NRC expert panel later said the commission should review its "patchwork" approach to safety in light of that disaster (Greenwire, July 20, 2011).

Even so, Boxer said the NRC has "simply no excuse" for not using such assessments now and called for immediate safety changes.

"While the NRC has agreed to study the issue, action is needed now to ensure that standards are in place that best protect the health and safety of the American public," she said in a statement.