The federal Government Accountability Office has concluded that the methods used to determine natural hazard risks at the nation’s nuclear power plants could be improved.
In a highly technical report issued this week, the GAO recommends that the Nuclear Regulatory Commission should examine the benefits of requiring that nuclear plants add probabilistic risk assessment (PRA) to the methods used to evaluate and prepare for natural hazards such as earthquakes and tsunamis.
Probabilistic risk assessment is a broader method for assessing what can go wrong, its likelihood and its potential consequences. Findings would help to determine levels of risk in order to provide insights into the strengths and weaknesses of the design and operation of a nuclear power reactor.
“Without conducting the analyses necessary to determine whether a PRA requirement is warranted for operating reactors, NRC does not have a strong basis to judge whether or not such a requirement should be implemented,” said Frank Rusco, author of the GAO’s report.
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The report was written at the request of U.S. Sen. Barbara Boxer, D-California, and Rep. Edward Markey, D-Massachusetts, who questioned the effectiveness of risk assessment at nuclear plants in the aftermath of the March 2011 earthquake in Japan and subsequent tsunami damage at the Fukushima Dai-ichi plant, which resulted in the release of significant amounts of radioactive material.
Plant managers at Diablo Canyon nuclear power plant already use probabilistic techniques for evaluating seismic and fire risks at the plant, said Jearl Strickland, director of nuclear projects at Diablo Canyon. But the criteria had not been updated since they were first developed in 1988, so an update was begun last year.
“The key is that Diablo was on the forefront of developing a PRA program and has effectively used it since 1988,” Strickland said. “It’s been a valuable tool.”
Additionally, Diablo managers agreed to delay license renewal until high-resolution studies of the earthquake faults around the plant can be completed over the next several years.
The PRA technique is particularly useful when analyzing the risk of taking a particular piece of safety equipment out of service for a period of time for maintenance, Strickland said.
In a letter to the GAO, the NRC agreed that probabilistic risk assessment has advantages and will consider requiring such analysis. Boxer wants swift action. “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.
The GAO is the investigative arm of Congress. In the course of researching the report, GAO investigators visited Diablo Canyon and four other nuclear plants and interviewed 15 national experts.
The report concluded that protecting against natural hazards at the 104 nuclear reactors in the nation rely primarily on deterministic risk assessment.
This approach uses historical experience, testing and experts to establish a set of potential accidents that specific nuclear plants face, as well as steps operators must take to protect public health and safety in the event of an accident.
Experts told the GAO that adding probabilistic risk assessment would be a more comprehensive approach and might allow plants to better assess all the threats posed by natural hazards, particularly for extreme natural hazards with a low likelihood of occurring but potentially high consequences, such as what happened in Japan.