Last month, Japanese authorities released an initial investigative report on the causes and consequences of the March 11 earthquake and tsunami disaster at Fukushima Daiichi that documented weaknesses in planning for extreme emergencies and anticipating severe damage to multiple reactors at the site, and chaotic conditions confronting operators as because of equipment and instrumentation failures.
Following the emergency, the NRC ordered inspections of emergency preparedness at the 104 U.S. reactors. While the results did not reveal imminent threats to reactor safety, the commission said, the inspections documented gaps in emergency planning and cases where emergency equipment might not withstand earthquakes or other severe events.
For example, the NRC reported that the Diablo Canyon nuclear power plant on southern California's coast had a single diesel-driven pump to provide emergency cooling water to a single reactor in case an earthquake cut off normal water flow. The pump could not have serviced both of the plant's reactors if they lost normal water supply simultaneously, the NRC staff said.
The plant's six emergency diesel generators were located in the same plant area, and thus vulnerable to a "common mode" failure. An earthquake could cause a structural failure in the building where the fire truck is stored, and debris could block crews from using the truck.
Entergy's Arkansas Nuclear One plant safety plan was directed against the loss of offsite power to one of its units, and did not anticipate a simultaneous additional threat such as an earthquake.
At Duke Energy's Oconee Nuclear Station in South Carolina, pumps required to remove water from auxiliary buildings in a flood could not be used because the plugs did not fit any outlets in the area. Instrumentation on spent fuel pools would be unavailable if power were lost, which would require workers to visually inspect water levels -- "an unacceptable requirement under some scenarios," the NRC said. One such scenario would be a loss of water in the pool to a level that permitted fuel rods to ignite and release perilously high radiation levels.
The NRC said that all issues have been fixed or put on schedule for correction, and that the safety of the reactors was not compromised.
The task force recommendations included proposals to strengthen and standardized plant defense in response to the inspection findings.
The task force also called for a more "coherent" approach to reactor safety issues to improve what it called a "patchwork of regulatory requirements and other safety initiatives."
The NRC's review of the Japanese accident has centered on a key distinction in NRC regulations that cover "design basis" or anticipated accidents, and "beyond design basis" threats from extreme disasters that are considered less likely but are potentially more dangerous. "I am really bothered by this separation between design basis and beyond-design basis," NRC Commissioner George Apostolakis said at a commission meeting in May.
"I appreciate the need for a design basis. Licensees know what they have to do. We impose all sorts of conditions. This particular pump must deliver this flow rate under these conditions. And then we are going to inspect. We asking them to test it and tell us what they find, all that."
The plant owners' responses to beyond design basis threats are usually voluntary. "We keep saying, 'Oh, these are beyond basis events therefore we don't' get involved.' We are happy that the industry responded. We look at it once. That's it. In the future it's up to them. I am really bothered by that," Apostolakis said.
"It is a constant challenge we have to deal with," NRC director of operations Bill Borchardt responded. "There is a balance."