Among the findings in other reports:
Entergy's Arkansas Nuclear One plant safety plan is directed against the loss of offsite power to one of its units, and does not anticipate a simultaneous additional threat such as an earthquake.
Numerous manhole inspections in the past year have revealed safety-related cables submerged in water, a problem the NRC inspectors identified as minor.
At Duke Energy's Oconee Nuclear Station in South Carolina, pumps that would be used 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 Palo Verde nuclear plant, operated by the Arizona Public Service Co., determined that some seals that were not hardened to withstand seismic shocks could fail in an earthquake, allowing water to enter rooms containing electrical equipment used to shut down the plant. Three tanks at the plant could rupture, leaking water into the plant, and a backup diesel generator and electrical switch gear were vulnerable to flooding in such an emergency.
The report on Dominion Resources' Millstone Power Station in Connecticut noted that some equipment is classified as "seismically qualified" and must function during and after the maximum earthquake anticipated for the site (based on historical data plus a safety margin).
However, most sump pumps and flooding detectors are considered "non-safety related" and thus are not hardened to withstand earthquakes, the report said. Firefighting equipment staged to respond to severe fires or explosions was not stored in hardened buildings because a severe fire and an earthquake "were not assumed to occur coincidentally."
An "isolation valve" for unit 1 would have to be operated to pressure the fire main to fight fire. But the valve would be under water following an anticipated flood that occurred at the same time as a fire. These issues are under review, the NRC said.
At Entergy's Indian Point 2 station on the Hudson River above New York City, inspectors reported that fire fighting equipment is not designed to withstand earthquakes, which could compromise the fire protection system. Generally, plants are not required to survive a simultaneous loss of outside and internal alternating current power ("station blackout") and an earthquake, the NRC said.
In a severe accident at Indian Point, where it was crucial to relieve pressure inside the reactor containment, high pressures could damage equipment required to carry out the venting and "potentially prevent containment depressurization," the NRC said. Workers at Fukushima were forced to vent hydrogen and steam after fuel assemblies melted in order to prevent an even more catastrophic damage to reactor containment structures and a far greater radiation release.
Ameren's Callaway nuclear plant in Missouri assigns operating staff to make up the fire brigade, but trying to fight two fires at once would be "very difficult" because of limited staffing, the NRC said.
The company had not assessed the capability of a halon fire suppression system that protects essential switchgear rooms. "The licensee determined that this equipment does not need to be evaluated based on an industry frequently asked question," the NRC said. The company has trained workers to use water to fight electrical switchgear fires if halon is not available, the report said, raising the risk of flooding in adjacent rooms with electrical controls because flood doors have not been established. The NRC said issues at both Indian Point and Callaway are being evaluated.