NRC says it has adequate safeguards in place for battery backup and spent-fuel pools.
A 1998 tornado knocked out power to the Davis-Besse plant near Toledo, Ohio, for more than a day. An NRC analysis (pdf) of the event said that the outage brought the plant perilously close to meltdown.
Regulators were alarmed by a blackout that hit the northeastern United States in 2003, cutting power to nine reactors and prompting a wide-ranging review (pdf) by NRC.
And in 1992, Hurricane Andrew caused the Turkey Point Nuclear Reactor near Miami to lose access to the grid for more than six days.
A 2005 NRC report (pdf) shows there were 24 "loss of offsite power" events between 1997 and 2004, including the nine in the Northeast blackout.
Some nuclear critics say the situation is even more dangerous with spent fuel -- uranium-bearing rods that no longer produce enough energy to sustain a nuclear reaction in the reactor.
In the United States, most spent fuel remains on site at nuclear plants because the country has not developed a facility to store it.