According to the AP one of the berms holding back flood waters at Fort Calhoun Nuclear Generating Station failed at 1:30 AM Sunday (June 26, 2011):
...This resulted in backup generators being temporarily used to power both the already shut down main reactor (having been shut down since April) and spent fuel cooling systems, as the main power was cut. Quoting a second AP article:
The 2,000-foot berm collapsed about 1:30 a.m. Sunday, allowing the swollen river to surround two buildings at the plant. The NRC says those buildings are designed to handle flooding up to 1014 feet above sea level. The river is at 1006.3 feet and isn't forecast to exceed 1008 feet.
...According to CNN news wire the berm was punctured by machinery:
The berm's collapse didn't affect the reactor shutdown cooling or the spent fuel pool cooling, but the power supply was cut after water surrounded the main electrical transformers, the NRC said. Emergency generators powered the plant until an off-site power supply was connected Sunday afternoon, according to OPPD.
...In this case there was not any lengthy electrical outage as the power was diverted immediately to working backup generators and reconnected promptly from the outside. Unfortunately, at the Fukushima Daiichi nuclear power plant once battery backup systems were exhausted, the diesel generators had been damaged and made non-functional by the tsunami. No external power was able to be brought in to the Fukushima Daiichi nuclear plant until the cores had already melted.
Some sort of machinery came in contact with the berm, puncturing it and causing the berm to deflate, said Mike Jones, a spokesman for the Omaha Public Power District (OPPD), which owns the Fort Calhoun plant.
For further reading: another article from CNBC, or this article from Omaha World Herald.
 Flood berm collapsed at Nebraska nuclear plant, AP via Action 3 News
 Flood berm collapses at Nebraska nuclear plant, AP via Victoria Advocate
 Flood berm bursts at Nebraska nuclear plant, CNN news wire