Have the cooling systems inside a nuclear reactor in the U.S. ever failed before?

Yes, but it didn't last long and the impact was minimal.

In March 1990, a truck backed into a transformer at the at the Alan Vogtle nuclear plant near Augusta, Georgia. The resulting local blackout caused the heat pump for one of the two reactors at Vogtle to switch off. One of the backup diesel generators was down at the time for scheduled maintenance and another failed to start. With both the regular and backup sources of power for cooling out, the temperature began to rise in the reactor.

Within 37 minutes, however, an emergency backup generator came online and restarted the cooling system.

So in total, the U.S. has had one cooling failure. It lasted 37 minutes and the accident was man-made, said Tony Pietrangelo, chief nuclear officer and senior vice president at the Nuclear Energy Institute, the industry's domestic trade group, during a conference call with reporters. 

The U.S. has 3,500 cumulative years of experience of operating reactors, he said. (In all, the U.S. has 104 commercial nuclear reactors and around 104 in the military.)

If you wanted to put it another way, you could say the incident constituted 0.0000482 percent of the total operating hours of the U.S. nuclear fleet.

But from another perspective, the number to keep in mind is 12 miles. That is the size of the official evacuation area around the Fukushima nuclear power plant. The unofficial evacuation area has expanded to 19 miles and the U.S. government is advising its citizens to stay 50 miles away. The Fukushima disaster occurred because of multiple failures with the cooling systems and backup generators. Only one hour passed between the time the earthquake damaged the plant and when the tsunami knocked out the backup generators. Harsh critics thus could argue that the real difference between Vogtle and Fukushima was 23 minutes.

Last year alone, the U.S. nuclear industry had 14 "near misses" -- significant safety lapses which resulted in special investigations, according to the Union of Concerned Scientists.

The Vogtle incident was also not the only time the backup cooling systems have failed in the U.S. Vogtle marked the only time that both the operating and backup cooling techniques failed. In 1998, a tornado struck the Davis-Besse plant outside of Toledo. The transmission lines to the plant went down and the diesel generators kicked in. But 26 hours later, the sweltering 90-degree heat caused the backup generator to shut off. Luckily, power had been restored only one hour before. The diesel generators were still running but thankfully weren't absolutely necessary.

"Had that not been the case, then you would have been relying on your battery capacity for the safety of Davis-Besse," David Lochbaum of the UCS recently said in testimony before Congress.

More nuclear or not? That's the big question facing the U.S. and other nations. Nuclear plants can provide baseline, carbon-free energy, say proponents. The Nuclear Energy Institute says that between four and eight new nuclear reactors might be in various states of completion in the U.S. between 2016 and 2020. Pietrangelo further added that the industry as a whole will begin to take precautions against multiple, simultaneous safety hazards. Since 2001, the U.S. nuclear industry has looked at what would happen if one reactor in a series got knocked out by a plane. Fukushima will force the industry to contemplate what would happen if two or more reactors go out due to multiple causes.

Critics, meanwhile, note that Fukushima and subsequent reviews of the U.S. fleet have underscored the risks and potential safety hazards. Investors also are wary: nuclear plants often cost more and take more time than anticipated. Interviews conducted by us with various experts on the cost of nuclear plants yielded results ranging from $4,000 a kilowatt to $10,000.

"Folks on Wall Street have long memories on cost overruns," said Pietrangelo.

Germany has already vowed not to expand its nuclear fleet. France, by contrast, promotes nuclear, although power plants in recent years have had to shut down on some days during droughts due to a lack of water to be used for cooling them.