There are 99 nuclear reactors producing electricity in the United States today. Collectively, they're responsible for producing about 20% of the electricity we use each year. But those reactors are, to put it delicately, of a certain age. The average age of a nuclear power plant in this country is 38 years old (compared with 24 years old for a natural gas power plant). Some are shutting down. New ones aren't being built.