Nuclear reactors have one job: to split atoms in a controlled reaction and use the released energy to generate electrical power . Over the years, reactors have been viewed as both a miracle and a menace. When the first U.S. commercial reactor went on line in Shippingport, Pa., in 1956, the technology was hailed as the energy source of the future , one that some believed eventually would make electricity too cheap to meter. Countries around the world built 442 nuclear reactors, and about a quarter of those reactors were built in the United States [source: Euronuclear.org ]. The world has come to depend upon nuclear reactors for 14 percent of its electricity [source: Nuclear Energy Institute ]. In fact, futurists fantasized about having nuclear-powered automobiles [source: Ford ]. Then, 23 years later, when Unit 2 at the Three Mile Island power plant in Pennsylvania suffered a cooling malfunction and a partial meltdown of its radioactive fuel, feelings about reactors change