The worst nuclear disaster in U.S. history occurs at the Three Mile Island nuclear power plant in Dauphin County, Pa., when the core of a reactor undergoes a partial meltdown. Blame is placed on the nuclear industry and the Nuclear Regulatory Commission for failing to have proper regard for nuclear safety and worker training. The $1 billion cleanup takes more than six years. In 1986, an explosion and fire at the Chernobyl nuclear plant in the Ukraine spread tons of radioactive material, killing 31 people immediately and affecting thousands more.