Japan 's electric power companies run 54 nuclear reactors at 17 power plants . The Fukushima Daiichi plant is the most dangerous, and it was built in a tsunami zone that has been prone to earthquakes for decades. An explosion caused by hydrogen buildup Saturday blew the roof off the No . 1 reactor 's containment structure and injured four workers , but the reactor was not reported to have sustained damage . Most Japanese plants -- including the three facing emergencies -- use boiling-water reactors .
Japan 's electric power companies run 54 nuclear reactors at 17 power plants . The Fukushima Daiichi plant is the most dangerous, and it was built in a tsunami zone that has been prone to earthquakes for decades. Most Japanese plants -- including the three facing emergencies -- use boiling-water reactors .