National Geographic : 1991 Aug
REACTOR DESIGNS: NEW AND IMPROVED? SINCE THE 1979 MELTDOWN at Three Mile Island in Penn sylvania (right), emergency shutdowns at the nation's 113 reactors have been reduced by three-fourths. Yet some analysts say the chances of a severe accident within the next 20 years remain as high as 25 percent. To improve the odds, designers have gone back to the drawing boards. When they fail Though most water-cooled reactors have operated safely during the past three decades, some components have proved troublesome. Among them: steam-generator tubes prone to crack, valves that fail to open or close, and backup electric sources vulnerable to interruption. When emergencies arise, reactor safety depends entirely upon backup mechanical and electric devices-themselves vulnerable to breakdown. By contrast, proposed reactors would use gravity, natural air flow-convection-and evaporation to give operators more time to respond. In addition, the reactors would operate at reduced power, sacrificing output for added safety.