Gregory B. Jaczko (/ˈjɑːskoʊ/; born October 29, 1970, Norristown, Pennsylvania) is a physicist who was chairman of the U.S. Nuclear Regulatory Commission (NRC) from 2005 to 2012. While at the NRC, he voted against the opening of new nuclear plants and an inspector general report found that he unilaterally and improperly sought to block the Yucca Mountain nuclear waste repository project from advancing. After leaving the NRC, Jaczko called for a global ban on nuclear power.