Kristine Svinicki knew just how to warm up hundreds of nuclear physicists, engineers and other experts who had gathered for a nuclear regulatory conference in Maryland earlier this year. She cracked a neutron joke. "A neutron walks into a bar, and the bartender says, 'What'll you have?'" the 45-year-old Nuclear Regulatory Commission member said, her small frame tucked behind a wooden lectern at a hotel in downtown Bethesda, Md. "The neutron says, 'I think I'll have a beer, but how much is it?' and the bartender answers, 'For you, no charge.'" Svinicki's wit doesn't surprise her former colleagues, even if it's seldom seen in public settings.