Tubes, capsules, and pellets of used radioactive material are piling up in the basements and locked closets of hospitals and research installations around the country, stoking fears they could get lost or, worse, stolen by terrorists and turned into dirty bombs. For years, truckloads of low-level nuclear waste from most of the U.S. were taken to a rural South Carolina landfill, but a state law that took effect July 1 ended nearly all disposal of radioactive material at the landfill, leaving 36 states with no place to throw out some of the stuff.