 Recent concerns have been expressed about safety issues affecting a large part of the nuclear fleet of reactors.In 2012, the Union of Concerned Scientists, which tracks ongoing safety issues at operating nuclear plants, found that "leakage of radioactive materials is a pervasive problem at almost 90 percent of all reactors, as are issues that pose a risk of nuclear accidents".The U.S. Nuclear Regulatory Commission reports that radioactive tritium has leaked from 48 of the 65 nuclear sites in the United States.