sentences:  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".Following the Japanese 2011 Fukushima Daiichi nuclear disaster, according to Black & Veatch’s annual utility survey that took place after the disaster, of the 700 executives from the US electric utility industry that were surveyed, nuclear safety was the top concern.