Until recently, concerns about attacks on commercial nuclear power plants focused mainly on the vulnerability of reactor containment buildings. But nuclear power plants may have a weaker link?spent fuel ponds. ?Reactors are inside steel vessels surrounded by heavy structures and containment buildings,? says Gordon Thompson, senior scientist at the Institute for Resource and Security Studies. ?Spent fuel pools, containing some of the largest concentrations of radioactivity on the planet, can catch fire and are in much more vulnerable buildings.?