"It's one of the greatest, and most disturbing, questions of the Fukushima disaster: What happened to the nuclear fuel inside the plant? Now physicists are trying to shed some light on the problem using particles from the edge of space."

"The Fukushima accident was broadcast around the world. On March 11, 2011, an earthquake and tsunami struck the plant, knocking out cooling in three working reactors. The uranium fuel inside melted down."

"But nobody's quite sure where it went."

"'Right now we don't know where the fuel is,' says Christopher Morris, a fellow at Los Alamos National Laboratory in New Mexico."