Two security experts from the Department of Energy’s Idaho National Laboratory drove to San Antonio, Texas, in March 2017 with a sensitive mission: to retrieve dangerous nuclear materials from a nonprofit research lab there. Their task was to ensure that the radioactive materials did not fall into the wrong hands on the way back to Idaho, where the government maintains a stockpile of nuclear explosive materials for the military and others. To ensure they got the right items, the specialists from Idaho brought radiation detectors and small samples of dangerous materials to calibrate them: specifically, a plastic-covered disk of plutonium,...