U.S. President-elect Donald Trump has picked Brandon Williams, a former Navy officer and one-term member of Congress, to become the keeper of the nation’s arsenal of thousands of nuclear bombs and warheads. Trump’s selection is a shift from a tradition in which the people who served as administrator of the National Nuclear Security Administration typically had deep technical roots or experience in the nation’s atomic complex. What’s unknown publicly is the extent of Williams’ experience in the knotty intricacies of how the weapons work and how they are kept reliable for decades without ever being ignited. Terry C. Wallace Jr.,...