Reactors bristle with subatomic complexity. Especially in the early days, devilishly complex reactor physics calculations had to be undertaken because the underlying physical phenomena are so difficult to model. As a former actuary, I knew something about mathematics, but I struggled, and still struggle, with nuclear physics. Witness this page from a wonderful, but tough, basic text, E. E. Lewis’s “Nuclear Reactor Physics.” I can’t pretend that I mastered this or many other pages, but, in my defense, where a physics or engineering principle is crucial for understanding how a reactor works, I found I can muddle along though the physics until I grasp the essence.