A spent fuel assembly pulled from a reactor after six years of operation looks nearly identical to the fresh one that went in. The mass has changed by a tiny fraction of a percent, nuclear alchemy has occurred in which half the periodic table has been generated in the form of fission products within the ceramic pellets but the volume and geometry is essentially identical. This one fact, that nuclear fuel must be preserved rather than destroyed, that the job of every layer of every system surrounding the core is to maintain the integrity of a material through years of radiation bombardment and extreme temperature gradients, shapes much of nuclear engineering. It explains the cladding materials, the obsessive quality control in fabrication facilities, and the decades of slow, painstaking improvement that have transformed a fleet that routinely operated with failed fuel elements into one where a single leaker triggers a formal investigation.