Reprocessing of used nuclear fuel is a key step in enabling the deployment of advanced nuclear reactors and minimizing the amount of radioactive material that must be disposed of as waste. Pyroprocessing safely and efficiently recycles used nuclear fuel into usable components by applying electrochemical operations in molten salt electrolytes. For light water reactor (LWR) fuel, the oxide is electrochemically reduced to metal, then electrorefined to recover a pure uranium product. The fission products are removed from the salt electrolyte and immobilized in durable waste forms for disposal. These processes are well understood at the laboratory scale, but the impacts of salt chemistry and cell geometry on process efficiencies must be determined on scaled-up systems to optimize production methods, process controls and monitoring, and product quality. Recent work at Argonne National Laboratory on understanding aspects of the process electrochemistry and engineering at multiple scales will be discussed.