The Advance of Reactors examines reactors differing from the large, water-cooled plants that provide the preponderance of today’s nuclear electricity; they fall into overlapping categories of breeder, modular, and advanced reactors. The need for reactors that breed more fuel than they burn waxes and wanes with perceived future shortages of uranium fuel. Small modular and micro reactors should bring lower costs from factory mass production and incremental financing. Adapting naval reactor technology may provide rapid ramping capabilities. Advanced reactors—often modular—utilize gas, sodium, or molten salt coolant to achieve efficient high temperature operation.