after the PFBR criticality thread a couple days ago, spent some time digging into what actually determines whether breeders scale from one working reactor to energy independence. the answer surprised me. the reactor physics works fine. breeding ratio around 1.2 means you're producing more fissile material than you burn. that part is settled. the binding constraint is the out-of-pile fuel cycle. you breed plutonium in the blanket, great. but then you need to cool the spent fuel, chemically separate the plutonium from fission products (reprocessing), and refabricate it into new fuel assemblies. only then can it start another reactor. the whole-system metric is doubling time: how long for one reactor to breed enough surplus fuel to start a second. for oxide-fueled breeders like PFBR, system doubling time is around 30 years, and most of that lag is cooling + reprocessing + refabrication, not the breeding itself.…