Portable nuclear is usually discussed as a single technology, which hides the interesting structure. It is a ladder. At the bottom, betavoltaic cells deliver milliwatts for decades and power sensors that must never be visited. Above them, radioisotope nodes deliver watts for a working generation and can sit on a seabed as a dock that vehicles visit. Then tens of kilowatts, then containerised megawatts that replace a diesel generator, then installation-scale units under twenty megawatts, then port-scale plants mounted on barges. One rung per class of machine, from a sensor you bury and forget to a terminal that runs itself. The useful question is never which rung is best; it is which rung is smallest for the job, because every step up buys power and pays for it in licensing, security and siting.