>The fact the nuclear reactor hasn't been miniaturized to the size of a coin But it has, or at least close to: RTGs[1] weighting a couple dozen kilograms are

>The fact the nuclear reactor hasn't been miniaturized to the size of a coin But it has, or at least close to: RTGs[1] weighting a couple dozen kilograms are used in space exploration and in rare cases in cold climates. Their main problem is low power density. Nuclear pacemakers[2] of just a few centimeters across used to be a thing - long lasting and reliable. Also used thermoelectric generators. Replaced by cheaper lithium-based batteries due to high costs of making the nuclear capsule resilient against various possible sources of damage - like body cremation, car accidents, etc. Nonetheless we have reached certain natural limit of nuclear fission, and only use of nuclear fusion[3] (or major scientific and engineering break-throughs) could change the status quo. The three main unsolved problems for wide scale commercial deployment of fission power: 1) radiation shielding - right now we only have passive shielding via use of structural materials, weights a lot 2) critical mass for active chain reaction (heavy and requires active management) vs passive generation via spontaneous fission (low power density) 3) operational safety and non-proliferation issues: a fission device requires radioactive material, and radioactive material is by its nature dangerous Notably all three issues are either minimized or entirely absent for nuclear fusion. Lastly, in typical nuclear power plant, the power generation facilities are usually of comparable size to, or even larger than, the nuclear reactor building itself - so miniaturizing the reactor would yield only diminishing results. High-temperature superconductors ought to be of some help here. -- [1] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... [2] https://www.medicaldesignandoutsourcing.com/medtech-memoirs-the-plutonium-powered-pacemaker/ https://www.medicaldesignandoutsourcing.com/medtech-memoirs-... [3] "Typical fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel." - https://en.wikipedia.org/wiki/Inertial_confinement_fusion https://en.wikipedia.org/wiki/Inertial_confinement_fusion