Nuclear microreactors like Westinghouse's eVinci are small enough to fit on a flatbed semi-truck, and NuScale Power's small modular reactor (SMR) is relatively small, too; but are there smaller nuclear power generators? What limits the size of a nuclear power generator?
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1$\begingroup$ livescience.com/technology/engineering/… $\endgroup$– Pete WCommented Apr 13 at 21:27
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$\begingroup$ @PeteW Wow, fascinating. Thanks. I knew there had to be smaller reactors, such as those used in the Voyager etc. space probes. $\endgroup$– GeremiaCommented Apr 13 at 21:29
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$\begingroup$ Where will it be used? if it does not need shielding then small is "easy"... but if you don't want offspring with two heads then shielding becomes important. Unless you are Zaphod of course. $\endgroup$– Solar MikeCommented Apr 13 at 21:47
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$\begingroup$ @SolarMike In a car, someone's home, etc. $\endgroup$– GeremiaCommented Apr 13 at 21:59
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1$\begingroup$ those batteries are not reactors, they are harnessing the heat of radioactive decay. $\endgroup$– Tiger GuyCommented Apr 14 at 18:35
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2 Answers
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The minimum size of a reactor will be based on material constraints and manufacturing. We can only make control rods so small, can only have a stable fuel source so small until it won't support itself. Your other constraints will be based on the overall design and things like how reflective you need the water jacket to be. The atoms in a .5 cm water jacket won't be hit by enough neutrons to bounce them back into the reactor.
As the size drops, eventually the costs per power output will go up to the point of being impractical, and warrant being replaced by items like radioisotope batteries, which harness the heat of decay, not of fission.
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The smallest nuclear reactor ever put into use probably was US Navy' NR-1 submarine.
With a hull of 96 feet and 12 feet wide was only 1/12th its practical volume.
Another contender in size is NASA' KRUSTY or Kilowatt scale, stirling driven mini-reactor about the size of a trash can. 
