alt Hacker NewsThere were some crazy ideas for nuclear powered cars, but there are hard physical limits how small you can make a nuclear reactor.
1. Smaller the nuclear reactor is more neutron leakage you get. Each neutron which escapes a nuclear reactor is a neutron which can not be used to sustain the chain reactor. To compensate this you have to put more fissionable U-235 isotope into the reactor and as a result you need higher enriched nuclear fuel. A nuclear reactor in nuclear submarine can have the size of a dining table but it's running on nuclear fuel enriched to a weapon grade enrichment.
2. Even a small nuclear reactor with few kW thermal output needs a thick and heavy radiation shielding. This is not problem for power plant, or nuclear powered submarine, or nuclear powered ship. But the shielding requirement were problem for nuclear powered airplanes or trains.
https://en.wikipedia.org/wiki/Nuclear-powered_aircraft
In case of the mobile ML-1 experimental nuclear reactor, built as part of the US Army Nuclear Power Program, extensive shielding was omitted in favor of a personnel exclusion zone of 500 feet (150 m) while in operation.
https://en.wikipedia.org/wiki/ML-1
Chicago Pile-1 (CP-1), the first artificial nuclear reactor, didn't have shielding. But, to keep the dose of ionizing radiation for the staff within reasonable limits, it operated only for very short time periods and the total output of CP-1 was only few Watts.