* they have a high power density in a small volume and run either on low-enriched uranium (as do some French and Chinese submarines) or on highly enriched uranium (>20% U-235, current U.S. submarines use fuel enriched to at least 93%, compared to between 21–45% in current Russian models, although Russian nuclear-powered icebreaker reactors are enriched up to 90%),
* the fuel is not UO2 but a metal-zirconium alloy (c.15% U with 93% enrichment, or more U with lower enrichment),
* they have long core lives, so that refueling is needed only after 10 or more years, and new cores are designed to last 25 years in carriers and 10–33 years in submarines,
* the design enables a compact pressure vessel while maintaining safety.
Terrestrial Energy (of Canada) is trying to develop integral molten salt nuclear fission reactors. These nuclear reactors would have about 20-200 times less volume than conventional nuclear fission reactors. The US, Europe and China are trying to develop supercritical carbon dioxide turbines that would have 100 times less volume than regular steam turbines. The Hammer's Slammers Science fiction nuclear hovertank would be enabled with the two technologies that are under development (molten salt reactors and supercritical CO2 turbines.
By shrinking the nuclear reactor and the turbine by 100 times, plenty of other vehicles are made possible. Various nuclear ships and submarines can be revamped. Also, space bases with nuclear become more possible with one launch.
The 650 MWth IMSR (Integrated Molten Salt) reactor is about the same size as the smAHTR (125 MWth) reactor.
The smAHTR reactor is 9 meters tall (30 feet) by 3.5 meters (12 feet) in diameter.
The 220 MWth S8G reactor for the Ohio submarines is 42 feet in diameter, 55 feet long; 2,750 tons
So the IMSR with supercritical CO2 turbines would have almost 3 times as much power in an area about 16 times less area. In the range of 150-200 cubic meters and about 200-400 tons.