The main purpose behind asking this question is to ascertain the potential power-to-weight ratio of reactors built using modern-derivative technology (i.e. either Generation III derivatives or Generation IV derivatives with experimental operational experience, as well as declassified marine reactor elements) and lifted using present-capacity vehicles for use to power space habitats and to power ion engines for high-delta-V, high-mass missions†. Obviously, the higher the power-to-weight ratio the better, especially as a higher PWR leads to a higher TWR in ion engines, leading to more efficient and expedient burns, which is extremely valuable in the case of crewed missions.
The design goal is absolute maximum continuous electrical power fulfilling these design conditions. (Note that the radiation conditions may be more stringent than what would be actually needed for acceptance, but safer is always better—for example, if you just "shadow-shield" the reactor, what happens if you need an EVA for repairs to its rear? Also, while strict for a space application, look at the second-to-last sentence...):
- Full assembly (including working fluid) must mass less than 24,400 kg, making it liftable by the in-service Delta IV Heavy and Falcon Heavy as well as the post-Challenger-disaster Shuttle.
- Going in hand with latter element of the first requirement, the full, folded assembly must fit inside the Shuttle cargo bay. (Likely foldable elements would include radiators.)
- The system must be closed-cycle. (Of course.)
- The radiation environment in the direct vicinity of the reactor assembly shall nowhere exceed the non-emergency limits established for US radiation workers in standby.
- Low-positive or negative void coefficient, or alternatively a gas- or liquid metal-cooled design.
- Redundant SCRAM shutdown modes.
- Reactor core must not suffer a containment loss or meltdown from a ballistic reentry from a Martian trajectory. (Note: As the emergency heat shielding will likely do double-duty as radiation shielding, limit of 4. in the case of a reactor core crash from ballistic Martian reentry will be that for emergency work.)
- Reactor core must not suffer a containment loss or meltdown from a crash at its terminal velocity at any shielding mass.
- The working fluid must not come into direct contact with nuclear fuel.
Note this is talking about electrical (≈brake) power, not thermal power.
My estimate is somewhere in the range of 1–5 MW, but what do I know...
Lastly, about how much could the mass be reduced for operation in an atmospheric environment, for use, say, in medium-output generators in a similar form factor to the Army ML-1 and large vehicles (by replacing the large radiator panels with a small air radiator and combined radiation/heat shielding with more specialized radiation shielding)?
†Actually, the specific impetus was to flesh out an alternate history where an International Mars Spacecraft is constructed instead of the International Space Station...