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...):

  1. 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.
  2. 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.)
  3. The system must be closed-cycle. (Of course.)
  4. 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.
  5. Low-positive or negative void coefficient, or alternatively a gas- or liquid metal-cooled design.
  6. Redundant SCRAM shutdown modes.
  7. 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.)
  8. Reactor core must not suffer a containment loss or meltdown from a crash at its terminal velocity at any shielding mass.
  9. 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...

  • $\begingroup$ "look at the second-to-last sentence..." -- I'm lost here, and don't know which sentence you mean. I'm afraid I'm lost through much of this... it's a bit like a government request-for-proposal, very long and detail and a little difficult to wrap my mind around. $\endgroup$ Commented Apr 7, 2020 at 3:42
  • $\begingroup$ Is the external radiation requirement normal for space reactors? My impression is that they are typically shadow shielded only. $\endgroup$
    – ikrase
    Commented Apr 7, 2020 at 11:27
  • $\begingroup$ Recommend closing because it's overdefined in some areas and badly underdefined in others. For example, the amount of waste heat that needs to be dumped in a reactor is overwhelming by spacecraft standards $\endgroup$ Commented Apr 7, 2020 at 12:35
  • $\begingroup$ The Gen III and Gen IV articles that you linked describe Nuclear/Thermal power stations. I'm no expert, but I'd imagine that the operating environment and the requirements are so completely different... It's like you asked about building a race car, and then linked to an article about next generation railway locomotives. $\endgroup$ Commented Apr 7, 2020 at 18:08

1 Answer 1


There are some examples of space reactors:

3kW@385kg: BES-5

5kW@1000kg: TOPAZ

At your 24400kg weight budget that scales to 100-200kW electrical output. But these are thermionic(3% efficiency) and you may be able to do better with gas turbine (30%-50% efficiency), or not (secondary loop, turbine and generator machinery are heavy).

Let's make a rough guess here:

A 5MW gas turbine plus electric generator (not including the reactor) is about 35000kg already(SIEMENS SGT-100). Say turbine+generator is 7kg/kw electrical(SIEMENS SGT-100, 5MW@35000kg), and reactor is 3.85kg/kw thermal(BES-5, 100kW@385kg), and you don't need secondary loop (e.g. HTGR) and gas turbine is 50% efficiency, you are looking at 7+3.85*2=14.7kg/kw weight/power ratio, which translates to 1.7MW@24400kg.

The finished product be quite a piece of engineering but it sure looks promising.

  • $\begingroup$ Where are you going to get the fuel for your turbine? The whole point of RTG and their ilk is the energy source is self-contained. $\endgroup$ Commented Apr 7, 2020 at 12:33
  • 1
    $\begingroup$ @CarlWitthoft I'm using fossil fuel gas turbine electric generator as an estimate of weigh/power ratio for the non-reactor components of an HTGR power pant. The heavy components are very similar between the two (shaft, compressor, turbine, electric generator and structural support). With some effort the overall design could be even identical: take an existing fossil fuel gas turbine power plant, replace air with Helium, combustion chamber with reactor core, add exhaust cooler and close cycle plumbing. To directly answer your question: use the reactor core. $\endgroup$ Commented Apr 7, 2020 at 14:31

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