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500 MW Thermoelectric Fission Reactor

ah didnt know that. tyvm

Regarding update, you can just modify the old design and reupload, if you use the in-game upload function (rather than manual uploading of exports). Workshop id is stored in the blueprint.

Your reactor doesn't really run all that hot, most ceramics should handle those temperatures easily.
If you try driving radiators almost twice as hot as your core is now, then you might need to make some questionable choices like hafnium carbide control rods, not because it's any good, but because pretty much everything else melts:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1945930655

As for moderator, I mostly build fast neutron stuff anyway (apart from NTRs).

much appriciated on the control rods suggestion. found that boron nitride was actualy a bit more efficient here but hey both were an improvement over 10 tons of graphite. as far as the tungsten carbide goes, im not sure if its a case of a value being off somewhere but tungsten was actualy one of the few materials that made a dent in shifting this thing towards supercritical. the only two materials i could find to actualy get it there were tungsten carbide and magnetic metal glass, and the glass isnt capable of standing up to reactor temperatures. the turbopumps are already in their sweet spot but i did change the material on the blades to shave off a bit of mass. the coolant was chosen for thermal conductivity and, as mentioned, being a good moderator on its own. as far as the thermocouple goes its prettymuch as small as i could get it. anything smaller runs into cracking from thermal stress.

AtomHeartDragon 11 hours ago
Feedback 2/2:
- Could use some shielding (boron carbide recommended). A lot of people don't use it and just place radshields, but it's massive already and added freedom of putting reactors where you want them helps make your ship harder to kill.
- Very heavy thermocouple. Good materials for the working temperature, but try to reduce dimensions and thickness.
- Might be helped by bigger but slower turbopumps - find the sweet spot that makes them lighter. Pretty heavy turbopump material too, but that's not much of a problem and it's a pretty realistic one.
? Odd choice of coolant. Not wrong in itself (note, this coolant will already be a moderator on its own), but there might be a bug in thermal properties values for ethane, fixing which might break your reactor.

AtomHeartDragon 11 hours ago
Feedback 1/2:
+ Nice efficiency. Kind of comes with the territory at low temps, but you still need to do things right to get it.
- Low exit temperature. At such output it'll need massive radiator area to get rid of the waste heat. OTOH it's pretty far on the safe side of realism, but then you'd probably want less power and smaller reactor to make do with smaller radiators.
- Very massive, especially given low temperature (less stresses) and lack of any kind of shielding.
- Weird and inefficient choice of moderator/control rods material (tungsten carbide is very heavy, but only the carbide part is doing the work, for control rods having something that actually catches the neutrons is better - at those temperatures boron carbide should work just fine).