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212 MW Thermoelectric Fission Reactor

This is a very hot reactor with nice overall properties.

Its very high outlet temperature means that you can get away with very small heat sinks.
(This makes it less efficient in terms of nuclear fuel consumption, but the game does not care about that.)

It is highly optimized in the sense that changing two or less design variables is very likely to either decrease its power output or make the game declare it broken.

Mass, cost, radiation hazard and size are all quite low.