Another, more experimental, complicated idea for a “coolant”:
- Molten fuel-salt mixture: Basically turns the heat vents into fuel, emitting heat, energy and neutrons and all, so no cooling with them. This is only possible when non-coolant heat vents are a thing, or cooling would be limited to coolant cells/condensators (if available). Produces hot fuel-salt mixture based on heat, which can be heat exchanged (but a bit of it is lost, so no infinite fuel, but no nuclear waste). Coolant heat vents filled with the fuel-salt mixture also act as a core heat exchanger (averages out percentage-wise), so if they get hot, your core also, if it gets cool, so does your core. Neutron output is lowered with higher temperatures. High heat capacity and no heat transfer/cooling.
This “coolant” is really a liquid fuel. It’s very safe (hot temperatures will make the reactor stop working and no coolant automatically means no fuel) and produces no waste, but cooling it and keeping it efficient is really hard, because it takes away the ability to have a dedicated coolant.
Edit: Would probably be a good idea to just rename the “heat vents” that accept fluids, because they are so mechanically different than heat vents. Something like “Reactor Pipe” or “Reactor Channel”.