The ZIRNOX and research reactors are both single things you place down (although multiblocks) that accept different fuel rods and radiation sources in a grid. Research reactors are just fancy radiation sources for breeding reactors (the process is not entirely realistic since the conversion using neutrons is way too clean but if you wanted realism you’d stick to RBMK fuel recycling).
The ZIRNOX (inspired by real life MAGNOX reactors but using zirconium instead of magnesium because I really didn’t feel like adding an entirely new element just for one reactor) is gas-cooled with a similar fuel pattern as the research reactor which turns regular water into super dense steam (i.e. double compressed so you need three turbines to fully decompress it, I believe IC2 has a similar system where you could double stack turbines).
The fusion reactor is also a fixed structure similar in operation as the ZIRNOX (but instead of using nuclear fuel it needs a separate plasma heater that injects the desired fuel type), creating triple compressed steam.
The RBMK (probably a fan favorite at this point) is made up of many smaller pillar-like parts that you can arrange in different ways to fully customize the size, power and purpose of your reactor. There are some parts that you usually wouldn’t see in a real RBMK (like dedicated boilers or evaporation-cooling elements) but those are purely optional. While I haven’t played with GT6’s reactors too much but I think in general design they are somewhat comparable.
There’s also a generic “large nuclear reactor” which can be expanded upwards in size but honestly the thing sucks in almost all aspects, it can deplete fuel really fast I guess but that’s about it.
Steam goes directly out of the reactor core into turbines, there’s no coolant cycles or heat exchangers, mostly because I never had the time to add them and I don’t feel like adding this extra step that doesn’t really add anything functionally (also because I find the idea of having radioactive steam pouring through your turbines mildly humorous). After steam is fully decompressed, it can be piped into either a compact condenser or a cooling tower to turn it back into water, allowing you to close the reactor’s water cycle.