EDIT: It's now a public challenge with a small prize for the winner:
https://forums.factorio.com/viewtopic.php?f=5&t=83935
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Oof, I got nerdsniped by a comment about making a nuclear reactor for a 9-tile high ribbon world (meaning the entire reactor setup can be only 7 high), and it got me wondering what is the maximum possible. During the process I learned a lot about fluid/heat mechanics, and ultimately I came up with this design, which I don't think can be improved anymore.
Prove me wrong! (I'm probably wrong!)
I'll probably make a public challenge out of it to improve my design, with a small prize for the winner (like a quarantaine challenge or smt similar. If I do, I'll let you know)
Short rules: 7-tile high design, all reactors must be connected as a single unit, must be refuelable by train, 'real' output counts (must be able to output MW-value indefinitely)