Yet another idea, this one I actually tried out: make neutron flux spread pollution-like, not cardinal directions only. I find it really counter-intuitive when I have 5 fuel rods in playing dice pattern (in "legendary quality pattern") yet each rod interacts with at most 2 others and one of them - only with itself.
I have tried prototyping it (not in lua yet) and it seems to work just fine. Each cell emits "flux" in 4 cardinal directions to its direct neighbors (and also itself). The neighbors then reemit the flux on the next tick (also back to the original cell) and so on. The system reaches equilibrium in less than 10 ticks (assuming no runaway reaction). The control rod is then just converting the flux into heat.
I think this could even be implemented using Factorio's native pollution mechanics - one would only need to create a dedicated surface and populate it with special machines that emit and consume pollution.