Apologies for badgering you with questions, but it did make me wonder - why is there no neighbour bonus for Fusion reactors? I know they're pretty powerful already, with a single one being as strong as 12 Nuclear reactors. However, as few as 4 Nuclear reactors can outperform a single Fusion reactor and 8 of them in a double line (which is what I'm using) can outperform 3 Fusion reactors.

I get that Fusion reactors heat up much faster, but I'm not sure that's enough to offset the ease of using Nuclear with a preexisting setup. Some amount of neighbour bonus would help offset this, as it could reduce the size of the currently way oversized nuclear power plants.

While developing the mod, I thought for a long time what to do with neighborhood bonus. In the end I decided against a neighborhood bonus because, as you said yourself, the individual fusion reactor is already very powerful. With neighborhood bonus, the fusion energy would then be OP.

I'm not sure it would be OP, and I have a couple of reasons to feel that way.

Firstly as it stands, I'd argue that your Fusion reactors are if anything a bit underpowered. Due to the neighbour bonus on Nuclear reactors, their output can exceed that of Fusion reactors with very few actual reactors present. Considering the reactors themselves are the smallest part of a reactor complex, that's a problem. Outside of "for the hell of it," I'm hard-pressed to justify investing in the processing infrastructure.

Secondly, your Tritium Breeder is a balancing factor as it is. It needs to be built right next to a Fusion reactor, the more the better. This means that the player needs to choose between a neighbour bonus by slapping two reactors together, or a tritium processor, instead. Considering one side of each reactor needs to be open for heat pipe access, standalone reactors can support a lot more breeders than reactors in stacks. As it stands now, there is no decision. Reactors gain no neighbour benefit so I've no reason to even consider sticking them close together.

Mind you, I'm not saying you HAVE to add a neighbour bonus, just that your mod already has the mechanics to counterbalance it :)

edit
Since I was already fiddling with your code, I decided to test out a 50% neighbour bonus. We'll see how that goes. If it's too extreme, I could cut that down to 25% or some such. Hope you're OK with me changing settings on my own end. Probably screw multiplayer compatibility but oh well.