when having 248k reactors next to each other they they crash the game and i think its the neighbour bonus that has something todo with it.
https://imgur.com/a/aiK7Lv4

https://imgur.com/a/6ZK5B9J

I assume next to each other means directly attached without a heat pipe in between?
Thats not possible with this mod, the neighbour bonus function can't handle that.

The issue still persists.

You can place Plutonium Power Mox Reactors directly touching each other, and their adjacency bonuses work just fine.
Likewise, you can have Realistic Reactors touching each other and profiting from adjacency bonus.

But when you either place 248k reactors next to each other, or connect 2 already placed 248k reactors with a heat pipe, you get a lua crash:
The mod Realistic Reactors (3.1.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RealisticReactors::on_built_entity (ID 6)
RealisticReactors/scripts/heat/network.lua:498: attempt to index local 'q' (a nil value)
stack traceback:
RealisticReactors/scripts/heat/network.lua:498: in function 'add_entity_to_network_cell'
RealisticReactors/scripts/heat/network.lua:618: in function 'add_heat_pipe'
RealisticReactors/scripts/events/construct.lua:35: in function 'on_heatpipe_added'
RealisticReactors/scripts/events/construct.lua:104: in function 'entity'
RealisticReactors/control.lua:51: in function <RealisticReactors/control.lua:50>