I want to build an RBMK capable of rapidly burning up fuel for the production of bismuth and such. But regardless of how much cooling I put into it, it will always melt down with the extreme levels of flux I produce in the core. The fuel doesn't dissipate the heat from the rod's skin fast enough. I wonder if there's a setting to modify the diffusion ratio?

It would be also neat if you can add RBMK rods that do not produce heat, but rather modify the neutron flux in some way. During my attempts to create a fuel burning RBMK, I have, for example, had multiple unobtanium rods channeling their neutron flux directly into a thorium rod which I intended to act as a kind of lens to focus that neutron flux toward a receiving plutonium rod. But that didn't work as planned as the rod acted more like a moderator/absorber than anything else, actually lowering the overall amount of neutron flux despite emitting flux.

Use alot more cooling, for example: if you want use balefire in reactor you need around 8 boilers in ultra-dense mode for only ONE fuel rod.
You can't change heat diffusion i think.
And tip: for bismuth use irradiation channel and make lead-209, than make RTG pellet from it and wait ~20000 tick in RTG generator for depletion to bismuth pellet - now you have 3 bismuth billet. More easier if you have high flux for irradiation channel. One problem - lead-209 is 10000 rad\sec for ingot.

And also maybe you can run reactor in ReaSim mode because in this mode all columns can be use as boilers in ultra-dense mode. Also this mode changes some principles of how rbmk works.

you need setting /gamerule dialDiffusionMod or dialPassiveCooling or dialHeatProvision

But even when set to the maximum, If you use 1-degree fuel rods, e.g MEU. its upper limit still cannot exceed 1500/t, maxHeat is 1500., it is a hardcode