Hey! This issue was reported on my tracker: EnigmaticaModpacks/Enigmatica2ExpertSkyblock#221
I'll copy paste it here, I hope you don't mind.

Issue Description
Large Nuclearcraft fission reactors, with active cooling, can overflow the cooling rate value, as it can go over 65535, and NC doesn't handle that. Apparently default NC config cooling values were "recently" updated, but not sure when, so the config may need a regen. Or if the heat values were intentionally boosted, they may need to just be set to normal. If the coolers are set to half efficiency that could maintain the 'difficulty' if that was the case

Just an example:
Nearly overflowing:

Then with one more cooler:

Version

1.12.2-2.12f

\o/ thanks!

Finally fixed for the next version :)

Cheers Niller - yep, this is a known problem. Will get it sorted! The reactor should still run fine, it's just the GUI numbers that mess up :)

Ah alright, thanks!