MC Version:1.12.2
MOD Version: NuclearCraft-2.12d--1.12.2

It should be -5280K/t when Liquid Helium is pumped into the active cooler at a rate of 2mB/s.
Because 52800K/20t*2mB=5280K/t
However: the cooling rate keeps increasing and finally stops at the maximum possible cooling rate of a single active cooler with liquid helium.

After some tests, it's clear that the fusion structure dose not remove the used coolant in active cooler.

Also, when I changed version back into 2.11g, the magnets does not work properly, and the reactor will finally explode.

The switching-back caused an issue because the way NBT data was saved and loaded changed between 2.11 and 2.12. It was forwards-compatible but I didn't think to make it backwards-compatible.

I will look for the cause of the cooling error, though!

Here's version 2.12e :)

Found the problem - bit of a sneaky one. Will try to get a fix out for this and the fusion issue tonight if I can get TeamViewer working ;)