In original tekkit-era, and even beta-era, IC2 you could passively cool or heat the reactor with the environment, fire/lava/water. You could also input buckets of lava/water or ice as well. Neither feature works in current ic2c

This is a feature from 1.2.5 IC2.
IC2classic is based on the 1.4.7 version of IC2 where that was no longer a thing.