This item has been removed in IC2 Experimental Build 500?
Experimental Build 499?
Heating Cells were used to heat up Nuclear Reactors in order to make them produce Re-Enriched Uranium Cells faster and more efficiently. They replaced the Lava Bucket working to achieve the same thing pre v1.106.
Heating Cells were the only item capable of stacking inside reactors, as this was because the heating effect in the reactor is <stacksize> times 1000. Although, Heating Cells did not actually heat the reactor up directly, they would only heat up components adjacent to them, meaning a Reactor Heat Exchanger would be needed to actually pass the heat onto the reactor. Any adjacent Heat Vents would also try to dissipate the heat, and would be unable to do so, causing them to melt and be destroyed.