The core reactor is for crafting other stuff. It is a custom modeled block, and was modeled by Guybot1001. He was modeling for a different block, when he started to screen share on Skype and MCE saw it as a use for something else, then coded in this block and set the model to it.
The smelter has modes, or upgrades that you can add. You place these in the fuel slot of the smelter. Note, you still need to have lava in the smelter.
Item
Mode
Damageable
Changes
Notes
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Einsteinium, Neptunium, Uranium Core Reactors
Nuclear
Yes (All 250)
Reduces speed from 400 to 200.
400 speed is the normal smelter speed, twice as slow as a furnace. 200 is normal furnace speed.
Super Reactor
Nuclear
Yes (750)
Reduces speed from 400 to 150.
400 speed is the normal smelter speed, twice as slow as a furnace. 200 is normal furnace speed. This is slightly faster.