Minecraft wiki entry for Nether_Reactor_Core, associated text: Nether reactor cores are currently unobtainable in new worlds except through inventory editing or add-ons. It is not available through commands or Creative. The block is, however, still usable for decoration in older worlds that had the block prior to it becoming unobtainable. In Survival Mode, new Nether Reactor Core cannot be crafted, including in worlds with the block, so once placed, the block would be lost if broken. In Creative Mode it is still possible to use Pick Block on existing Nether Reactor Cores to get the block into the inventory.