Minecraft wiki entry for Nether_reactor, associated text: The spire was composed of over 25 stacks worth of netherrack.
Excluding the core, any part of the reactor could be mined after activation and it would still function. This way, the player could recycle the expensive gold blocks. It was possible to recover all 4 blocks provided the player worked quickly.
Because the spire could not replace blocks past the world border, when the reactor was activated near the edge of old worlds the player would be able to see the time set to night.
If the nether reactor core was mined out while the nether reactor is active, the glowing obsidian would not turn into obsidian after the reactor deactivates. This also caused the reactor to stop spawning items and zombie pigmen, and make the day/night cycle freeze at nighttime for several in-game days.