This is a recreation of the Nether Reactor from early Minecraft: Pocket Edition, now as a Minecraft: Java Edition datapack! It aims to be as accurate as possible to the original experience, except for a few minor quality-of-life changes.

This a vanilla Minecraft 1.21.6 datapack, no mods required. Though there is a small resource pack with the textures for the two new blocks.

Features

• Adds two new blocks, the Nether Reactor Core and Glowing Obsidian.
• Adds a crafting recipe for the core (3 diamonds down the middle, 6 iron ingots fill in the rest of the slots).
• Adds the Nether Reactor, a player-built structure that spawns a large netherrack spire, hostile pigmen, and some loot.
  
  • See the screenshots for the build instructions.
  • The reaction begins when a player interacts (right-clicks) with the core, summoning the Nether Spire.
  • While the reaction is going:
    
    • The blocks of the structure are replaced by Glowing Obsidian. These are obtainable via Diamond Pickaxe.
    • Zombified Piglins will spawn occasionally, typically towards the beginning of the reaction. These are automatically hostile to players.
    • Loot items will spawn. Each drop is equally weighted between one of the following options:
      4x Nether Quartz
3x Glowstone Dust
Red Mushroom
Brown Mushroom
Cactus
Sugar Cane
Pumpkin Seeds
Melon Seeds
    • The world will progress to midnight and stay there until the reaction is complete.
      
      • The old "infinite night glitch" is intentionally recreated here. If the reaction is interrupted by breaking the core, the reaction will end but the night will remain until another reaction is completed.
  • When the reaction is complete, the center structure is replaced by obsidian, and the outer walls of the spire decay in a random pattern. The dead core is still retrievable inside the obsidian casing.

Changes/additions compared to MCPE

• There's a bossbar on screen to show the reaction progress.
• The Nether Reactor structure can be built with either Blackstone or Cobbled Deepslate instead of Cobblestone.
• Not a change per se, but the Nether Reactor will work in any dimension.
• Expanded the vertical range in which the core can be placed, depending on the build height of the current dimension. The limit is between 8 blocks above the lower build limit and 30 blocks below the upper build limit.
• Removed the requirement for all players in the world to be near the Nether Reactor to activate it.