An Extreme Reactors reactor is a box: casing on the outside, fuel rods inside. It burns Yellorium and makes energy, leaving Cyanite behind. There is no fixed size — the reactor assembles itself as soon as the box follows the rules. This page builds the smallest one, 3 × 3 × 3, with the recipes of Extreme Reactors 2.4 (Minecraft 1.21.1).
1. Materials
Smelt Yellorite ore (or Raw Yellorium) into 
Yellorium Ingot. Smelt coal or charcoal into 
Graphite Bar — the casing needs it.
Yellorite Ore → Yellorium Ingot
Coal → Graphite Bar
Iron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphiteIron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphite
Sand#c:sands/colorlessGraphite Bar#c:ingots/graphiteIron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphiteIron Ingot#c:ingots/iron
4 × any Iron Ingot + 4 × any Graphite Bar + any Sand → Reactor Casing (Basic)
2. The smallest reactor
Tap or click a block to see what it is.
Lay a 3 × 3 floor of Reactor Casing and build the casing walls up around the middle, except the four middle blocks of the side walls.
The single block inside the reactor is a Reactor Fuel Rod.
The middles of the four side walls: the Reactor Controller (north), a Passive Forge Energy Power Tap (south) for the energy, a Solid Access Port (west) for the fuel, and Reactor Glass (east) to look inside.
Close the top with casing. The middle of the top is a Reactor Control Rod, right above the fuel rod. The reactor assembles by itself as soon as the last block is in; the controller's screen shows it.
Materials 7 kinds · 27 items
- Reactor Casing (Basic)step 1×21
- Reactor Fuel Rod (Basic)step 2×1
- Reactor Controller (Basic)step 3×1
- Reactor Glass (Basic)step 3×1
- Reactor Passive Forge Energy Power Tap (Basic)step 3×1
- Reactor Solid Access Port (Basic)step 3×1
- Reactor Control Rod (Basic)step 4×1
The layer plan appears with the 3D view.
- Every edge and corner of the box is Reactor Casing (Basic). The walls, floor and roof may also hold glass, ports and the controller.
- Inside,
Reactor Fuel Rod (Basic)s stand in columns from the floor to the roof. Each column has a 
Reactor Control Rod (Basic) in the roof directly above it. - One
Reactor Controller (Basic) per reactor. A 
Reactor Passive Forge Energy Power Tap (Basic) gives the energy out as Forge Energy; a 
Reactor Solid Access Port (Basic) takes the fuel in and gives the Cyanite out. - Bigger reactors use the same rules: in the default settings a reactor may be up to 32 blocks wide and long and 48 blocks high.
On our test server the 3 × 3 × 3 reactor assembled by itself as soon as its last block was in. Ten Yellorium Ingots put into the access port: the reactor took 4 of them — 4,000 mB of fuel, all one fuel rod holds — and 6 stayed in the port. Switched on at the controller, it filled its power tap with energy and turned some of the fuel into Cyanite within 200 ticks.
Yellorium Ingot#c:ingots/yellorium
Diamond#c:gems/diamondYellorium Ingot#c:ingots/yellorium
Redstone Dust#c:dusts/redstone4 × Reactor Casing (Basic) + Redstone Comparator + 2 × any Yellorium Ingot + any Diamond + any Redstone Dust → Reactor Controller (Basic)
Iron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphiteIron Ingot#c:ingots/iron
Any #c:glass_blocks- Glass
- White Stained Glass
- Orange Stained Glass
- Magenta Stained Glass
- Light Blue Stained Glass
- Yellow Stained Glass
- Lime Stained Glass
- Pink Stained Glass
- Gray Stained Glass
- Light Gray Stained Glass
- Cyan Stained Glass
- Purple Stained Glass
- + 6 more
Yellorium Ingot#c:ingots/yelloriumAny #c:glass_blocks- Glass
- White Stained Glass
- Orange Stained Glass
- Magenta Stained Glass
- Light Blue Stained Glass
- Yellow Stained Glass
- Lime Stained Glass
- Pink Stained Glass
- Gray Stained Glass
- Light Gray Stained Glass
- Cyan Stained Glass
- Purple Stained Glass
- + 6 more
Iron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphiteIron Ingot#c:ingots/iron4 × any Iron Ingot + 2 × any Graphite Bar + 2 × any Glass (#c:glass_blocks) + any Yellorium Ingot → Reactor Fuel Rod (Basic)
Redstone Dust#c:dusts/redstoneIron Ingot#c:ingots/iron
Iron Ingot#c:ingots/ironGraphite Bar#c:ingots/graphite4 × Reactor Casing (Basic) + any Redstone Dust + 2 × any Iron Ingot + Piston + any Graphite Bar → Reactor Control Rod (Basic)
4 × Reactor Casing (Basic) + 4 × Redstone Dust + Block of Redstone → Reactor Passive Forge Energy Power Tap (Basic)
Iron Ingot#c:ingots/iron
Any #c:chests/woodenIron Ingot#c:ingots/iron4 × Reactor Casing (Basic) + Hopper + 2 × any Iron Ingot + any Chest (#c:chests/wooden) + Piston → Reactor Solid Access Port (Basic)
Any #c:glass_blocks- Glass
- White Stained Glass
- Orange Stained Glass
- Magenta Stained Glass
- Light Blue Stained Glass
- Yellow Stained Glass
- Lime Stained Glass
- Pink Stained Glass
- Gray Stained Glass
- Light Gray Stained Glass
- Cyan Stained Glass
- Purple Stained Glass
- + 6 more
Any #c:glass_blocks- Glass
- White Stained Glass
- Orange Stained Glass
- Magenta Stained Glass
- Light Blue Stained Glass
- Yellow Stained Glass
- Lime Stained Glass
- Pink Stained Glass
- Gray Stained Glass
- Light Gray Stained Glass
- Cyan Stained Glass
- Purple Stained Glass
- + 6 more
2 × any Glass (#c:glass_blocks) + Reactor Casing (Basic) → Reactor Glass (Basic)
3. Passive or active
A reactor with a power tap is passive: it makes energy directly. An active reactor is cooled by water through fluid ports and makes steam for a turbine instead — the fluid ports exist only for the Reinforced tier, so the Basic reactor is always passive. How a turbine is built: How to Build a Turbine.
4. Cyanite and Blutonium
The spent fuel comes out of the access port as 
Cyanite Ingot. Cyanite is needed for turbine parts, and in the 
Reprocessor Controller multiblock 2 Cyanite and 1,000 mB of water make 1 
Blutonium Ingot — a fuel that burns like Yellorium.
2 × Cyanite Ingot + Water 1,000 mB → Blutonium Ingot