The Fission Reactor is the strongest steady power source in Mekanism, and the only build in the mod that can level your base if you get it wrong. It burns Fissile Fuel to make heat, that heat boils water into steam, and an Industrial Turbine turns the steam into power.
The reactor makes no power on its own, so you need the turbine before you light it. It comes from Mekanism: Generators, a separate download that installs alongside Mekanism, and without that add on none of these blocks exist in your world.
Obtaining
Everything is built from 
Lead, 
Steel and a few 
Elite Circuits, so the reactor is cheap compared to what it powers. Four Lead Ingots around a Steel Casing gives you four Fission Reactor Casing at a time, and the ports and Logic Adapter are then made from that casing.
4
2
You also need Fissile Fuel before the reactor will run. It has no crafting recipe and comes off the end of a chemical line that starts with Uranium Ore, so build that first and store a buffer of fuel before you switch anything on.
Building It
A Fission Reactor is a hollow box of Fission Reactor Casing with an empty 3 by 3 by 3 space inside it. A 5 by 5 by 5 shell is the size almost everyone settles on, because it holds ten Fission Fuel Assembly blocks and still boils steam at full efficiency. Gather every block before you start, since the ports have to go in as you lay each layer.
| Block | Count | What It Does |
|---|---|---|
| Fission Reactor Casing | 84 | Makes up the shell |
| Reactor Glass | 9 | One see through wall |
| Fission Reactor Port | 4 | Water in, fuel in, steam out, waste out |
| Fission Reactor Logic Adapter | 1 | Connects the reactor to redstone |
| Fission Fuel Assembly | 10 | Holds and burns the fuel |
| Control Rod Assembly | 5 | Caps each stack of assemblies |
| Configurator | 1 | Sets what each port does |
One rule governs the whole shell. Every block running along the twelve edges of the box has to be Fission Reactor Casing, and nothing else is accepted there. The flat 3 by 3 middle of each of the six sides is yours to fill with Reactor Glass, a Fission Reactor Port or the Fission Reactor Logic Adapter.
Lay the Floor
A solid 5 by 5 of casing
Place a flat 5 by 5 square of Fission Reactor Casing on the ground. All 25 blocks are casing, with nothing else mixed in. The 3 by 3 middle of this square is the floor your fuel sits on.
Raise the Walls
Three rings of 16 blocks
Build three more layers on top, each one a ring of 16 blocks with the middle left open. The four corner columns have to be Fission Reactor Casing the whole way up. This leaves a 3 by 3 window in the middle of all four walls.
Fill the Wall Windows
Glass, four ports and the adapter
Fill one window with all nine Reactor Glass so you can watch the fuel burn. On a second wall, put a Fission Reactor Port in the bottom middle and another in the top middle, then do the same on a third wall. Drop the Fission Reactor Logic Adapter anywhere in the fourth window and fill every slot you have not used with casing.
Fill the Inside
Five stacks, ten assemblies, five rods
You are now looking at a 3 by 3 by 3 hole. Put a Fission Fuel Assembly on each of the four corners of its floor and one more in the centre, giving you five. Stack a second Fission Fuel Assembly directly on top of each of those five, then cap every stack with a Control Rod Assembly.
That is ten Fission Fuel Assembly blocks and five Control Rod Assembly blocks. The four columns you skipped, the ones in the middle of each side, stay empty from the floor to the ceiling.
| Row | Left | Middle | Right |
|---|---|---|---|
| Back | Stack | Empty | Stack |
| Middle | Empty | Stack | Empty |
| Front | Stack | Empty | Stack |
Cap It Off and Form It
Roof, then right click
Close the top with another solid 5 by 5 of Fission Reactor Casing, then right click any casing block. If the reactor formed you get its interface, and if it did not the chat tells you which block is wrong.
Silence means the reactor is working from a stale scan of itself rather than rejecting a block. Break any single casing block and place it back to force a fresh check, and it will usually form straight away.
| Layer | The Shell | Inside |
|---|---|---|
| 1 | Solid 5 by 5 of casing | Floor, nothing goes here |
| 2 | Ring of 16, corners must be casing | Fission Fuel Assembly on the four corners and the centre |
| 3 | Ring of 16, corners must be casing | A second Fission Fuel Assembly on top of each |
| 4 | Ring of 16, corners must be casing | A Control Rod Assembly capping each stack |
| 5 | Solid 5 by 5 of casing | Roof, nothing goes here |
A Control Rod Assembly goes directly on top of each stack of Fission Fuel Assembly blocks, and the blocks below it cannot have gaps. A missing rod is the most common reason the reactor refuses to form.
Why the Corners and the Centre
This layout is not a formation rule, it is an efficiency one. The reactor counts how many of each Fission Fuel Assembly’s six faces are exposed, and every face touching another assembly costs the pair two points. Spread over the corners and the centre, your ten assemblies average five exposed faces each, which clears the target of four and gives you 100 percent Boil Efficiency.
Packing all nine columns instead would give you 18 assemblies and a higher maximum burn rate, but the average falls to about 2.3 exposed faces and Boil Efficiency drops to roughly 58 percent. You would be burning far more fuel to heat water you cannot boil fast enough.
Hooking It Up
Crouch and right click each port with a Configurator to cycle what it does. Every port only has one face pointing out of the reactor, so each one carries a single line and you need all four.
| Line | Set the Port To | Use |
|---|---|---|
| Water in | Input | Mechanical Pipe |
| Fissile Fuel in | Input | Pressurized Tube |
| Steam out | Output Coolant | Pressurized Tube |
| Nuclear Waste out | Output Waste | Pressurized Tube |
Mekanism counts steam, Fissile Fuel and Nuclear Waste as gases, so all three travel in Pressurized Tubes. Only the water coming back is a fluid. Put a Mechanical Pipe on the steam line and nothing moves at all, with no error to tell you why.
Every 1 mB/t of burn shifts 20,000 mB of water per tick, which is more than any pipe can carry past 6.4 mB/t. Above that, stand the reactor and the Industrial Turbine against each other so the ports touch and feed each other directly.
Mechanics
Burn Rate and Boil Efficiency
Each Fission Fuel Assembly lets the reactor burn 1 mB/t, so ten of them cap out at 10 mB/t. New reactors start at 0.1 mB/t, and you raise the number yourself in the statistics tab.
Boil Efficiency shows how well the coolant is reaching your fuel. Assemblies standing on their own expose every side and give 100 percent, while assemblies pushed together lose faces to each other and drag it toward 50 percent, running the reactor hotter on the same fuel. Spacing them out costs you nothing, so always do it.
Heat and Meltdowns
The interface shows core temperature on a coloured bar. Green below 600K is completely safe, yellow runs to 1000K, orange to 1200K, and red above that means the reactor is taking damage at roughly 2 percent per second. Damage repairs itself once you drop back below 1200K.
Once damage passes 100 percent while the core is still that hot, the reactor can melt down at any moment. Almost every meltdown starts with a full energy buffer: the turbine stops taking steam, steam backs up, water stops coming back, and the core cooks in seconds.
Nuclear Waste
The reactor makes Nuclear Waste as fast as it burns fuel, so 1 mB/t of burn is 20 mB every second. A Radioactive Waste Barrel only clears 1 mB per second, which is fine at the starting burn rate and hopeless above it. Feed the waste into Polonium or Plutonium production instead, where ten units become one.
Logic Adapter Modes
| Mode | What It Does |
|---|---|
| Disabled | Ignores redstone |
| Activation | Turns the reactor on with a signal, off without one |
| Temperature | Fires at 1200K, the moment damage starts |
| Excess Waste | Fires only when the waste tank is completely full |
| Damaged | Fires only at 100 percent damage, far too late to help |
| Depleted | Fires when the fuel tank runs empty |
Frequently Asked Questions
18 blocks along every side. The smallest one that actually works is 3 wide, 4 tall and 3 deep, because you need two blocks of space inside to stack a Fission Fuel Assembly with a Control Rod Assembly on top. Most players build a 5 by 5 by 5.
Pipe Fissile Fuel into a port set to Input using a Pressurized Tube, since Fissile Fuel is a gas rather than an item. You make it by processing Uranium Ore through a chemical line that ends at an Isotopic Centrifuge.
Mostly Lead and Steel. A 5 by 5 by 5 build takes 84 Fission Reactor Casing, 9 Reactor Glass, 4 Fission Reactor Port, 1 Fission Reactor Logic Adapter, 10 Fission Fuel Assembly and 5 Control Rod Assembly. You also need Mekanism: Generators installed, since none of these blocks come with Mekanism on its own.
Right click any casing block to open the interface, then press the activation button. Fill the coolant tank and connect the Industrial Turbine first, because a reactor burning with nowhere to send its heat reaches damaging temperatures in under a minute.
