Feed The Beast Wiki

This page is for Solid-Fuel Fission Reactors in NuclearCraft. You may also be interested in reading the Solid-Fuel Fission Reactor section of the NuclearCraft tutorial or the Reactor Designs page for more specific info on designs that others have come up with.

Fission Reactors

4x3x5 Solid-Fuel Fission Reactor, complete with Reactor Ports for fuel handling and redstone activation

A solid-fuel fission reactor consists of a controller and a reactor core. The core is then filled with your choice of Reactor Cells, Coolers and Moderator blocks. Building the frame of the core is simple, but choosing the right combination and configuration of parts to go inside is tricky!

  1. Start by building the reactor core frame out of Reactor Casing and Fission Ports if wanted.
  2. Place a Fission Controller alongside or within one of the walls of the frame.
  3. Place Reactor Cells, Moderator blocks (currently the only moderators are Graphite and Beryllium blocks) and Coolers inside the core.
  4. Insert a Fuel Rod into the controller.
  5. Apply a redstone signal to the controller to turn the reactor on.
  • The interior of the reactor core may have any size from 1x1x1 to 24x24x24. Any cuboid shape is valid; it doesn't need to be a cube.
  • Note that only the faces consist of reactor casing blocks - placing reactor casing along the edges will most often result in the controller being unable to determine the position and size of the reactor.

Constructing Your Reactor

This section details the mechanics of how NuclearCraft's fission reactor components interact.

Managing heat and power, while getting the best efficiency, is the core challenge of NuclearCraft's fission reactor.

  • Each block inside a Fission Reactor either creates power (RF) and heat (H), removes heat, or modifies the way in which the blocks interact.
  • The position of blocks relative to each other is very important. Some blocks get boosts from being adjacent to others, some won't work unless adjacent to another.

Fuel Calculator

Jox, a member of the NuclearCraft Discord server, created a spreadsheet that both assisted with Fission Reactor calculations and tabulated the products of fuel reprocessing. It was updated by the mod author to include information on which fuel is most efficient in the design being tested, and can be found here

Reactor Core Blocks

The following blocks should be placed down inside the reactor core:

Reactor Cell

This is the main component that creates energy and heat.

  • When not adjacent to any other cell or moderator, each cell produces the base power and heat listed for the fuel you are using. For example, if using TBU fuel, which has a base power output of 60 RF/t and a base heat output of 18 H/t, a reactor containing a single cell will produce 60 RF/t and 18 H/t. A reactor containing two non-adjacent cells and using TBU fuel, will produce 120 RF/t and 36 H/t.
  • Any cell adjacent to at least one other has a higher efficiency and their power and heat output increases dramatically. In general, each cell adjacent to other cells produces times the base fuel power output and times the base fuel heat output. For example, a reactor with two adjacent cells and TBU fuel will have an efficiency of 200%, producing 240 RF/t and 108 H/t.

Moderator Blocks

Moderator blocks such as Graphite provide an efficiency boost but will also produce additional heat. Although moderators will generate additional heat wherever it is placed in the structure, it will only generate extra power if adjacent to at least one Reactor Cell.

The additional power generated per moderator block is equal to RF/t for each adjacent cell, where cellEff is the efficiency of the cell. Similarly, the extra heat generated is H/t. If it is not adjacent to any cells, additional power will not be generated and extra heat equal to the fuel's base heat output will be generated. In addition, if two cells are connected by a straight line of at most four moderator blocks, the cells behave as if they were adjacent, becoming more efficient.


Coolers remove heat from the reactor core. There are several different types of cooler and each has its own set of conditions under which it operates.

Water Cooler
60 H/t Must be adjacent to at least one Reactor Cell or active moderator block.

Redstone Cooler
90 H/t Must be adjacent to at least one Reactor Cell.

Quartz Cooler
90 H/t Must be adjacent to at least one active moderator block.

Gold Cooler
120 H/t Must be adjacent to at least one valid Water Cooler and one valid Redstone Cooler.

Glowstone Cooler
130 H/t Must be adjacent to at least two active moderator blocks.

Lapis Cooler
120 H/t Must be adjacent to at least one Reactor Cell and one Reactor Casing.

Diamond Cooler
150 H/t Must be adjacent to at least one valid Water Cooler and one valid Quartz Cooler.

Liquid Helium Cooler
140 H/t Must be adjacent to exactly one valid Redstone Cooler and at least one Reactor Casing.

Enderium Cooler*
120 H/t Must be adjacent to exactly three Reactor Casings at exactly one vertex.

Cryotheum Cooler*
160 H/t Must be adjacent to at least two Reactor Cells.

Iron Cooler
80 H/t Must be adjacent to at least one valid Gold Cooler.

Emerald Cooler
160 H/t Must be adjacent to at least one active moderator block and one Reactor Cell.

Copper Cooler
80 H/t Must be adjacent to at least one valid Glowstone Cooler.

Tin Cooler
120 H/t Must be at least between two valid Lapis Coolers along the same axis.

Magnesium Cooler
110 H/t Must be adjacent to at least one Reactor Casing and one active moderator block.

  • Active Fluid Cooler: This cooler requires liquid coolant to be piped into the reactor core via a Buffer. Inside the chamber, the fluids must be piped from the Buffer to the coolers. Active coolers cool the reactor at a maximum of 500% the rate of the regular passive coolers of the same type but any liquid piped into them is consumed at a maximum rate of 0.25 mB/t (0.5 mB/t in v2.8 onward). Water will be the most practical for most players, but more expensive coolants are extremely effective at removing heat.


  • If a cooler's conditions are not met, the block will not cool anything!
  • Sub-conditions must also be met! E.g., a Gold Cooler must be placed against an active water cooler. If the water cooler isn't against the cell, then it isn't active so it does nothing, and neither does the gold cooler.
  • Enderium and Cryotheum coolers require Thermal Expansion to be installed.

Reactor Configuration

The main challenge in NuclearCraft is figuring out how to arrange the cells, moderator blocks, and coolers. Placing cells next to each other boosts efficiency but results in a lot more heat. Using graphite blocks can increase the efficiency and add space to insert more coolers. Diamond, Emerald and Cryotheum coolers do a lot of cooling, but may be too expensive for you before late mid-game.

This subsection is for suggestions and tips on how to proceed with this task. Your mileage will vary so be careful. In this process you'll have to go in and out of the reactor core quite often so you might want to replace two Reactor Casing blocks with a Reactor Door.

After you've built the reactor and chosen which fuel you want to use, place down a single reactor cell then put in a Fuel Rod in the Controller. The Controller will show the figures for efficiency, power, and heat. Now go back in and place another Reactor Cell. Check the controller and see how the numbers have changed. You'll need to experiment a lot!

Saving Fuel

A fission reactor will continue to operate even when the internal energy buffer is full. To avoid wasting fuel, a tool such as the RF Monitor from RFTools can be employed to send a redstone signal to the Fission Controller if the buffer is below a certain percentage. Note that the presence of a redstone signal turns the reactor on.

Preventing a Meltdown

Monitoring the heat level with a redstone comparator loop

A fission reactor that overheats will undergo a meltdown and random blocks of the structure will melt into radioactive corium. For heat-positive reactors, a comparator placed against the controller block will emit a signal proportional to the heat level of the reactor, and output a full signal if it exceeds 50% (this can be altered in the mod's config file). By inverting this signal and feeding it back into the controller or a fission port, the reactor will only operate if the heat level is below 50%.

For heat-negative reactors, a comparator placed against the controller block will emit a signal proportional to the energy stored in the reactor, and output a full signal if the stored power reaches 100%.