Feed The Beast Wiki
Molten Salt Reactor
TypeMultiblock structure

Molten salt reactors use fission of nuclear fuel in a FLiBe solution to produce steam in a multi-stage cycle involving molten NaK-based coolant. They operate on the following mechanics:

  • Fluid fuel created by making FliBe and mixing it with molten nuclear fuels.
  • Fluid cooling with NaK-based solution.
  • Reactors turn fuel into depleted fuel, and the coolant into "hot" coolant.
  • Heat Exchangers turn the hot coolant back into regular NaK coolant, and water into Superheated Steam.
  • Superheated Steam can power special turbines and be processed in a second Heat Exchanger to create normal Steam.
  • Steam can be used in any other steam-accepting system to produce additional power.


Fuel and Coolant

Unlike conventional solid-fuel reactors MSRs use molten FLiBe salt solutions of nuclear fuel, and are actively cooled with molten Eutectic NaK alloy. To create these fluids, significant chemical engineering is required:

  • FLiBE salt solution is created in the Salt Mixer with Lithium Fluoride and Beryllium Fluoride
    • Lithium Fluoride and Beryllium Fluoride are created in the Chemical Reactor with molten Lithium and molten Fluorine
      • Lithium can be obtained from mining ore
      • Beryllium can be obtained by processing Andesite in a Rock Crusher
      • Fluorine is obtained by processing Hydrofluoric Acid in an Electrolyzer
        • Hydrofluoric Acid is created in a Chemical Reactor with Sulfuric Acid and Fluorite Water
  • Eutectic NaK alloy is created in the Salt Mixer with Molten Sodium and Molten Potassium
    • Molten Sodium is electrolyzed from Sodium Hydroxide
    • Molten Potassium is electrolyzed from Potassium Hydroxide
  • Fluoridated isotopes are created in the Chemical Reactor with Fluorine and the isotope of choice (in molten form)

FLiBe salt solution is then mixed with the appropriate fluoridated isotopes, which are in turned mixed to create a FLiBe salt solution of fluoridated nuclear fuel. This is what will generate heat in the reactor. Eutectic NaK alloy is pumped through the reactor as a coolant. It can be used on its own or mixed with an additive to improve its cooling properties.

The Fluorine and FLiBe salt solution can be recovered by centrifuging the depleted fuel solution, and then electrolyzing the molten fuel fluoride.


Build rules

  • Structure must be cuboid
  • Corners and edges must be Frame
  • Unused blocks in faces must be Wall or Transparent Wall
  • Must have exactly 1 Controller in some face
  • Faces can contain any number of Vents, Fuel Distributors, Fuel Retrievers, and/or Redstone Ports
  • Interior can contain any number of Fission Vessels, Coolant Heaters, Moderator Blocks, or internal piping
  • Don't build reactors adjacent to each other


  • Right-click on a malformed reactor structure with an empty hand to find out what's wrong with it.
  • Internal arrangement of fission vessels, coolant heaters, and moderator blocks works exactly like solid fuel reactors.
  • "Efficiency" governs how quickly coolant heaters produce hot coolant. This is a multiplier on top of the base rate, which is 20 mB/s by default (changeable in config; for instance, Enigmatica 2 and E2E set this to 16). It has no effect on actual cooling power of the coolant!
  • "Heat multiplier" multiplies how much heat the fission vessels produce, and thus how much heat your coolant heaters must disperse.
  • While filled with cold coolant, coolant heaters produce hot coolant, and also remove a certain amount of heat from the reactor. For plain NaK this is 3600 H per mB of coolant converted. Molten additives create coolants that remove more but have more stringent placement restrictions.
  • Coolant heaters must be placed correctly to have any effect! The placement rules depend on the liquid coolant used and work the same way as the corresponding solid fuel coolants.
  • While filled with fuel, fission vessels produce heat and slowly turn fuel into depleted fuel. The heat amount for a fuel per tick is equal to the solid fuel's energy output per tick (NOT the solid fuel's heat output!).
    • Notably, this makes it much, much easier and more profitable to use highly enriched fuel than in a solid fuel reactor.
  • You can use normal piping solutions to move fuel and/or coolant to and from reactor vents.
  • Reactor vessels and coolant heaters also have the ability to push input and/or output products by themselves! You can reduce the internal piping required this way.
    • Right-click on the side of a reactor vessel or coolant heater to change its side mode. Sneak-right-click to change the opposing side's mode.
    • "OUT" sides will push products to a "DEFAULT" or "SPREAD" neighbor.
    • "SPREAD" sides will push input to a "DEFAULT" or "OUT" neighbor, and will also push products to a "DEFAULT" neighbor.
    • Sneak-placing the same type will link the target's side settings to the placed block's. Afterwards changing one will change the other as well.
    • Vents will push fuel and/or coolant to a neighboring "DEFAULT" reactor vessel or coolant heater.
  • Distributors will push up to 4 mB/t of fuel (changeable in config) to as many different reactor vessels as possible, but cannot push less than 1 mB to any given reactor vessel. They tend to fill up reactor vessels before moving to the next ones if you have too few. 1 for every 4 reactor vessels is a good number.
  • Retrievers will pull up to 4 mB/t of depleted fuel from as many different reactor vessels as possible, but cannot pull less than 1 mB from any given reactor vessel. They tend to leave reactor vessels unaddressed (causing eventual stalling) if you have too few. 1 for every 4 reactor vessels is a good number, but you can get away with less as long as you respect the depletion time per batch of 4 mB.
  • Controllers and redstone ports enable the reactor when receiving a redstone signal. Comparators from these output signal based on internal heat.

Heat exchangers


  • You currently need 2 stages of heat exchange.
  • In the first stage you need 3.33 pairs of interacting basic heat exchanger tubes per 480 H/t of efficiency-multiplied reactor output, producing 41.67 mB/t of HPS.
  • In the second stage you need 3.33 pairs of interacting basic heat exchanger tubes per 2160 H/t of efficiency-multiplied reactor output, producing 750 mB/t of steam.
  • If you're playing Enigmatica 2, you instead need 1 pair of interacting advanced heat exchanger tubes, producing 12.5 mB/t of HPS and 225 mB/t of steam respectively.
  • If you're recycling HPS from the second stage, you need X pairs per 1767 H/t instead.
  • Don't stall the second stage exchanger!

Build rules

  • Similar to the build rules for reactors.
  • Enable the heat exchanger with a redstone signal.


  • Heat exchangers take in a cold fluid and put out a hot fluid, or take in a hot fluid and put out a cold fluid, depending on the recipe set by the fluid in the input tank
  • Heat exchangers have a flow direction dictated by the first DUNSWE facing to have either a connected "input" vent (flow away from), "output" vent (flow toward), or valid and active "output" or "spread" target (flow toward).
  • Heating heat exchanger tubes advance based on the total cooling provided by neighboring cooling heat exchanger tubes. Cooling heat exchanger tubes advance based on the total heating provided by neighboring "heating" heat exchanger tubes.
  • Heating and cooling heat exchangers should never flow in the same direction (Unless one recipe is strictly hotter than the other, but this never applies to most cases).
  • More advanced heat exchanger tubes require less heating / more cooling to process their recipes. More heat exchanger faces are needed to process the hot fluid but produce much more HPS in exchamge.
  • It is recommended that the tubes' own fluid pushing capabilities are used to avoid issues with lack of contraflow. These work the same way as the coolant heaters and fission vessels.

Primary heat exchange

  • In the first stage transfer heat from the NaK coolant to the water. The amount of total cooling required is set by the config as a multiplier (default 125; 30 in E2). The batch size is set by the config and is equal to the cooling base rate in mB.
  • The heated NaK will arrive at a slow rate, so it is acceptable to have a small number of intakes into the NaK piping.
  • The high-pressure steam will be generated at a somewhat faster rate, so it is recommended to have more outputs from the water-HPS piping to prevent stalling. (The amount cells can push per tick is limited by tank capacity.)
  • It is generally acceptable to build the primary exchanger in a balanced cuboid shape. Interleaf tubes as long as flow happens; doing so increases the number of interacting faces and processing speed.

Secondary heat exchange

  • NuclearCraft does not currently have turbines and nothing else runs on high pressure steam (HPS), so it must be converted into standard steam. Water cooling can turn HPS into ordinary steam at a 1:4 ratio.
  • Unlike the primary heat exchanger, it is strongly recommended that the secondary heat exchanger be in a planar configuration to maximize steam egress and prevent stalling. (Steam takes slightly more than 4 ticks to be created and is produced 1000 mB/t at a time.)
  • Multiple vents may be needed to prevent rate limiting of the output steam.
  • HPS can be recycled from this exchanger back into itself, though doing so in an exchanger undersized for the total HPS production will eventually stall the exchanger as the water→HPS stage will not process. If unsure, dump the water→HPS output into a nullifier or another trashcan.
  • The output steam can be utilized at the user's discretion but Mekanism turbines are generally the best option as they can achieve much, much greater than 10 RF/mB, can consume absurd amounts of steam, and do not need an attached PID controller.