Guide: Designing an RBMK Reactor

RBMK reactors vary wildly in how unstable they are but are broadly some of the most difficult reactors to control in this mod. As such, great care must be taken in constructing one to make it as safe to operate as possible and to account for all possible risk factors in the operation of one so that it does not melt down violently. This guide will not address the principles of designing a ReaSim reactor as the mechanics surrounding them are greatly more complicated and volatile than standard RBMK mechanics.

Designing reactors should be done in creative mode in a test world with the gamerule dialDisableMeltdown set to true so any mistakes made in the operation or design of a reactor will not result in a lengthy and annoying cleanup process.

Fuel Types

Each fuel type is primarily identified by one statistic, that being the flux function. This function takes in a given value x determined by the amount of flux that the rod is receiving at a given moment at then outputs an amount of flux that corresponds to that function. For example, a fuel with a flux function of x + 5 that is being given 20 flux from other rods will experience 20 flux itself and then output 25 flux in all 4 cardinal directions around it. For a detailed description of each flux function, refer to the main page on RBMKs.

Fuel Danger

Fuels can be broadly categorized into 3 different levels of danger, those being:

Low

Includes passive flux output fuels, fuels that output exponentially less flux the more flux they receive, and fuels that act as strong neutron absorbers. This class of fuel is quite safe to run reactors with if sufficient cooling is provided but can still easily melt down if cooling is cut off, especially LEAus as its self igniting nature means that it can still technically operate without other rods to react with, though its flux function is so weak that this is practically not the case. Some of these fuels also are slightly difficult to operate reactors with due to their sheer unreactivity, meaning that it may be better to run a reactor using a mix of these fuels and more dangerous fuels.

Name	Relative Danger	Output Type	Notes
Ra226Be (Radium-226 Beryllium)	Low	Constant	Not intended for use as a fuel, instead as a neutron source.
Po210Be (Polonium-210 Beryllium)	Low	Constant	Same as Ra226Be but with a slightly higher constant flux.
ThMEU (Thorium with MEU Driver)	Low	Euler	Absolute upper bound on flux output, cannot physically output more than 20 flux to nearby rods.
LEAus (Low Enriched Australium)	High	Sigmoid	Self igniting. Similar to ThMEU but requires a significant starting flux to stabilize, making it slightly more dangerous.
NU (Natural Uranium)	High	Logarithmic	One of the least reactive fuels, often run with a driver fuel.

Flux function graphs below cover outputs up to 60 flux (y axis) and inputs up to 200 flux (x axis).

Ra226Be
Po210Be
ThMEU
LEAus
NU

Medium

Includes stronger logarithmic flux output fuels and square root output fuels. This class of fuels can quite easily result in a meltdown if not properly controlled and designed around with large amounts of cooling being a requirement for more dense arrangements. As well as this, some fuels in this category are now dangerously self-igniting, meaning that even if control rods can prevent any fuel rods from reacting with each other, the reactor can still heat up and subsequently melt down if the fuel is left inside of the fuel rods.

Name	Relative Danger	Output Type	Notes
MEU (Medium Enriched Uranium)	Low	Logarithmic	One of the most basic and also least reactive medium danger fuels.
LEP239 (Low Enriched Plutonium-239)	Low	Logarithmic	MEU but significantly more reactive.
MOX (Mixed LEP and MEU Oxide)	Low	Logarithmic	LEP239 but slightly more reactive
Pu241 ZFB (Plutonium-241 ZFB)	Low	Square Root	Breeding fuel rod, not designed for effective use in reactors but is still quite reactive anyways.
BiZFB (Bismuth ZFB)	Low	Square Root	Same as Pu241 ZFB
MEN (Medium Enriched Neptunium)	Moderate	Square Root	Very similar to MEU but square root rather than logarithmic, meaning it outputs far more flux at higher fluxes than MEU.
HEAus (Highly Enriched Australium)	Moderate	Square Root	MEN but slightly more reactive.
HEN (Highly Enriched Neptunium)	Moderate	Square Root	HEAus but slightly more reactive. Starts off slower than others.
LES (Low Enriched Schrabidium)	Moderate	Square Root	HEN but slightly more reactive. Emits slow neutrons and thus does not require moderator columns.
HEU (Highly Enriched Uranium)	Moderate	Square Root	Same as LES
MEP239 (Medium Enriched Plutonium-239)	High	Square Root	Self igniting. Starts off slighty reactive but grows more slowly.
Pu238Be (Plutonium-238 Beryllium)	High	Square Root	Self igniting. Starts off very reactive and grows quite quickly.
LEAm (Low Enriched Americium)	High	Square Root	Self igniting. Starts off slightly reactive and grows very quickly.
HEA241 (Highly Enriched Americium-241)	High	Square Root	Self igniting. Starts off very reactive and grows very quickly.

Flux function graphs below cover outputs up to 100 flux (y axis) and inputs up to 200 flux (x axis).

MEU
LEP239
MOX
Pu241 ZFB
BiZFB
MEN
LES
HEU235
MEP239
Pu238Be
LEAm
HEA241

High

Includes the strongest of fuels, including all linear, negative quadratic, and the single quadratic fuel. This class of fuels will nearly instantaneously result in a meltdown if not strictly controlled and moderated and practically prohibit certain designs of reactors due to the sheer reactivity of certain fuels. Using neutron reflectors directly on any of these fuels is strongly discouraged due to their ability to rapidly form positive feedback loops, especially the final two fuels which will always input either an amount of flux as much or greater than what they are given.

Name	Relative Danger	Output Type	Notes
AmZFB (Plutonium-241 Americium ZFB)	Low	Linear	The least powerful linear fuel. Still quite dangerous.
HEU233 (Highly Enriched Uranium-233)	Low	Linear	AmZFB but significantly more reactive.
HEP239 (Highly Enriched Plutonium-239)	Low	Linear	HEU233 but slightly more reactive.
HEP241 (Highly Enriched Plutonium-241)	Low	Linear	HEP239 but slightly more reactive.
HEA242 (Highly Enriched Americium-242)	Medium	Linear	HEP241 but significantly more reactive.
MES (Medium Enriched Schrabidium)	Medium	Negative Quadratic	HEA242 but greatly more reactive.
MEA (Medium Enriched Americium)	Medium	Negative Quadratic	Self igniting. Less reactive than MES but starts off more reactive and is self igniting.
HES (Highly Enriched Schrabidium)	High	Linear	MES but greatly more reactive.
Flashgold	High	Negative-Quadratic	Self igniting. MEA but greatly more reactive.
Flashlead	High	Negative-Quadratic	Self igniting. Flashgold that starts more reactive but is overall slightly less reactive
Balefire	Insane	Linear	Self igniting. Starts off highly reactive and is 1:1 linear with an added constant (100 input flux = 100+30 output flux)
Digamma	Incomprehensible	Quadratic	Self igniting. Impossibly highly reactive, will nearly immediately achieve absurd flux values if allowed to react with itself.

Flux function graphs below cover outputs up to 300 flux (y axis) and inputs up to 200 flux (x axis).

AmZFB
HEU233
HEP239
HEP241
HEA242
MES
MEA
HES
Flashgold
Flashlead
Balefire
Digamma

Fuel Summary

The specific kind of fuel that a reactor should use is greatly dependent upon various factors, including availability of resources in a world, the amount of power required from a reactor, and any number of other factors that may influence the decision of what fuel to use in a reactor. Despite this, there are some fuels that are generally the best option for most use cases on account of either being very safe, very cheap, or just all round practical in many fields. A few particularly good fuels are:

ThMEU. Composed entirely of thorium fuel which can be easily bred from thorium in an RBMK. Has an incredibly safe flux function that physically cannot exceed an output of 20 flux.
MEU. Composed of uranium fuel which only requires a 2 stage gas centrifuge to refine from uranium. Has a very safe flux function.
MEP239. Composed of reactor grade plutonium which can be bred from natural uranium in a chicago pile or in a ZIRNOX with natural uranium fuel. Does not require a neutron source due to its self igniting properties and is generally relatively safe to operate with proper cooling.
LES. Composed of a small amount of schrabidium and various other fuels and binder materials. Splits into slow neutrons which means that no moderator columns are required and has a relatively safe flux function.

Heat

Proper use of an RBMK steam connector. Placed one block below the bottom of a boiler column with a water fluid duct above it and a steam fluid duct below it.

The most important metric for measuring the power of a reactor and the entire reason behind constructing a reactor, but also the factor which causes meltdowns. Heat in a reactor must be carefully balanced and dealt with such that no single column goes above the meltdown temperature of 1,500C or any fuel rod's skin temperature exceeds its melting point. The primary method of transferring heat away from a reactor is with boiler columns and steam, but fluid heater columns connected to a heat exchanging heater can also work quite well for transferring heat. Boiler columns have multiple steam settings from standard steam to ultra dense steam, with each level of steam being 10 times denser than the previous level of steam and needing to be run through a turbine to be reduced in pressure. As such, any reactor operating at a higher steam pressure will require more turbines in total but less cooling infrastructure, meaning that it can be possible to be more efficient with the use of highly efficient industrial steam turbines for high level steam as compared to the inefficient leviathan turbine. Cooler columns can also be used to remove heat from a reactor but in doing so they completely void the heat and consume cryogel, making them less useful as means of sustainably maintaining a reactor.

Broadly, boiler columns are the most efficient option on account of not needing a separate assembly for transferring around hot and cold heat transfer fluids as well as the fact that there exists a dedicated block for making fluid routing for boiler columns far cleaner: the steam connector, whose function is displayed to the right.

Closed Loop Cooling

As with any turbine based power generation system, there needs to be a point at which the hot steam is cooled down and condensed back into water. Players that have little experience in designing RBMKs will often attempt to construct a system wherein they have some infinite water input (provided by pumps or infinite barrels) and then an output where they void the low pressure steam. This is a highly inefficient design due to the fact that consistent large sources of water are either expensive or require power to run and voiding fluids in large quantities is incredibly difficult. As such, it is nearly always better to construct a system where water that is obtained from condensing steam is put directly back into the system for reuse. This design is known as closed loop cooling due to the fact that once the system is full of water, it will never run out of water due to the 100% efficiency of conversion of water to steam and then back into water.

Control Rods

Primarily used with incredibly reactive fuel arrangements to limit reactivity to a certain level, though are also used in reactors designed for multiple different types of fuel so that the reactor does not need to be reconstructed to handle a variety of fuels. In a properly designed reactor that is running off of a specific type of fuel, control rods are generally not necessary as the majority of useful fuels will reach a stable flux equilibrium point which they will not significantly exceed, meaning that there is no need for additional moderation. Automatic control rods are also an option for fuel arrangements where a stable flux equilibrium cannot be reached but they should be used with extreme caution as some fuel arrangements can flip from rapidly decaying in reactivity to a runaway reaction even in the span of a single point of control rod extension.

Reflectors

Very useful with lower power fuels but becomes immensely dangerous with powerful fuels like balefire or digamma which can rapidly enter into a positive feedback loop. Should be used with extreme caution with highly reactive fuels and ideally with a control rod column between any fuel assembly and a reflector in such a case.

Templates

For more design see at: RBMK#Designs or #rbmk-designs in the discord server.

Template Non-Reasim Reactor
Template Non-Reasim Reactor Starter Center