PWR

The PWR (Pressurized Water Reactor) is a modular fission reactor added in 1.0.27 X4704 (1.7.10) that is designed to be a replacement for the old multi block fission reactor. The behavior of the reactor is similar in principle to the RBMK except that it is in 3D and does not iteratively calculate flux values, meaning that no runaway flux feedback loops can occur. It is more limited than the RBMK in terms of capability and fuel types but it is much safer and easier to use due to it melting down more gently and being far more stable in its operation. All statistics are pre-calculated upon construction, meaning that it has better computational efficiency than the RBMK which calculates everything in an iterative fashion.

Component Blocks

(Each block is prefixed with "PWR" in game.)

Fuel Rod

Holds PWR fuel which can react with other fuel rods or neutron reflectors to produce flux and therefore heat. Each fuel rod can hold a single piece of PWR fuel which is inserted into the reactor through its GUI or through an access port. Fuel rods will emit flux in all 6 cardinal directions for 15 blocks in each direction and can only interact with one other fuel rod or neutron reflector in any given direction. Unlike the RBMK, PWR fuel rod flux calculations cannot result in positive feedback loops, meaning that when the quantity of flux a PWR produces with a given fuel stabilizes, it will stay at that level of flux unless the conditions of the reactor are modified. As such, powerful fuels in PWRs are highly stable and do not experience similar issues to RBMK fuels such as linear and negative quadratic type flux function fuels.

Unlike the RBMK, there can only be 1 fuel type at a time despite the ability to have multiple separate fuel rods inside of the reactor.

Control Rod

Restricts the amount of neutron flux that passes through it to other fuel rods, thereby limiting the reaction. Must be placed between two reactive blocks (two fuel rods or a fuel rod and a neutron reflector) in order to work. Lacking this block may lead to excessive reactivity and prevent the reactor from shutting down in an emergency.

Unlike the RBMK, all control rods operate in unison and cannot be individually controlled.

Heat Exchanger

Transfers heat out from the reactors core for use by the coolant channels to heat the hull. Lacking a sufficient proportion of this block will inhibit the reactor's ability to move heat to the coolant channels, thus reducing efficiency and increasing the risk of a meltdown.

Coolant Channel

Transfers heat pulled from the core by heat exchangers to the hull to heat the cooling fluid of the reactor. Lacking a sufficient proportion this block will inhibit the cooling fluids ability to fully utilize the heat the reactor generates, thus reducing efficiency and increasing the risk of a meltdown.

Heatsink

Increases the maximum core temperature by 5%. PWRs are normally limited to a maximum core heat capacity - thereby capping their heat generation. Heatsinks allow for the creation of extremely powerful (and hot) designs that would result in meltdown otherwise. However, they reduce the effect of heat exchangers and coolant channels.

Neutron Source

Provides a constant flux level inside of the reactor to initiate a reaction. Quantity and use may vary based upon reactor design but broadly PWRs do not need more than one. The flux production of a single neutron source will not heat the reactor sufficiently to cause a meltdown if not cooled meaning that a reactor can be safely shutdown.

Neutron Reflector

Valid casing block that reflects incoming neutrons back to their origin. Useful for reducing lost neutron flux on the outside of the reactor or for increasing reactivity inside of the reactor without needing to fill more fuel rods.

Pressure Vessel

Valid casing block that has no other function but to enclose the reactor. Blocks neutron radiation.

Access Port

Valid casing block that allows for access to the reactor's fluid and item input/output. Each port can serve as many functions as can be attached to it with current options including:

Inserting fuel.
Extracting spent fuel.
Inserting cooling fluid
Extracting hot cooling fluid.

Controller

Valid casing block of which only one can exist in a reactors casing in a single reactor. Necessary for assembly of the reactor. Allows for access to the PWR GUI which displays things such as current flux, core heat, hull heat, fuel depletion amount, control rod extension, and various manual IO functions.

Construction

The PWR is fully modular and has no single set design with the only requirements for a valid reactor being:

All internal blocks (fuel rods, control rods, coolant channels, heat exchangers, neutron sources) must be entirely enclosed by a shell of casing blocks (neutron reflectors, pressure vessels, access ports, controller).
Must have only a single controller block in the casing as any other controller block in a reactor besides from the first will not act as a valid casing block.
Must have a single fuel rod and a single neutron source. The neutron source's position in the reactor does not matter and does not need to be adjacent to a fuel rod to operate.

With these requirements met at the absolute minimum, the reactor will assemble but will not operate as the fuel rod does not have a neutron reflector or other fuel rod to react with. Additionally, it does not have the necessary blocks to transfer heat away from the fuel rods, meaning that the reactor will build up heat until melting down. As such, the recommended requirements for a fully functional PWR are:

Fuel rods arranged in such a way that there are other fuel rods or neutron reflectors in the 6 cardinal directions surrounding it.
Access ports for pulling hot cooling fluid out of the reactor for use in an external heat exchanging heater and putting spent cooling fluid back into the reactor for reheating.
Heat exchangers and coolant channels to transfer heat from the core to the cooling fluid. The position of heat exchangers and coolant channels in relation to the fuel rods and each other inside of the reactor does not matter. Additionally, reactors typically require far more coolant tubes than heat exchangers to operate effectively.
Control rods placed between cardinally adjacent fuel rods or a neutron reflector and a fuel rod to moderate their reaction to allow for reactor shutdown or use of higher power fuels without meltdown.
Sufficient quantity of cooling fluid inside of the reactor, usually in the range of 10,000 to 30,000mb depending on the amount of heat generated.
Optionally, heatsinks can be used to prevent meltdowns in reactors with high heat generation. To maintain the same level of cooling in the reactor, more heat exchangers and coolant channels are needed for each heatsink; every 4 heatsinks can be treated as 1 fuel rod.

To complete construction, right click the controller and the PWR will assemble itself (indicated by the casing blocks besides from the controller turning into PWR blocks). Any issues will be highlighted in red with text describing the issue.

User Interface

The GUI for the PWR is accessed by interacting with the controller and shows all needed information about the reactor's operation.

Cold Cooling Fluid Buffer. Contains up to 128,000mb of a specified cooling fluid which will be heated by the reactors hull temperature.
Hot Cooling Fluid Buffer. Contains up to 128,000mb of a specified cooling fluid that has been heated by the reactor.
Fluid Identifier. Specifies the type of cooling fluid the reactor will use. Must be a valid cooling fluid.
Fuel Input. Used to place fuel inside the reactor. Will automatically place fuel in empty fuel rods when available. Will only accept fuel of the type that is already inside of the reactor or any if the reactor is empty.
Fuel Type/Count. Displays the icon of the current type of fuel in the reactor as well as the quantity of fuel rods in the reactor that have a piece of fuel inside of them.
Depletion Progress. Displays the amount the that the current piece of fuel has been depleted with the arrow filling with white until completely full.
Spent Fuel Output. Automatically receives hot spent fuel from the reactor for extraction.
Control Rod Extension. Amount the control rods are removed from the reactor with 0 indicating full insertion and 100 indicating full removal. Takes in numerical input when selected to specify an amount of control rod extension.
Control Rod Set. Sets the control rods to move towards the amount of extension specified in 8.
Heat Warning. Glows an ominous yellow and orange if the reactor is above 80% of its heat capacity. Does nothing otherwise.
Core Heat. Displays the current core heat with a visual dial and a precise numerical display when hovered over. Base capacity of 10,000,000TU
Hull Heat. Displays the current hull heat with a visual dial and a precise numerical display when hovered over. Maximum capacity of 10,000,000TU
Total Flux. Displays the amount of flux that is currently occurring between fuel rods and reflectors in the reactor.

General Operation

Fuel

Fuel is inserted into the reactor through the GUI or access ports and decays at a rate specified by the amount of flux that the reactor is currently producing. Any fuels added to the fuel input slot will be automatically added to the reactor if any space is available, meaning that adding a stack of fuel at a time may lead to a rapid uncontrolled rise in reactivity. The amount of space available depends on the number of fuel rods currently present in the reactor with a fuller reactor leading to a greater amount of flux produced. Different fuels cannot be mixed unlike other reactors such as the RBMK, so arrangements featuring low enriched fuels with highly enriched driver fuels are impossible. Fuel cannot be removed from the reactor once inserted and fuel reactivity drops greatly as fewer fuel rods are occupied, resulting in the last few fuel rods inside of a reactor taking an exponentially longer amount of time to fully deplete which makes the full shutdown of a reactor difficult. This issue can be "resolved" by simply breaking the controller block but this also voids all currently present cooling fluid and stored fuel so it should only be used for disassembly.

Cooling

Cooling is performed passively by the reactor by pushing and pulling to and from internal buffers for hot and cold cooling fluid. Fluid can only be added to and removed from the PWR by fluid ducts attached to access ports. During operation, the amount of cooling fluid that is present in the cooling fluid buffer will not affect how efficiently the reactor cools itself with the only requirement being that enough cooling fluid is left inside the reactor such that it can fully cool itself every tick. The amount used each tick varies based upon the heat production of the reactor so testing is highly recommended for operation to ensure that the quantity of cooling fluid in the buffer is sufficient. This is especially important because due to how fluids are handled in this mod as there will always be a certain quantity of fluid in the heat exchanging heater each tick. This means that a reactor can potentially lose all of its cooling fluid on every other tick which will result in a rapid meltdown due to the 20 ticks per second update rate of Minecraft.

Any fluid with the tag [PWR Coolant] in its extended tooltip can be used as a cooling fluid with some providing different benefits over others such as increasing the amount of flux produced in the core and/or being better at absorbing and transferring heat.

Cooling Fluid Types

Pros and Cons

Pros

Incredibly safe once the flux level has reached an equilibrium.
Far fewer variables to keep track of at any given moment, making it more friendly for new players to use.
100% modular, allowing for a wide variety of designs to fit various purposes.
No xenon poisoning at low operating temperatures allows it to run at incredibly low powers reliably.
The shell can be of any shape as long as it fully encloses the interior, meaning that it can be constructed to fit in a variety of spaces.
Safest meltdown out of all fission reactors, coming solely in the form of corium dripping down from the reactor and radiation being spread around with little block damage and no debris.
Refueling and processing of spent fuel can be easily automated with conveyors and access ports.
Simple waste processing chain that does not require SILEX or waste barrels to fully utilize.
Does not require the inclusion of a neutron source fuel rod or self igniting fuel in order to start its reaction unlike the RBMK.
3D design allows for highly vertical reactors which allows for large reactors to be contained in a smaller amount of chunks, reducing performance load in games with chunk loading.
Heat exchanging heaters are naturally self balancing such that even if not given a machine to transfer its heat to, it will build up heat and reach a point of equilibrium such that it dissipates enough heat to fully cool the reactor.

Neutral

Requires a separate heat exchanger and boiler setup in order to utilize the heated cooling fluid.
Makes a nice noise.

Cons

Expensive, though less so than the RBMK in terms of rare resources such as zirconium.
More performant cooling fluids can be expensive, particularly liquid thorium salt which requires a separate enrichment process to be reused.
3D design can be more difficult than the 2D design of the RBMK.
Fewer options for late game high power fuels compared to the RBMK (no balefire, flashgold, flashlead, or digamma fuel)
More difficult to run on medium enriched fuels such as MEP, MEU, and MOX compared to the RBMK
Cannot be used to breed bismuth or to irradiate items.

Trivia

Despite the name, it is technically not a PWR, since the coolant in the cooling circuit doesn't necessarily have to be water.
- Because of this, there was an unofficial competition to give the reactor a comically excessive acronym to fully capture its capabilities, but ultimately the name remained "PWR".
- With the different possible coolant types, it could function as a PHWR (Pressurized Heavy Water Reactor), LMCR (Liquid Metal Cooled Reactor), or even a breeder reactor.
- Ironically, it cannot use plain light water as coolant.