Building a Reactor

A reactor is assembled in the world, block by block, on a station or a rented room — never on a ship. There is no menu that spawns one. You place a sealed shell of plating around a core of fuel, exchangers, and coolant; wire in a Core Console; and right-click the console to commission it. Once live, it burns fissile fuel and turns captured heat into Drive Cells.

This page is the construction half: every module block, the containment shell and its ports, what the commission gate checks, and the reference layouts. How the sim behaves once it's running is on Physics; what you feed it is on Fuel & Isotopes.

Placing or breaking a reactor block requires BUILD_MODULE rights on the host; using the console or Fuel Port requires MODULE_USE. See Stations for rooms, ACL, and renting a base.

The module blocks

Each reactor block has an inventory icon in hand and an in-world donor block once placed. Every block is a module.reactor_* item; the console reads your build from the placed blocks, so the donor block is the identity the sim sees. This table is generated live from the module templates, so the block you must actually place is always accurate:

Two of these are not part of the reactor structure at all. The Enrichment Centrifuge is a standalone isotope separator (it splits raw uranium ore into Uranium-235 and Uranium-238 tailings in one run — see Fuel & Isotopes), and it never joins the core. Everything else is a cell in the reactor lattice: a role the heat-and-neutron sim understands.

The cells split into three jobs:

• The core — Fuel Assemblies (the heat source), Control Rods (the throttle), and the neutron shapers (Moderators, Reflectors) that tune how the fuel behaves.
• The heat path — Heat Exchangers pull heat off the core into Coolant Cells, which carry it to a Turbine that mints Drive Cells. Heat Vents dump the waste heat you don't capture — they are your safety margin, not your output.
• The shell — Reactor Plating and its see-through / walk-in variants (Viewports, Blast Hatches, Blast Doors) wrap the whole thing in containment.

Containment: sealed & shielded

The reactor's radioactive, hot guts must be fully enclosed. Every sealed-core cell — Fuel, Exchanger, Coolant, Control Rod, Moderator, and Reflector — must not touch open space. If any one of them has a face on the outside air, the reactor is unshielded and won't commission. The console flags the exact blocks you left exposed so you know which faces to cover.

The shell around that core can be built from anything in the containment class:

• Reactor Plating — the default wall. Inert to heat and it never melts; it also raises the meltdown ceiling, but only modestly (a big shell won't buy you much headroom — see Physics).
• Reactor Viewport — plating you can see through. It seals and shields exactly like plating; use it to watch the core.
• Blast Hatch — an openable plating panel. It seals whether open or closed (the scan reads the placement, not the state), so you can climb in without breaking containment.
• Blast Door — a full-height (2-tall) walk-in door. Both halves seal like plating.
• The Core Console counts as valid skin too — it's an armored access panel, inert and never-melting, the same class as plating.

Vents and Turbines are ports, not containment. They sit on the wall and must each have a face on open space so they can exhaust outside the shell. A Heat Vent buried in the core can't dump heat anywhere; a Turbine needs to vent spent steam. So every Vent and every Turbine must reach exterior air — bury one and the commission gate rejects the build. (These two are the only cells allowed to face outward; the sealed-core cells above never may.)

Commissioning

With the shell sealed and the ports on the wall, right-click the Core Console and confirm the Commission prompt it opens — a right-click alone never auto-commissions, so a stray click can't lock in a half-built reactor. If the build falls short, the console prints every shortfall at once — each with the current count and the fix — so you can correct them in one pass. The minimum block counts are generated live from the config the sim runs on:

Counts are only half the gate. On top of the minimums, the console checks the build's physical coherence:

• Connectivity. One Console, a Turbine, and an Exchanger that touches both fuel and coolant must all be joined into a single connected structure — no stray or detached blocks.
• Shielding. Every sealed-core cell must be enclosed (above). Nothing hot or radioactive may face open space.
• A live coolant loop. The coolant must actually reach a Turbine: at least one capture Exchanger has to sit on coolant that floods, coolant-to-coolant, all the way to a Turbine. Coolant that dead-ends short of a turbine passes the block-count minimum but fails this liveness check — a distinct, single-cause error the console names for you.
• One Control Rod per Fuel. Every Fuel Assembly needs a face-adjacent Control Rod. The rods are the throttle: they slide out of the fuel as you throttle up and slam back in on SCRAM. A fuel cell with no rod is uncontrollable, and the gate refuses it.

When all of it passes and you confirm, the reactor commissions and the console becomes the live status readout. Right-click it again afterward and it reopens that dashboard, not the prompt — so a reactor commissions once, never twice. From there you drive it: throttle, meters, and SCRAM on the Core Console; fuel and live internals on the Fuel Port. See Operating a Reactor.

Reference builds

Two shapes cover almost every reactor, and the difference between them is pure geometry — how tightly the fuel is packed. The schematics below are top-down grids, one per vertical layer (floor y=0 upward), colour-coded to the donor block you place. Control rods (one per fuel) and the outer Plating box are left off for clarity — wrap the finished guts in a sealed shell as described above.

The burner (start here)

A burner spreads its fuel out so no two fuel blocks touch. That keeps every fuel at amplification 1 and its net packing at zero, so it breeds nothing — it just makes power, and Doppler feedback holds it steady enough to walk away from. This is the shape the Starter Kit unpacks.

The breeder

A breeder does the opposite: it packs fuel into a tight cube with the moderator pulled back, so each fuel sees three fuel neighbours (net packing 3). That crosses the breeding threshold and lets loaded Uranium-238 transmute to Plutonium-239 — but the same packing runs it hot and twitchy, so a breeder is attention-hungry where a burner is idle-safe. See Fuel & Isotopes for the harvest-timing.

See also

• Neutronics: the overview and where this fits
• Fuel & Isotopes: enrichment, the fuel chain, and the breeder
• Physics: criticality, Doppler cruise, xenon, and meltdown
• Operating a Reactor: throttle bands, the console, and the Fuel Port
• Waste & Byproducts: fission products and Nuclear Waste
• Stations: where reactors live; rooms and ACL
• Crafting wiki: full bootstrap tree for every block