Fuel & Isotopes
A reactor eats fissile fuel and turns captured heat into Drive Cells. That fuel doesn't come out of the ground ready to burn: you mine ore, enrich it into a loadable rod, burn it, and breed what's left over. This page is the fuel chain end to end, plus the one number that separates a forgiving fuel from a twitchy one: β.
Enrichment runs at an Enrichment Centrifuge — a generic isotope separator, not the ore refinery. The last step, breeding, happens inside a reactor built as a breeder. See Reactor Physics for what the fuel does once it's in the core.
The uranium chain
The whole chain, read live from the actual recipes:
| Step | Inputs → outputs | Where |
|---|---|---|
| Enrich | 3× Raw Uranium + 1× Refining Flux → 1× Uranium-235 + 2× Uranium-238 | Enrichment Centrifuge |
| Burn | Uranium-235 → Drive Cell + a depleted Uranium-238 rod | Reactor Core Console |
| Breed | Uranium-238 → Plutonium-239 | Reactor Core Console (breeder geometry) |
Walk it start to finish:
- Mine. Raw Uranium Ore comes from rare deep-planet veins — mostly fertile U-238 with a trace of fissile U-235. You can also buy it, but veins are the cheap source.
- Enrich. Feed raw ore into an Enrichment Centrifuge and one run splits it two ways: it concentrates the fissile fraction into Uranium-235 — the workhorse rod you load into a Fuel Assembly — and drops the leftover Uranium-238 as a tailings byproduct in the same batch. The Centrifuge runs while you're away, up to its buffer cap; right-click to collect.
- Burn. A loaded rod feeds the chain reaction for roughly three hours at cruise throttle. A burnt-out rod is depleted uranium: when it's spent the Fuel Assembly ejects Uranium-238 directly — no separate spent-fuel item, no reprocessing step. U-238 isn't fissile on its own, so it won't burn — but it's the feedstock for breeding.
- Breed. Load U-238 as fertile into a reactor built as a breeder (a tightly-packed, under-moderated core — a geometry, not a mode toggle) and under neutron flux it transmutes into Plutonium-239: a premium fissile you breed yourself — and the twitchiest of the three to run. Breeding is covered in full on Reactor Physics.
So U-238 reaches you two ways: as enrichment tailings, and as the ejected remains of every burnt-out rod. Both are the same item and both feed the breeder.
Breeding is a harvest-timing skill. Left cooking too long, bred Pu-239 walks on to Pu-240 — a poison, not a fuel. Pull the plutonium before it degrades.
Fuel types & β
A reactor runs one fuel type at a time, and each fissile fuel carries its own β — the delayed-neutron fraction. β is the safety margin between a controllable reaction and a prompt-critical runaway: control comes from the small fraction of neutrons that arrive late, and β is how large that fraction is. A smaller β means a smaller margin — the fuel reacts faster to every throttle nudge, sits closer to the prompt-critical cliff, and is far less forgiving to leave unattended.
| Fuel | β | Handling |
|---|---|---|
| Uranium-235 | 0.0065 | The workhorse. Largest margin, most forgiving — the default a fresh reactor loads. |
| Plutonium-239 | 0.0021 | Genuinely premium: a Pu-239 rod produces 1.5× the Drive Cells of a U-235 rod. Burn tracks fissions, so the rod lasts just as long — it simply makes more power over its life. It also carries the smallest β here, so it's the twitchiest and most attention-hungry. More power and less margin: you earn it by breeding it, and you pay for it at the console. |
| Uranium-233 | 0.0028 | Reserved. There's no thorium feedstock item yet, so U-233 isn't currently breedable. |
Run U-235 while you're learning the console; graduate to Pu-239 only once you can hold a power level by hand.
Loading & the Fuel Port
Fuel goes in through the Fuel Port — a container surface separate from the Core Console. Hold a stack of rods and shift-click (or drop them onto the load slot) and they feed into the core's Fuel Assemblies through a single-fuel gate: the reactor takes only rods matching its current fuel type, and hands back anything it can't fit, so you can never lose or dupe a rod. Fertile U-238 loads the same way into a breeder.
The Port is also where you watch the core's live internals. A running reactor accrues a whole zoo of byproducts, and you see every species tick up in real time here. Beyond the five fission products (Cs-137, Sr-90, Tc-99, I-131, Mo-99), the xenon poison Xe-135, and its precursor I-135, an over-breeding core walks the actinide chain: Pu-240 → Pu-241 → Am-241 → {Cm-244, Pu-238}, with Np-239 surfacing as the breeding intermediate along the way.
Three of these you can pull out and use:
- Bred Plutonium-239 withdraws as its loadable fuel item, so you can intercept it before it walks to Pu-240.
- Americium-241 and Plutonium-238 withdraw as themselves — dedicated feedstock (neutron source and RTG), not garbage.
Everything else — the five fission products, Xe-135, I-135, Np-239, Pu-240, Pu-241, and Cm-244 — consolidates on withdrawal into one generic Nuclear Waste item, so those isotopes are things you see ticking up but can't collect as distinct items. Nuclear Waste is terminal for now (a use comes later).
The Core Console — throttle, SCRAM, meters — is a separate surface entirely. The two are never fused: fuel and internals live at the Port, control lives at the Console. See Reactor Physics for the Console.
The full roster
For every fuel, byproduct, isotope, and waste item in one place, see the Neutronics Roster. The reactor Fabricator recipes — the Enrichment Centrifuge and the rest of the reactor blocks — live in the Industry tab (Neutronics was folded into Industry to fit the 7-tab UI cap).
See also
- Neutronics: the overview and where the fuel chain fits
- Building a Reactor: the blocks, breeder vs burner geometry, and commissioning
- Reactor Physics: what the fuel does in the core, and breeding in full
- Waste: fission products, the poison isotopes, and Nuclear Waste
- Neutronics Roster: every fuel, isotope, and waste item
- Mining: where Raw Uranium Ore comes from
- Crafting wiki: full bootstrap tree for every item