I've finally got a nuclear reactor up and running. In order to do this, you need both Uranium-235 and Uranium-238 to create a Uranium fuel cell (as well as iron plates, but that part is easy). To make both Uranium-235 and Uranium-238, you need to use a centrifuge.

From observing the centrifuge as it works, it produces much more Uranium-238 than Uranium-235. This is having a major impact on producing the fuel cells needed to power my reactor since the production of Uranium-235 is so little.

What are the odds of producing both Uranium-235 and Uranium-238? Is it random with a heavier weight towards Uranium-238 or is there a defined odds such as 100:1?

1 Answer 1


Since version 0.15.8 the game displays these probabilities to enough precision (using percentage notation) to see usable values for the uranium processing recipe, but if you're still interested, the exact definition can be found in the game's files. data/base/prototypes/recipe/recipe.lua contains:

    name = "uranium-235",
    probability = 0.007,
    amount = 1
    name = "uranium-238",
    probability = 0.993,
    amount = 1

For each batch of ore you process, the result is independently selected according to those probabilities. On average you will get 7 235U and 993 238U out of every 1000, but there is no guarantee of exact numbers.

Since this is much, much larger than the ratio needed for fuel production (1:19), you must do something about the extra 238U.

  1. First, create a generous storage buffer (I recall needing about four steel chests) to keep 238U from clogging things up until you have a better solution.

  2. Research the technology Kovarex enrichment process. This will allow you to convert 238U into 235U, which will not only solve your ratio problem but also allow for immensely more efficient use of uranium ore (and fewer centrifuges).

    Note that the actual enrichment process requires that you have at minimum 40 235U as a catalyst which you keep circulating, and preferably more to get the cycle going better despite items sitting idle in belts and input slots. (Players have come up with a wide variety of designs for this.) Therefore, it is wise to save up some 235U early on and not turn it all into fuel, so you don't have to wait again to bootstrap your enrichment.

  3. (Optional:) Research the technology Uranium ammo. This allows you to use extra 238U for another purpose. (The technology Atomic bomb uses 235U so it is not helpful here.)

  • I saw the Kovarex enrichment process, but haven't obtained it yet. Still have you to start making productivity science packs since they are rather difficult now. 7 out of 1000 are terrible odds :(
    – Timmy Jim
    May 3, 2017 at 10:44
  • If you check (just did myself), those numbers (0.7% and 99.3%) are the same as in naturally occuring uranium deposits. Of course, this isn't actually Earth, so... maybe it should be random. Actually, it should be. May 17, 2017 at 6:27
  • 1
    @TimmyJim Yep, that was changed in version 0.15.8.
    – Kevin Reid
    May 17, 2017 at 13:36
  • 1
    @JürgenA.Erhard The natural isotope ratios aren't random but stem from the nucleosynthesis processes that created the matter the planet is made of. They shouldn't be too different on other habitable planets.
    – AndreKR
    May 26, 2017 at 14:52
  • 1
    @Luaan, it also depends on whether or not the deposit experienced the right conditions to become a natural nuclear fission reactor, which did happen on Earth, almost 2 billion years ago in Africa. Because some of the 235-U was used up, the ratio there is closer to 0.6%. en.wikipedia.org/wiki/Natural_nuclear_fission_reactor
    – DaCheetah
    Mar 5, 2019 at 11:51

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