How do the odds of producing Uranium-235 and Uranium-238 work?

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?

Solution 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.)