The recipe takes 40 U-235 and 5 U-238 and gives 41 U-235 and 2 U-238. In its essence, you convert 3 U-238 to 1 U-235, rest of the input ingredients are returned.
However with productivity modules, filled productivity bar gives all of the resulting products as bonus, disregarding the fact the overall conversion is only 3 U-238 into 1 U-235.
So for example, if we take 2 Productivity module 3 into Centrifuge, every 5th craft (because of 20% productivity bonus) we will get for free 41 U-235 and 2 U-238.
Which means, we spend 3 * 5 - 2 = 13 U-238 and get 1 * 5 + 41 = 46 U-235. This is almost 10x productivity boost, and it makes U-235 supply abundant once you got first initial recipe runs.
I'm attaching small sandbox test setup with this issue. It contains one Centrifuge with 2 productivity module 3s and 40 U-235 and 17 U-238, which are exact requirements for 5 runs. After it runs 5 times, total amount of U-235 will be 86, which is 46 if we subtract initial 40 (81 will be in Centrifuge, 5 more in chest). To start the Centrifuge, turn on the power switch on the right.
![Image](https://forums.factorio.com/images/ext/0bf27c555372f2874cbf266d360023e9.jpg)