I had made a mathematical model that predicts the stability of a configuration and its "stability" flux. Except that when I tested a configuration supposed to be stable based on Medium Enriched Schrabidium, the reactor meltdown.
So I make a small test RBMK configuration : a MES rod, with two absorbers (here, irradiation channels) and 2 Ra226 Be neutron sources, which are passives, so the input flux of the MES rod is fixed at about 40 (there is a small effect of Xe135 and depletion but I check the input flux and it was correct). And I check the output flux which was recieved by the neutron sources, which are also surrounded of absorbers. And I have a flux of about 40, and not (40-40²/100000)*75%≈30.

Looking at the code I also wondered why the reactivity of the MES and MEA was less than 1 when this is supposed to be the flux when the input flux is about 100.
In https://github.com/TheOriginalGolem/Hbm-s-Nuclear-Tech-GIT/blob/Custom-1.12.2/src/main/java/com/hbm/items/machine/ItemRBMKRod.java, I found the formula used for flux calculation, which is in this case Math.max(flux - (flux * flux / 100000D) / 100D * reactivity, 0D); (L246), but with the priority, this formula is x-((x²/100000)/100×reactivity), and not (x-(x²/100000))/100×reactivity (the intended function), and so give a unexpected result more or less equal to an unobtainium rod. That's why my reactor meltdown