The math in line 535 of TileEntityReactorControlRod always results in 0.1. The code comment indicates this is supposed increase the number of fast neutrons as the reactor heats up.

neutronSplit = 0.1f + Math.max(0.0f, Math.min(0.9f, Math.min(0.0f, (float)Math.log(this.localHeat/75.0f)/9.0f) + Math.min(0.0f, (float)Math.log(this.localHeat/300.0f)/5.0f)));

Well spotted! Typos for the win.

I'm going to tag this for 0.3 as people are largely already used to the mechanics of the 0.2 release, and I'm planning to rewrite much of TileEntityReactorControlRod anyway, particularly the radiate() method.

Decided to fix it anyway because I hate bugs, in 0.2.13A.