I build the 2x2x3 Fission reactor per the tutorial, Using current version of FTB revelation. I do not get the negative heat modifier as shown in the wiki (-12H/T) and instead heat keeps steadily rising and rising..

Screens...

Using just the regular TBU Fuel cells as listed...

As hinted to at the top of that tutorial, the instructions are a little out of date as the reactor rules have changed - I will get them fixed up today if I can :)