Haha yeah the 1.9 MRF/t avg settings ARE scary.
"Burn mode" is not the final stage but more like the initial stage where it burns through a lot of fuel to get more RF/t earlier.
The reactor generates more and more energy, there more fuel it has converted to chaos. That's why you get higher average output at the cost of efficiency.
You can set "burnRFt" to 0 to disable that. That'll also make things less scary .
If you're using my settings It'll stay in burn mode until the reactor generates 2M RF/t (a few hours).
-XyFreak