My solution for Exploding head syndrome. Takes roughly 2400 cycles, but that's not the point here, it's about how to do the level without using controls.
1st reactor splits the water to H and O, with plenty of space on the H-pipeline, there is a lot more than needed.
2nd reactor does the magic: Fuses 10 O and 14 H to Plutonium; for every H, another H and an O is handed to reator 3. Also possible is 11 O and 6 H, this needs less symbols in the reactor, but needs even more space in the pipeline (4 tanks + pipe) and is slower.
3rd reactor takes H and O and outputs H2 and O2.
Maybe I'll build a better one sometime, less cycles, maybe 2 reactors. But I want to have it running without the manual controls.
