As a noob, I have been messing around with the designer all day..
After a lot of trial and error, I have created (the worlds first?) stable MK5 reactor.
Outputs 340 EU/t
I think the efficiency is terrible, 2.4?
It's been running for over an hour now and no components are degrading.
I am not sure why this works, as the designer indicates a failure.
If anyone knows, leme know.