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.
Looks like you've found what happens when you use an outdated reactorplanner to test reactor designs in an updated IC2!
I remember that reactor cooling systems got buffed by a factor of two a while back; it looks like you've found a design that makes use of this.
Also note that the component vents in the upper corners of your design are doing nothing at all- they can only vent heat from adjacent components that store heat, which uranium cells don't. You can just remove them, and the behavior of the reactor won't change.
If your reactor is in fact stable, then it's a Mark 1. Be hesitant about believing anything that the reactorplanner says when you're using IC2 Experimental.