Input and output are at the bottom of the system.
At this revision it will only input one fuel cell when it outputs 1 empty fuel cell, so you will have to prime each reactor with 1 fuel cell. If you like you can remove the arithmetic combinators at each reactor and it will fill reactor queue without priming.
I have a blueprint of a simple koravex system that is enough to run this system.
Update 10/05/18 Seems I published this with some errors in my combinator logic. It will only pick a uranium 235 if one is available, if not, you have to manually feed the reactor. Sry. However, if the belt is full, it will work as I had planned.
I have a robot version available that may suit you needs better.