Input base Uranium Hexafluoride into pump top left. Output in the multipurpose pipes on the right of the highly enriched uranium and lowest enrich uranium. This only inputs if your stock (can substitute buffer tank with a different one) of highly enriched uranium is below a threshold, using the input pipe. The key is the middle pump which only allows the higherest enrichment recipes to go if there is space in a middle buffer. That middle buffer really should be kept the same.
The point of it is to keep room for the lower processes to continue to go until there is no more required input uranium. Top up valves keep the system from bottlenecking in the process. I believe.