Why don't you just have a stack filter inserter pull the U-235 out and place it on a belt which pushes it exactly one space over to another stack filter inserter that immediately pulls it right back in? Just make the belt keep going past the in inserter and any extra it doesn't have room to put int...

this version works without combinators and chests
explained here: viewtopic.php?f=202&t=45871#p274367

A description how it works would be helpful well you feed the centrifuges it with 40 U235 in the beginning to kick them off then after the process is done the purple filter inserter takes out 1 of the 41 U235 and will be deactivated if any U235 is in the cicuit network (green wire) the white stack ...

my setup - keeps only the necessary amount of U235 - reduced amount of stack-inserters - tileable - power saving To get stuff done more quickly I usually build more factories instead of beacon-ing everything up 0eNrdWm1vmzAQ/i/+tEkwgW0SEmkf9yumChEwqVUCyDZRuyr/feeQl5aYBXtLG/alKQbf+Z67e3x+eUWrsmWN4JVC...