It uses steam storage and will only insert one fuel cell at a time, the next will only be placed in upon 2 conditions being met, the first is that an empty cell is removed, and the second can be what ever you want, in this case the input being when steam falls bellow a certain level.
By reading hand contents and overriding stack size it allows the inserter to input one fuel cell then stop until that empty cell is taken out.
the nice bit about this setup is that it requires no priming just place and off it will go
I set the second input to activate when steam falls below 2000 in one tank, this gives the reactors enough time to heat up to maintain power levels, it will switch off again when steam hits 4000 in the same tank this could be used for main power or backup or whichever combination you want.
Let me know what you think