Is it good to enrich near depleted in a SUC?

    I have this small water-bucket SUC reactor running fine:


    :Nuke TNT: Mark 1-1-SUC
    I was wondering if I could place some near-depleted uranium to be enriched without affecting it.
    Like this for example: :Nuke TNT: Mark 1-1-SUC-Breeder
    My two questions:
    1) Does it work? Is it a good idea?
    2) Why if I add another near-depleted uranium to be enriched in the second link above it shows: Cooldown time: Impossible?
    Thanks in advance.