Hi Dien,

Yes, basically Artemis / Atoms cannot handle partial occupancy for crystallographic sites because partial occupancy is not readily translated to a cluster of atoms.  So, what you have to do is start with a CIF without fractional occupancy, say that has only Ca or only U in that site, and generate a cluster of atoms from it.  Then, you can (and here, have to) edit that cluster of atoms to replace some of the Ca/U with U/Ca.  Which ones, and how many, is up to you.  

--Matt