A model of minimum size clusters in orthophosphates has been suggested. In the case of monazite, the cluster includes a cerium(III) cation with nine oxygen atoms of the first coordination sphere, from seven neighboring phosphate anions. Its charge consists of the cation charge Q _M and the charges of the oxygen atoms Q _O =–1 a. u. The results of quantum-chemical simulation assume the possibility of the Ce(III) cation substitution with the Th(III), Th(IV) and U(IV) actinide cations. Substitution with thorium is favorable over that with uranium.