We present our ab initio study of [Formula: see text] (X = La, Ce, Pu, Am) compounds. Depending on the ionization of the associated element, trivalent or tetravalent, either uranium stays tetravalent as in [Formula: see text] or it is pentavalent to compensate the charge. We study the evolution of the lattice parameter as a function of X content and in the presence of oxygen vacancies. We also calculate the formation energies of several vacancy defects and show that depending of the ionic state of X the most stable charge state can be either positive (tetravalent: Ce, Pu) or negative (trivalent: La, Am). We also consider the positron lifetime in the presence of these vacancies and we show that it is almost independent of the doping element and its amount. For uranium vacancies, the positron is always trapped, while for oxygen vacancies, the trapping seems to depend on the surrounding cation oxidation states. Finally, we show that different defects have similar positron lifetimes, resulting in extremely difficult defect identification with dedicated experimental studies as the positron annihilation spectroscopy.