With their high chemical and self-irradiation stability, crystalline monazites are among the most promising materials for the encapsulation of nuclear wastes. Yet, the local and magnetic structures of the matrices doped with low-content actinide cation, depicted as most resistant, are still unclear. This limits the development of theoretical approaches predicting their behavior under extreme conditions—self-irradiation and long-term leaching. Here, we characterize the model matrices La1–xMxPO4 (0 ≤ x ≤ 0.10)—with M = Sm, 239Pu, 241Am—by X-ray diffraction and solid-state 31P NMR. As an example, we confirm that La0.96241Am0.04PO4 has higher self-irradiation resistance compared to 241AmPO4. Further, computational analyses show that magnetic properties of the Pu complex are strongly affected by the J-mixing and the paramagnetic NMR shifts are dominated by the Fermi contact contribution, arising from delocalization of the spin density of the cation toward the phosphorus through the bonds.