Monazite (Ln³⁺PO₄) and related solid solutions are a well-known source of rare earth elements on earth. They may also accommodate large amounts of thorium and uranium without sustaining damage to the structure by self-irradiation. Such observations led to monazite-type structures being proposed as a potential host matrix for sequestering long-lived radionuclides produced during the nuclear fuel cycle and/or plutonium and americium from dismantled nuclear weapons. Monazite has two main advantages as a matrix for the containment of radioactive waste (or “radwaste”). The first is a highly flexible structure that permits accommodation of high concentrations of actinides. The incorporation of trivalent elements may be achieved by direct synthesis of An³⁺PO₄ (An³⁺ = plutonium, Pu to einsteinium, Es), while tetravalent cation incorporation requires coupled substitutions, either on the anionic site (leading to monazite-huttonite solid solution) or on the cationic site (monazite-cheralite solid solution). Various methods developed for the preparation of such compounds are summarized here, as well as the experimental conditions required for the production of sintered pellets, with a particular focus on plutonium-bearing compositions. The second highly favorable property of monazite is its high chemical durability. Several experimental procedures developed to determine normalized leaching rates are reviewed, as well as results obtained from natural and synthetic monazite. Potential phases formed during dissolution were considered because they also partially control the concentration of actinides in the media. A preliminary list for such phases of interest, as well as corresponding thermodynamic data, is presented.

Several La_1 − xGd_xPO₄ solid solutions were prepared in the monazite- or rhabdophane-type structures for various x values using three methods of preparation (direct evaporation, synthesis in closed PTFE containers on a sand bath or in autoclaves). Samples of rhabdophane-type La_1 − xGd_xPO₄·nH₂O (n ≈ 0.5) were prepared at 150 °C only for x ≥ 0.4. For x ≤ 0.3, the solids were precipitated as the monazite-type structure. These results were confirmed by the study of pure rare earth phosphates synthesized under the same conditions. By these means, well-crystallized and monophase samples of MPO₄·nH₂O (n ≈ 0.5–1) in the monazite (La, Ce), rhabdophane (Nd, Sm, Eu, Gd, Tb, Dy) or churchite (Ho, Er, Tm, Yb, Lu) forms were prepared. On the basis of the variation of the specific area versus the holding temperature and of the dilatometric studies, the optimal temperature of sintering for these solids was found to be between 1250 and 1400 °C. The effective relative densities of the pellets of GdPO₄ prepared using a two-step procedure (pressing between 200 and 700 MPa, then heat treatment at 1300 °C) reached 96% of the value calculated from the XRD data. The chemical durability of sintered samples of GdPO₄ was evaluated in several acidic media between room temperature and 90 °C. The very low normalized dissolution rates R_L (between 10⁻⁶ and 10⁻³ g m⁻² day⁻¹) measured even in very acidic media confirmed the very good retention properties of this kind of phosphate-based matrix for the immobilization of radionuclides and especially of trivalent actinides.

In the field of the immobilisation of high-activity-level and long-life radwaste (HAVL) for a deep underground repository, several phosphate matrices were already proposed as good candidates to delay the release of actinides in the near-field of such disposal. Among them, thorium phosphate–diphosphate (TPD), monazites/brabantites, britholites, and TPD/monazite composites were extensively studied. The synthesis of samples doped with actinides (Th, U...) through wet and dry chemistry methods then their complete characterisation are reported. Their chemical durability is also examined. These materials appear as promising matrices to immobilise tetravalent and/or trivalent actinides.