Incorporation of Th4+ and Sr2+ into Rhabdophane/Monazite by Wet Chemistry: Structure and Phase Stability

Rhabdophane is an important permeable reactive barrier to enrich radionuclides from groundwater and has been envisaged to host radionuclides in the backend of the nuclear fuel cycle. However, understanding of how An4+ and Sr2+ precipitate into rhabdophane by wet chemistry has not been resolved. In this work, Th4+ and Sr2+ incorporation in the rhabdophane/monazite structure as La1-2xSrxThxPO4·nH2O solid solutions is successfully achieved in the acid solution at 90 °C. Some specific issues such as lattice occupation of Th4+ and Sr2+, precipitation reaction kinetics, and crystal growth affected by starting stoichiometry are discussed in detail, along with investigating the chemical stability of La1-2xSrxThxPO4·nH2O precipitations and associated La1-2xSrxThxPO4 monazite. The results reveal that the excess of Sr2+ appears to be a prevailing factor with a suggested initial Sr: Th ≥ 2 to obtain the stability domain of La1-2xSrxThxPO4·nH2O (x = 0∼ 0.1). A rapid ion removal associated with a nucleation process has been observed within 8 h, and Th4+ can be removed more than 98% after 24 h in 0.01 mol/L solutions. From structural energetics based on density functional theory, the lattice occupation of Th4+ and Sr2+ is energetically favorable in nonhydrated lattice sites of [LaO8], although two-thirds of lattice sites are associated with [LaO8·H2O] hydrated sites. Intriguingly, the crystal transformation from rhabdophane to monazite associated with the transformation from [SrO8] to [SrO9] polyhedra can greatly improve the leaching stability of Sr2+.

Monazite-Type SmPO4 as Potential Nuclear Waste Form: Insights into Radiation Effects from Ion-Beam Irradiation and Atomistic Simulations

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO₄ pellets and thin lamellae were irradiated with 1, 3.5, and 7 MeV Au ions, up to fluences of 5.1 × 10¹⁴ ions cm^-2 to simulate ballistic effects of recoiling nuclei resulting from α-decay of incorporated actinides. Threshold displacement energies for monazite-type SmPO₄ subsequently used in SRIM/TRIM simulations were derived from atomistic simulations. Raman spectra obtained from irradiated lamellae revealed vast amorphisation at the highest fluence used, although local annealing effects were observed. The broadened, but still discernible, band of the symmetrical stretching vibration in SmPO₄ and the negligible increase in P-O bond lengths suggest that amorphisation of monazite is mainly due to a breaking of Ln-O bonds. PO₄ groups show structural disorder in the local environment but seem to behave as tight units. Annealing effects observed during the irradiation experiment and the distinctively lower dose rates incurred in actinide bearing waste forms and potential α-radiation-induced annealing effects indicate that SmPO₄-based waste forms have a high potential for withstanding amorphisation.

Tailoring the Radionuclide Encapsulation and Surface Chemistry of La(223Ra)VO4 Nanoparticles for Targeted Alpha Therapy

The development of targeted alpha therapy (TAT) as a viable cancer treatment requires innovative solutions to challenges associated with radionuclide retention to enhance local tumor cytotoxicity and to minimize off-target effects. Nanoparticles (NPs) with high encapsulation and high retention of radionuclides have shown potential in overcoming these issues. This article shows the influence of pH on the structure of lanthanum vanadate (LaVO4) NPs and its impact on the radiochemical yield of 223Ra and subsequent retention of its decay daughters, 211Pb and 211Bi. An acidic pH (4.9) results in a high fraction of La(223Ra)VO4 NPs with tetragonal structure (44.6–66.1%) and a 223Ra radiochemical yield <40%. Adjusting the pH to 11 yields >80% of La(223Ra)VO4 NPs with monoclinic structure and increases the 223Ra radiochemical yield >85%. The leakage of decay daughters from La(223Ra)VO4 NPs (pH 11) was <5% and <0.5% when exposed to deionized water and phosphate-buffered saline, respectively. Altering the surface chemistry of La(223Ra)VO4 NPs with carboxylate and phosphate compounds resulted in a threefold decrease in hydrodynamic diameter and a 223Ra radiochemical yield between 74.7% and 99.6%. These results show the importance of tailoring the synthesis parameters and surface chemistry of LaVO4 NPs to obtain high encapsulation and retention of radionuclides.

Encapsulation and retention of 225Ac, 223Ra, 227Th, and decay daughters in zircon-type gadolinium vanadate nanoparticles

Unwanted targeting of healthy organs caused by the relocation of radionuclides from the target site has been one of the limiting factors in the widespread application of targeted alpha therapy in patient regimens. GdVO4 nanoparticles (NPs) were developed as platforms to encapsulate α-emitting radionuclides 223Ra, 225Ac, and 227Th, and retain their decay daughters at the target site. Polycrystalline GdVO4 NPs with different morphologies and a zircon-type tetragonal crystal structure were obtained by precipitation of GdCl3 and Na3VO4 in aqueous media at room temperature. The ability of GdVO4 crystals to host multivalent ions was initially assessed using La, Cs, Bi, Ba, and Pb as surrogates of the radionuclides under investigation. A decrease in Ba encapsulation was obtained after increasing the concentration of surrogate ions, whereas the encapsulation of La cations in GdVO4 NPs was quantitative (∼100%). Retention of radionuclides was assessed in vitro by dialyzing the radioactive GdVO4 NPs against deionized water. While 227Th was quantitatively encapsulated (100%), a partial encapsulation of 223Ra (∼75%) and 225Ac (>60%) was observed in GdVO4 NPs. The maximum leakage of 221Fr (1st decay daughter of 225Ac) was 55.4 ± 3.6%, whereas for 223Ra (1st decay daughter of 227Th) the maximum leakage was 73.0 ± 4.0%. These results show the potential of GdVO4 NPs as platforms of α-emitting radionuclides for their application in targeted alpha therapy.

Quantitative encapsulation and retention of 227Th and decay daughters in core-shell lanthanum phosphate nanoparticles

Targeted alpha therapy (TAT) offers great promise for treating recalcitrant tumors and micrometastatic cancers. One drawback of TAT is the potential damage to normal tissues and organs due to the relocation of decay daughters from the treatment site. The present study evaluates La(227Th)PO4 core (C) and core +2 shells (C2S) nanoparticles (NPs) as a delivery platform of 227Th to minimize systemic distribution of decay daughters, 223Ra and 211Pb. In vitro retention of decay daughters within La(227Th)PO4 C NPs was influenced by the concentration of reagents used during synthesis, in which the leakage of 223Ra was between 0.4 ± 0.2% and 20.3 ± 1.1% in deionized water. Deposition of two nonradioactive LaPO4 shells onto La(227Th)PO4 C NPs increased the retention of decay daughters to >99.75%. The toxicity of the nonradioactive LaPO4 C and C2S NP delivery platforms was examined in a mammalian breast cancer cell line, BT-474. No significant decrease in cell viability was observed for a monolayer of BT-474 cells for NP concentrations below 233.9 μg mL-1, however cell viability decreased below 60% when BT-474 spheroids were incubated with either LaPO4 C or C2S NPs at concentrations exceeding 29.2 μg mL-1. La(227Th)PO4 C2S NPs exhibit a high encapsulation and in vitro retention of radionuclides with limited contribution to cellular cytotoxicity for TAT applications.

Synthesis, Characterization, and Stability of Americium Phosphate, AmPO4

AmPO₄ was prepared by a solid-state reaction method, and its crystal structure at room temperature was solved by powder X-ray diffraction combined with Rietveld refinement. The purity of the monazite-like phase was confirmed by spectroscopic (high-resolution solid-state ³¹P NMR and Raman) and microscopic (SEM-EDX and TEM) techniques. The thermal and self-irradiation stability have been studied. The compound is stable under argon and air atmosphere at least up to 1773 K. It remains crystalline under self-irradiation for circa two months, with a crystallographic volume swelling of ∼1.5%, and then is amorphizing over a year. However, microcrystals are present in the amorphous material even after a two year period of time. All these characteristics are discussed in relation to the potential application of AmPO₄ as a stable form of Am in radioisotope power sources for space exploration and of behavior of the monazites under irradiation.

31P MAS NMR and DFT study of crystalline phosphate matrices

We present the study of the phosphorus local environment by using ³¹P MAS NMR in a series of seven double monophosphates M^IIM^IV(PO₄)₂ (M^II and M^IV being divalent and tetravalent cations, respectively) of yavapaiite and low-yavapaiite type crystal structures. Solid-state and cluster DFT calculations were found to be efficient for predicting the ³¹P isotropic chemical shift and chemical shift anisotropy. To achieve this performance, however, a proper computational optimisation of the experimental structural data was required. From the three optimisation methods tested, the full optimisation provided the best reference structure for the calculation of the NMR parameters of the studied phosphates. Also, a better prediction of the chemical shifts was possible by using a correction to the GIPAW calculated shielding.

Rare-Earth Orthophosphates From Atomistic Simulations

Lanthanide phosphates (LnPO₄) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

A Spectroscopic Investigation of Eu3+ Incorporation in LnPO4 (Ln = Tb, Gd1-xLux, X = 0.3, 0.5, 0.7, 1) Ceramics

We have investigated the incorporation of the luminescent Eu³⁺ cation in different LnPO₄ (Ln = Tb, Gd_1−xLu_x, x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO₄ phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd_1−xLu_xPO₄ solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO₄, Gd_0.3Lu_0.7PO₄, and Gd_0.5Lu_0.5PO₄ indicate Eu³⁺–incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd_0.7Lu_0.3PO₄ composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd³⁺–cation in this solid phase. Eu³⁺–incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu³⁺ incorporated in monazite and xenotime. The electronic levels of the Eu³⁺–dopant in Gd_0.3Lu_0.7PO₄ and Gd_0.5Lu_0.5PO₄ xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu³⁺ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu³⁺ was predominantly incorporated within the xenotime structure. Single crystals of Eu³⁺–doped LuPO₄ show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu³⁺ migration within the crystal structure and Eu³⁺ accumulation at grain boundaries or xenotime surface sites.

The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd3

In this study, we present a new concept based on the steady-state, laser-induced photoluminescence of Nd³⁺, which aims at a direct determination of the amorphous fraction f_a in monazite- and xenotime-type orthophosphates on a micrometer scale. Polycrystalline, cold-pressed, sintered LaPO₄, and YPO₄ ceramics were exposed to quadruple Au-ion irradiation with ion energies 35 MeV (50% of the respective total fluence), 22 MeV (21%), 14 MeV (16%), and 7 MeV (13%). Total irradiation fluences were varied in the range 1.6 × 10¹³–6.5 × 10¹³ ions/cm². Ion-irradiation resulted in amorphization and damage accumulation unto a depth of ~5 μm below the irradiated surfaces. The amorphous fraction created was quantified by means of surface-sensitive grazing-incidence X-ray diffraction and photoluminescence spectroscopy using state-of-the-art confocal spectrometers with spatial resolution in the μm range. Monazite-type LaPO₄ was found to be more susceptible to ion-irradiation induced damage accumulation than xenotime-type YPO₄. Transmission electron microscopy of lamella cut from irradiated surfaces with the focused-ion beam technique confirmed damage depth-profiles with those obtained from PL hyperspectral mapping. Potential analytical advantages that arise from an improved characterization and quantification of radiation damage (i.e., f_a) on the μm-scale are discussed.

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4)

This study investigates the incorporation of the minor actinide curium (Cm³⁺) in a series of synthetic La_{1- x}Gd _xPO₄ ( x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid-solutions. To obtain information on the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy and X-ray absorption fine structure spectroscopy investigations were conducted and complemented with ab initio atomistic simulations. We found that Cm³⁺ is incorporated in the monazite endmembers (LaPO₄ and GdPO₄) on one specific, highly ordered lattice site. The intermediate solid-solutions, however, display increasing disorder around the Cm³⁺ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid-solutions, Cm³⁺ is preferentially incorporated on nonhydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two-thirds of the lattice sites are associated with water of hydration (LnPO₄·0.67H₂O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.

Multifunctional GdVO4:Eu core–shell nanoparticles containing 225Ac for targeted alpha therapy and molecular imaging

Development of actinium-225 doped Gd_0.8Eu_0.2VO₄ core–shell nanoparticles as multifunctional platforms for multimodal molecular imaging and targeted radionuclide therapy.