Nina Huittinen

Single-source CVD enables the phase-controlled deposition of thorium dioxide/iron oxide nanocomposite thin films with exceptional long-term stability for alkaline hydrogen evolution.

Monazite (LnPO4) materials are promising waste forms for immobilizing minor actinides, but their radiation damage performance as mixed solid solutions remains poorly described. Herein, we have systemically examined the irradiation response of mixed monazite solid solutions, (La1–xLnx)PO4 (x = 0, 0.5, and 1; Ln = Pr, Nd, and Sm), using 14 MeV 197Au4+ ion irradiations at a fluence of 1 × 1015 ions/cm2 regarding structural, microstructural, and mechanical properties. Amorphization susceptibility was assessed by grazing incidence X-ray diffraction, with the optimal incidence angle determined from stopping and range of ions in matter simulations and X-ray penetration depth calculations. Complementary characterization via scanning electron microscopy, Raman spectroscopy, and Vickers indentation provided insight into microstructural evolution, local chemistry, and mechanical property changes, respectively. The results indicate an increasing radiation damage resistance of end-member LnPO4 materials with decreasing Ln cation size. In mixed solid solutions, (La0.5Ln05)PO4, the radiation damage resistance decreases as the size mismatch between the Ln cations increases, which is attributed to nonideal solid solution cation mixing during irradiation damage. We propose that irradiation-induced lattice displacements increasingly impede cation redistribution and promote structural disorder as the cation size mismatch increases, with measurable consequences for both chemical stability and mechanical integrity. The role of grain boundaries and the susceptibility to changes in local coordination chemistry of cations are further highlighted as determinants for specific irradiation damage response and their ability to act as sinks for ion and defect migration. The investigation underscores the need to evaluate complex solid solutions rather than idealized end-member compositions when assessing ceramic nuclear waste form performance for radionuclide immobilization.

Monoclinic zirconia ( m- ZrO ₂ ) forms on the internal surface of nuclear fuel Zircaloy cladding, acting as a critical barrier against radionuclide release at the fuel-cladding interface. However, the incorporation of minor actinide elements like americium in m- ZrO₂ and resultant structural chemistry remains poorly understood. Using a combination of diffraction and high-resolution X-ray spectroscopic techniques, we have examined m- ZrO ₂ with 5 mol% Am doping. We show Am enters m- ZrO ₂ tetravalently, where its solubility is approximately 1.0 mol%, m -(Am ⁴⁺ _0.011(7) Zr ⁴⁺ _0.989(7) )O ₂ , attributed to the large Am ⁴⁺ cation, where excess Am, that is predominantly trivalent, adopts a C-type (Am ^4+/3+ _1-x Zr ⁴⁺ _x ) ₂ O _3+x phase in space group Ia -3. The known reversible high temperature phase transformation of m- ZrO ₂ to tetragonal is further shown to be reduced from 1150 ^o C to 1050 ^o C via Am ⁴⁺ incorporation. The investigation provides critical insight into the chemical reactivity and speciation of minor actinide elements with nuclear fuel cladding related m- ZrO₂.

Curium is an artificial transuranic element with atomic number 96. It is typically found in its +III oxidation state, which is stabilized by a 5f⁷ electron configuration. Cm^III exhibits intense luminescence from its first excited to its ground state in the red part of the visual spectrum. Due to the nature of the 5f electron shell, this luminescence is sensitive to changes in the chemical environment of the Cm^III probe, while being detectable in the trace concentration range. This unique combination has established Cm^III luminescence spectroscopy as an ideal tool for speciation studies in complex systems, particularly those relevant to the nuclear fuel cycle. In this review, we present an overview of the developments and applications of Cm^III luminescence spectroscopy in the last 20 years since the last comprehensive review was published. The discussed studies have been categorized according to their chemical environment into reactions at the water/mineral interface, studies of solids containing Cm^III, aqueous complexation studies, spectroscopy in non-aqueous systems, and interaction of Cm^III with biomolecules and biota. These systems correlate in large parts with areas of application in nuclear waste disposal science, separation processes within current and proposed nuclear fuel cycles, and radioecological research. We summarize the most important findings in the studies, identify emerging trends and persistent challenges in the field of Cm^III luminescence spectroscopy. Finally, we offer an outlook on potential future developments and research directions in this area.

The environmental fate of radionuclides (RN), such as actinides and fission products, disposed of in underground nuclear waste repositories is a major concern. Long-term safety assessments of these disposal sites depend on the ability of geochemical models and thermodynamic databases (TDBs) to reliably predict the mobility of RNs over very long time scales. One example where TDBs still have large data gaps is related to the complexation of trivalent actinides and lanthanides with aqueous phosphates. Indeed, solid phosphate monazites are one of the candidate phases for the immobilization of specific high-level waste streams for future safe storage in deep underground disposal facilities, therefore potentially and locally increasing the presence of phosphate at the final disposal site.Recent work [1-3] obtained reliable complexation constants at 25 °C and at elevated temperatures and thus, closed some knowledge gaps. Laser-induced luminescence spectroscopy was used to study the complexation of Cm(III) and Eu(III) as a function of total phosphate concentration in the temperature regime 25-90 °C, using NaClO4 as a background electrolyte (I = 0.5 to 3.0 M). These studies have been conducted in the acidic pH-range (−log10 [H+] = 1.00, 2.52, 3.44, and 3.65) to avoid precipitation of solid Cm and Eu rhabdophane. For the first time, in addition to the presence of the 1:1 CmH2PO42+/EuH2PO42+ species [1-3], the formation of Cm(H2PO4)2+ [2] and Eu(H2PO4)2+ [3] was unambiguously established from the collected luminescence spectroscopic data. The conditional complexation constants of all aqueous complexes were found to increase with increasing ionic strength and temperature [1-3]. Extrapolation of the obtained conditional complexation constants to infinite dilution at 25 °C and elevated temperature was performed by applying the Specific Ion Interaction Theory (SIT). Using the integrated van´t Hoff equation, both the molar enthalpy of reaction ΔrHm° and entropy of reaction ΔrSm° values were derived.During complexation, depending on the concentration of phosphate, monodentate or bidentate Cm(III)/Eu(III)-phosphate complexes can form with different overall coordination numbers (8,9). Obtaining additional information about the complex structures from spectroscopic data only is often challenging, especially at the low metal-ion concentrations used in the experiments. Thus, relativistic quantum chemical (QC) methods can be considered as an additional tool to complement experimental observations. In this study, the structural properties, electronic structures, and thermodynamics of the 1:1 and 1:2 Cm(III) and Eu(III) phosphate complexes were solved using state-of-the-art QC calculations. In particular, the QC methods allowed i) to investigate the complexation strength of Cm(III) and Eu(III) with aqueous phosphate, ii) to understand the possible change of the coordination number with increasing temperature and iii) to investigate the nature (ionic/covalent) of the Cm/Eu bonds with water and phosphate. Combining the information obtained from quantum chemical calculations with the observed spectral changes facilitates the decisive determination of the structures of the formed phosphate complexes and their overall coordination [2,3]. [1] Jordan, N., et al., Inorg. Chem. (2018), 57:7015−7024.[2] Huittinen, N., et al., Inorg. Chem. (2021), 60:10656−10673.[3] Jessat, I., et al., Inorg. Chem. (in preparation).

The ZrO₂-CeO₂ system is fundamental to various technological applications, yet unresolved questions persist regarding cation miscibility and the occurrence of metastable phases in the Zr_1-xCe_xO₂ phase diagram. This work addresses these gaps through a comprehensive investigation of Zr_1-xCe_xO₂ compositions with varying cerium concentrations and incorporating Eu³⁺ as a luminescent probe. Synchrotron powder X-ray diffraction analysis unveiled a miscibility gap between 20 and 50 mol % cerium. Beyond this gap, the formation of solid solutions and multiple crystalline phases was observed, including tetragonal prime (t') and tetragonal double prime (t″) structures, depending on cerium content. Raman investigations revealed a unique distortion band in all compositions containing the t' phase. Our high energy resolution fluorescence detected X-ray absorption near edge structure spectroscopy (HERFD-XANES) analysis implies that this feature results from oxygen ion displacement in the t' structure. Luminescence spectroscopy of the europium environment revealed distinct excitation and emission spectra across the various crystal phases, enabling unambiguous identification of all metastable phases. These findings highlight the complex polymorphism of the ZrO₂-CeO₂ system. The ability to precisely control phase composition offers significant potential for optimizing properties for diverse applications, including oxygen sensors, three-way catalysts, and solid oxide fuel cells for clean, sustainable energy generation.

This study presents a comprehensive analysis of the microstructural characteristics and chemical composition of base and weld materials from reactor pressure vessels in the first (units 1 and 2) and second (unit 8) generations of Russian VVER 440 reactors at the Greifswald nuclear power plant. We measured the specific activities of ⁶⁰Co and ¹⁴C in activated samples from units 1 and 2. ⁶⁰Co, with its shorter half-life (t_1/2 = 5.27 a), is a key dose-contributing radionuclide during decommissioning, while ¹⁴C (t_1/2 = 5700 a) plays an important role in a geological repository for low- and intermediate-level radioactive waste. Our findings reveal differences in the proportions of trace elements between the base and weld materials as well as between the two reactor generations. Microstructural analysis identified Mo-rich precipitates and (Mn, S)-rich inclusions containing secondary micro-inclusions in the unit 1 and 2 samples. Raman spectroscopy confirmed iron oxides (γ-Fe₂O₃, Fe₃O₄), silicates (Mn-SiO₃), and Cr₂O₃/NiCr₂O₄ in the base metal as well as MnFe₂O₃ in the weld metal. X-ray photoelectron spectroscopy identified Mn inclusions as MnS, MnS₂, or mixed Mn, Fe sulfides, and the Mo precipitates as MoSi₂. These findings offer valuable insights into the speciation of elements and the potential release of radionuclides through corrosion processes under repository conditions.

Crystalline materials such as monazite have been considered for the storage of radionuclides due to their favorable radiation stability. Understanding their structural chemical response to radiation damage as solid solutions is a key component of determining their suitability for radionuclide immobilization. Herein, high-resolution structural studies were performed on ceramics of the monazite solid solution La_1-xCe_xPO₄ (x = 0.25, 0.5, 0.75, 1) in order to understand the role of structural chemistry on irradiation stability. Ceramic samples were irradiated with 14 MeV Au ions with 10¹⁴ ions/cm² and 10¹⁵ ions/cm² to simulate the recoil of daughter nuclei from the alpha decay of actinide radionuclides. The extent of radiation damage was analyzed in detail using scanning electron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray diffraction (GI-XRD), and high-energy-resolution fluorescence detection extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy. SEM and Raman spectroscopy revealed extensive structural damage as well as the importance of grain boundary regions, which appear to impede the propagation of defects. Both radiation-induced amorphization and recrystallization were studied by GI-XRD, highlighting the ability of monazite to remain crystalline at high fluences throughout the solid solution. Both, diffraction and HERFD-EXAFS experiments show that while atomic disorder is increased in irradiated samples compared to pristine ceramics, the short-range order was found to be largely preserved, facilitating recrystallization. However, the extent of recrystallization was found to be dependent on the solid solution composition. Particularly, the samples with uneven ratios of solute cations, La_0.75Ce_0.25PO₄ and La_0.25Ce_0.75PO₄ were observed to exhibit the least apparent radiation damage resistance. The findings of this work are discussed in the context of the monazite solid solution chemistry and their appropriateness for radionuclide immobilization.