A thermodynamic model of the molten salt system (Na,Cs,Mg,Pu,Nd)(Cl,I) has been developed in this work to assess the effect of CsI on the melting and vaporization behavior of the nuclear fuel in molten salt reactors. Investigation using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) of the binary systems NdCl₃–NdI₃ and MgI₂–NdI₃, as simulant systems for the analogous Pu system, is presented for the first time. Both systems were found to be binary eutectic systems, with solid solutions NdCl_3–3xI_3x (hexagonal in space group P6₃/m) and NdCl_3yI_3–3y (orthorhombic in space group Cmcm) in the NdCl₃–NdI₃ system, and Mg_1–xNd_xI_2+x (hexagonal in space group P 3 m1) in the MgI₂–NdI₃ system. Additionally, the system CsI–MgI₂ was scrutinized using DSC, confirming the available experimental data in the literature. Furthermore, the investigation of the reciprocal diagonals in the systems (Na,Nd)(Cl,I), (Cs,Mg)(Cl,I), (Mg,Nd)(Cl,I), and (Cs,Nd)(Cl,I) is presented, allowing the characterization of the quaternary behavior of these salts. Based on the experimental data obtained in this work, a CALPHAD model is presented using the quasi-chemical formalism in the quadruplet approximation for the liquid solution. With the aim of modeling the complete (Na,Cs,Mg,Pu)(Cl,I) system, the binary systems NaCl–CsCl, CsCl–MgCl₂, CsCl–NdCl₃, NaI–CsI, and CsI–NdI₃ were reassessed based on data from the literature. Furthermore, a CALPHAD model of the PuCl₃ and PuI₃ systems is also presented using Nd as a simulant for Pu in molten halide salts. With the developed thermodynamic models, calculations were finally performed to assess the fission product retention of Cs and I in a molten chloride environment. As opposed to their behavior in molten fluorides, the fission products are well retained in the fuel matrix up to concentrations of at least 5 mol%.

The low-temperature heat capacity and temperature-dependent DC magnetic susceptibility of neptunium-doped UO2 samples (U1−y[jls-end-space/]Npy[jls-end-space/])O2 with y = 0.01, 0.03 and 0.05 were measured. A strong effect was observed on the magnetic anomaly typical for UO2[jls-end-space/], both the Néel temperature and transition entropy. By comparison with data for other (U1−y[jls-end-space/], My[jls-end-space/])O2 solid solutions, it is suggested that this effect can be explained by the lattice strain resulting from substitution on the anion sublattice, and electron spin interaction.

A comprehensive thermodynamic assessment of the Cs-Pb system was performed with the CALPHAD method using the experimental thermodynamic and phase diagram data available in literature supplemented by density functional theory (DFT) calculations. The exact nature of the stable compounds in the phase diagrams reported in the literature is uncertain, except for CsPb and Cs₄Pb₉ whose crystalline structures are well known. Therefore, DFT calculations were performed to calculate the energy of formation at 0 K of different possible compounds with various crystalline structures. The enthalpies of formation of the compounds CsPb, Cs₄Pb₉, and CsPb₄, found to be the stable ones by DFT, were then used in the CALPHAD model. The result of this process has enabled the development of a more refined phase diagram comparing to experimental ones, providing more comprehensive insights into the phase equilibria in this system. Moreover, the CALPHAD model succeeded in describing the peculiar behavior of the heat capacity of the liquid phase, by using an ionic two-sublattice model (Cs+1)_P(Pb−1,Va,Pb)_Q which takes into account the short-range ordering taking place at the equimolar composition CsPb, related to the formation of clusters Cs₄Pb₄, modelled as (Cs+1) (Pb−1). The model allows for the prediction of important thermodynamic properties, which are of interest for a range of applications, including lead-cooled fast reactors and perovskite-based photovoltaics.

The chemistry following cladding failure in Lead-cooled Fast Reactors involves the interaction between lead (Pb) coolant and the Joint Oxyde Gain (JOG)-phase, mostly composed of dicesium molybdate (Cs₂MoO₄). A thermodynamic analysis of coolant-JOG phase chemical interaction as studied via the scenario of Pb-Cs₂MoO₄ chemical interaction is reported. Measurements of the standard thermodynamic properties of α -Cs₂Pb(MoO₄)₂ are presented. The enthalpy of formation of α -Cs₂Pb(MoO₄)₂ is measured to be -(2570.7 ± 2.3) kJ · mol⁻¹ using solution calorimetry, while the standard entropy is determined to be (399 ± 12) J · K⁻¹·mol⁻¹ using thermal-relaxation calorimetry. Thermodynamic calculations show that Cs₂Pb(MoO₄)₂ is in several cases thermodynamically stable under conditions typical for operation of Lead-cooled Fast Reactors. This means Cs₂Pb(MoO₄)₂ can form in cladding failure scenarios.

In this study, new insights into the solid state chemistry of the systems NaCl-RECl3 (RE = Ce, Nd) are presented, in which the intermediate compound suggested in the literature, i.e. Na3RE5Cl18, is investigated more closely. Our studies have revealed a solubility range around the intermediate composition in the form of the stoichiometry, and have allowed us to revisit the phase diagrams of the NaCl-RECl3 (RE = Ce, Nd) systems accordingly. Furthermore, we demonstrate that among the lanthanide chlorides, NdCl3 is the prime simulant candidate for the melting behaviour of PuCl3-based systems, while CeCl3 is most suited to simulate UCl3-based systems. This is corroborated in this work by comparing the melting profiles of the NaCl-MCl3, MgCl2-MCl3, and NaCl-MgCl2-MCl3 (M = Ce, Nd, U, Pu) systems. In doing so, the binary systems MgCl2-MCl3 (M = Ce, Nd) have been re-visited based on existing data in the literature and estimated mixing enthalpies. Extrapolations to the ternary systems NaCl-MgCl2-RECl3 (RE = Ce, Nd) have been made and compared to the available data in the literature, showing good agreement.

Graphical abstract: Simulant chemistry for uranium and plutonium molten fuel salts: crystallographic investigation and thermodynamic modelling assessment of the NaCl–RECl3 and NaCl-MgCl2-RECl3 (RE = Ce, Nd) systems

We show that a homogeneous single-phase mixed oxide is obtained by blending and sintering commercial UO₂ and nanocrystalline PuO₂ powder; whereas, under the same conditions, conventional PuO₂ powder obtained from oxalate precipitation and composed of platelets, yields two-phase mixed oxide. The use of nanometric PuO₂ results in a larger final grain size.

Correction for ‘Experimental investigation and thermodynamic modelling assessment of the AECl2–NdCl3 (AE = Sr, Ba) systems’ by D. C. Alders et al., Phys. Chem. Chem. Phys., 2024, 26, 24041–24057, https://doi.org/10.1039/D4CP01784F.

The reference list in the originating article was incorrect. The correct list of references is shown here.

The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

The thermochemistry of the quaternary molten salt system NaCl-NaI-MgCl₂-MgI₂ has been studied using an experimental and thermodynamic modeling approach. The binary subsystems NaCl-NaI and NaCl-MgCl₂ were reassessed based on existing data in the literature. The binary subsystem NaI-MgI₂ was subjected to a renewed experimental investigation, to complement and revisit the data in the literature. The subsystem MgCl₂-MgI₂ was investigated for the first time in this work using Differential Scanning Calorimetry (DSC). Furthermore, the phase equilibria in the pseudobinary phase diagrams of NaCl-MgI₂ and NaI-MgCl₂ in the quaternary system were investigated by DSC, while the condensed phases in the quaternary system were investigated using X-ray diffraction (XRD). A thermodynamic model of the quaternary system was developed using the CALPHAD (CALculation of PHase Diagrams) method with the quadruplet approximation in the modified quasichemical model for the liquid phase, and two-sublattice polynomial models for the solid solution phases. With this model, the liquidus surface of the NaCl-NaI-MgCl₂-MgI₂ quaternary system has been described for the first time.

The detailed crystal structure as well as the heat capacity at low temperature and standard entropy of Ba2MoO5 are reported for the first time. High-resolution X-ray and neutron diffraction were employed to reveal the structural features of this compound. Ba2MoO5 has a six-coordinated Mo and a strongly negative excess volume with respect to the binary oxides. X-ray absorption near edge structure (XANES) spectroscopy at the Mo K-edge shows Mo to be in the oxidation state 6+. The pre-edge peak in the XANES spectrum indicates a distorted octahedral environment, in line with the results from diffraction studies and FDMNES calculations. The standard entropy and heat capacity of Ba2MoO5 at 298.15 K, determined with a thermal-relaxation technique, are calculated to be respectively 223.2 ± 7 and 184.7 ± 5 J·K–1·mol–1. The obtained thermodynamic properties are discussed in the context of the literature reports on molybdate compounds.

We compare typical reductive sintering conditions for U, Pu mixed oxides (4 h at 1700 °C in Ar/6 % H2 + 1200 ppm H2O) with lower temperature and mildly oxidative conditions (2 h at 1200 °C in CO/CO2 = 1/9) and report on the resulting microstructures and homogeneity. We show that lower temperature and mildly oxidative conditions, without cover gas change, can give close-to stoichiometric, crack-free MOX pellets with a relative density of ∼ 95 %, and we propose ways to improve the homogenisations of PuO2 and UO2 and increase the grain size.

The structural, thermochemical, and thermophysical properties of the AnCl₄, and NaCl-AnCl₄ (An = Th, U) melts were investigated using molecular dynamics (MD) simulations based on the Polarisable Ion Model (PIM). New force fields were proposed and used to compute key properties including density, thermal expansion, enthalpy of mixing, heat capacity, as well as the local structure and chemical speciation in the molten (Na, An)Cl_x (An = Th, U) salts. Thermodynamic models were then developed based on the CALPHAD method, using both PIM-MD and experimental data as input. Employing the modified quasichemical formalism in the quadruplet approximation for the liquid solution, the models account for the chemical speciation in the melt as calculated by MD simulations, and reproduce well phase equilibria in those systems. In particular, the models included monomeric and dimeric species to represent the physical nature of the ionic melt, which shows progressive oligomerisation with increasing AnCl₄ fraction. Our studies confirm that the melt becomes highly volatile at high AnCl₄ fractions, which is discussed in light of the results obtained herein.

We present a two-step procedure for treating contaminated liquid waste generated during the processing of uranium, specifically residual uranium-contaminated liquid after precipitation. After precipitating U(IV) as sodium diuranate under alkaline conditions, the resulting solution was solidified by immobilising it in an alkali-activated material. Static leaching tests indicated excellent material stability in water. Exposing the material to aggressive chemical conditions (nitric acid) resulted in slow and incomplete dissolution of uranium (and of structural elements) from a thin superficial layer with further passivation. Characterisation of the solid phase was performed to assess the stability of the alkali-activated material under the tested conditions.

The presence of CsI in nuclear fuel has long been debated. Its formation significantly decreases volatility, thereby reducing the rate at which iodine and cesium are released from the reactor core during a nuclear accident. A series of samples were investigated by Knudsen Effusion Mass Spectrometry (KEMS) in order to determine whether CsI is present in irradiated nuclear fuel. The examined samples were pure CsI, CsI exposed to gamma radiation, CsI-doped UO2 simulated fuel and irradiated LWR fuel samples. The CsI and CsI-doped samples were examined to establish boundary conditions for the detection of CsI by KEMS. These samples indicated that the presence of CsI in fuel is characterized by three mass spectrometric signals Cs+, I+ and CsI+, with a peak ratio of CsI+ and I+ of 1:0.7. The examinations of irradiated fuels showed none of these characteristics and hence no evidence that CsI is present in irradiated LWR nuclear fuel, at least after a storage period of years.

This work provides an in-depth analysis of the extensive research and development activities on americium-based ceramics for space applications, particularly as heat source for radioisotope power generation. Our pioneering efforts focus on synthesizing and characterizing various americium ceramics with fluorite, monazite, perovskite, zircon, and pyrochlore structures, and assessing their potential for use in Radioisotope Power Systems (RPSs). This study identifies uranium-stabilised cubic americium oxide as the best candidate among the ceramic forms analysed, due to its superior stability and performance under extreme conditions relevant to space missions. The review emphasises the unique facilities and methodologies employed, including remote-handling techniques and advanced material characterization, to overcome the challenges posed by the high radiation dose and specific activity of ²⁴¹Am when working with gram quantities.

We measure the thermal conductivity of solid and molten tungsten using steady state temperature differential radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz law to capture the electronic thermal conductivity of metals in their molten phase. We further support this conclusion using ab initio molecular dynamics simulations with a machine-learned potential. Our results show that at these high temperatures, the vibrational contribution to thermal conductivity is negligible compared to the electronic component.

Lithium-based fluoride salts are one of the leading options as fuel matrix in Molten Salt Reactors. The understanding of their thermodynamic behavior, e.g. chemical stability, activity, as well as heat transfer properties, in the reactor’s environment is crucial for the safety assessment. In this work, the chemical equilibria of lithium fluoride with two important fission products dissolved in the salt matrix, namely barium fluoride and zirconium fluoride, are considered. The phase diagrams of the binary systems LiF-BaF2, LiF-ZrF4 and BaF2-ZrF4, as well as the ternary system LiF-BaF2-ZrF4 are assessed by the CALPHAD (CALculation of PHase Diagram) method using the quasichemical model in the quadruplet approximation for the liquid solution. These models are based on literature and new experimental data, and provide a good overview of the stability of intermediate compounds formed in the various systems and of the liquid solution.