We are investigating the use of Light Water Reactor claddings as precursor materials in zirconia-based matrices for the conditioning and transmutation/burning of actinides. This study examined two un-irradiated zirconium-rich claddings: Zircaloy-4 and M5®. After complete oxidation and characterization, both materials were converted into Yttria Cubic-Stabilized-Zirconia (YSZ) and pyrochlore (Nd2Zr2O7), with neodymium used as a surrogate for actinides. We present preliminary results and discuss a potential scenario for an innovative back-end fuel cycle.

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 Am-241 when working with gram quantities.

UO2 single crystals with (100), (110) and (111) oriented faces were dissolved in 2 mol.L-1 HNO3 at room temperature. The evolution of the topography of the surface was monitored and reliable dissolution rates corresponding to the three crystallographic orientations were determined under controlled hydrodynamic and chemical conditions. The dissolution tests of UO2 polished single crystals showed two different kinetic steps. During the first uncatalysed kinetic regime, the enhanced reactivity of the surface at defect sites was demonstrated. The second kinetic step was attributed to a catalysed dissolution mechanism involving species produced at the solid/solution interface during the first step.

A set of oxide fuel properties is needed to develop the fast neutron systems and to design prototypes of the Sodium Fast Reactor, the Gas-cooled Fast Reactor and a heavy liquid metal cooled Accelerator Driven System. The ESNII+ project (2014-2017) and then ESFR-SMART project (2017-2022) have a work package dedicated to this item : providing thermal and mechanical properties of MOX fuel through a catalog and performing complementary measurements on specific properties. The data and models available in the literature for each property were reviewed and experimental workplan was proposed for the European projects mentioned. Property measurements are done on existing fresh and irradiated fuel samples, identified to cover the fuel characteristics in term of plutonium content, density, stoichiometry, temperature and burn-up. Measurements have been performed and are continuing on thermal diffusivity, heat capacity, melting temperature, thermal expansion and Young modulus. The paper presents the results obtained on properties measurements and the comparison with existing and available data.

Using the modified quasi-chemical model in the quadruplet approximation, three new thermodynamic assessments of binary systems useful for the detailed operational design of the Molten Salt Reactor are presented: AF-NiF2 (A = Li, Na, K). These systems are particularly relevant for the study of the molten salt-structural materials interaction, as the salt containment is made of a Ni-based alloy. Using powder X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC), new experimental data were gathered for two of these systems, LiF-NiF2 and KF-NiF2, and compared to previous experimental assessments. Our data have confirmed the formation of a (Li1-2xNix)F solid solution. The three thermodynamic models show a very good agreement with the experimental data. The melting point of NiF2 was measured for the first time to be T = (1629 +/- 5) K, and the thermal expansion coefficient for Li2NiF4 was found to be alpha = 27.6.10(-6) K-1 in the temperature range T = (298-773) K. (C) 2018 The Authors. Published by Elsevier Ltd.

ASTEC-Na is a computer code system which evaluates protected and unprotected accidents in Sodium cooled Fast Reactors throughout the Initiation Phase. The fuel pin behavior models implemented in ASTEC-Na simulate the essential aspects of the thermal and mechanical behavior of SFR fuel pins in nominal and accidental conditions. Besides the improvement of existing ASTEC-Na models, a specific cladding mechanical model and an in-pin fuel relocation model have been developed during the JASMIN project, where the latter has been already implemented in the code. ASTEC-Na fuel pin behavior models are described in this paper and their transient predictions for four CABRI transient tests are compared to available experimental data as well as to SAS-SFR and SIMMER-III code calculations. Conclusions on the overall performance of ASTEC-Na fuel pin models are also presented. (C) 2017 Elsevier Ltd. All rights reserved.

We have recently proposed the hydrothermal decomposition of An IV -oxalates as a simple access to produce highly crystalline, reactive actinide oxide nanocrystals.The method could be easily applied at low temperature (95-250 °C) in order to produce highly crystalline nano-AnO2 (An= Th, U, Np, Pu).The size and shape of the crystals, together with their increased reactivity, enables the consolidation of homogeneous nanostructured mixed oxides as intermediates toward very dense nuclear fuels for advanced reactors.Spark plasma sintering studies indicates that such nanopowders have increased sinterability compared to powders obtained by conventional thermal decomposition of An IV -oxalates.We have studied the formation conditions, stability, and thermal expansion of AnO2 (An= Th, U, Np, and Pu).U1-xThxO2 solid solutions have been produced under similar conditions by hydrothermal treatment of U1-xThx(C2O4)2  n H2O obtained from oxalate coprecipitation.Ongoing experiments are aimed to show the validity of the method towards associate tri-, or even tetra-component associate solid solutions.The involvement of the water molecules in the oxalate hydrothermal decomposition mechanism has been proven by studying the isotopic exchange reaction during the thermal decomposition of An(C2O4)2  n H2O in H2 17 O through MAS-NMR and Raman techniques.

A solid state method was used to synthesize La1-xPuxPO4 (x = 0.01, 0.05, 0.10, 0.15, (0.5)) solid solutions with monazite structure. XRD measurements of the compounds with x = 0.50 revealed the formation of two phases: (La, Pu) PO4-monazite and a cubic phase (PuO2). Pure-phase La1-xPuxPO4-monazite solid solutions were obtained for materials with x = 0.00-0.15 and confirmed by a linear dependence of the lattice parameters on composition according to Vegard's law. X-ray absorption spectroscopy (XAS) analysis at the Pu-L-III and La-L-III edges confirmed the +III valence state of plutonium in the monazite solid solutions. The local environment of Pu is PuPO4-like along the solid solution series, except for the longest fitted cation-cation distance, which may be an indication of cluster formation consisting of a few Pu-atoms in the La-Pu-monazite lattice. (C) 2017 The Authors. Published by Elsevier B.V.

In the present studies, the thermal behaviour of NaUO2BO3 has been investigated. This compound is a potential product of interaction between the coolant (Na), control rods (B4C), and the oxide fuel, which could form under accidental conditions in sodium-cooled fast reactors. The thermal expansion, the heat capacity, and thermal diffusivity of NaUO2BO3 have been measured. The thermal conductivity of the material is derived from these results and presented here for the first time.

A new double neptunium zirconium phosphate of the type MxZr2(PO4)3 (M = Np), crystallizing in the structure type NaZr2(PO4)3 (NZP, NASICON), was synthesized by solid state reactions at high temperatures and characterized by X-ray diffraction, infrared spectroscopy and Mössbauer spectroscopy. The Rietveld refinement of the XRD pattern together with the analysis of the IR spectra of the sample confirmed the space group P3[combining macron]c, the same as that for the lanthanide analogues Ln0.33Zr2(PO4)3. However, Mössbauer studies revealed the presence of neptunium in the two oxidation states +3 and +4, indicating a two-phase NZP system with different crystallographic environments of the neptunium atoms. The thermal behaviour of the sample was followed up to 1400 °C by thermogravimetric analysis.

Na3.16(2)UV,VI0.84(2)O4 is obtained from the reaction of sodium with uranium dioxide under oxygen potential conditions typical of a sodium-cooled fast nuclear reactor. In the event of a breach of the steel cladding, it would be the dominant reaction product forming at the rim of the mixed (U,Pu)O2 fuel pellets. High-temperature X-ray diffraction measurements show that a distortion of the uranium environment in Na3.16(2)UV,VI0.84(2)O4 results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Nan-2Mn+On-1 - including Na3SbO4 and Na3TaO4, whose crystal structures are reported for the first time - has allowed us to assess the role played in the lattice expansion by the Mn+ cation radius and the Na/M ratio. On this basis, the thermomechanical behavior of the title compound is discussed, along with those of several related double oxides of sodium and actinide elements, surrogate elements, or fission products.

In this study we present a comprehensive thermodynamic description of the binary CsF-ThF4 system. The phase equilibria of several intermediate compositions in this system have been determined using the DSC technique combined with a post-analysis using powder X-ray diffraction. Considering all the novel experimental data, a thermodynamic model for the CsF-ThF4 system has been developed for the first time using the Calphad approach. The present model reproduces very well the measurements performed. Knudsen effusion mass spectrometry (KEMS) has further been used to investigate the vapour pressure over the molten CsF-ThF4 salt and to determine the thermodynamic activities of CsF and ThF4 in the liquid solution. As part of this study, the vaporization of pure CsF was examined and the results were compared with the literature showing a good agreement. Next, the vapour pressure of CsF-ThF4 in the liquid solution was investigated by measuring three samples with compositions X ThF4 = (0.4, 0.6, 0.8) mol/mol. A strong negative deviation from Raoult's law was observed for both species, more evident in case of CsF, and a good agreement with the predictions of our thermodynamic model was found. This article is a prerequisite for the assessment of the ternary LiF-CsF-ThF4 system. (C) 2017 Elsevier Ltd.

Cubic fluorite-type phases have been reported in the U(IV)O2-Bi2O3 system for the entire compositional range, but an unusual non-linear variation of the lattice parameter with uranium substitution has been observed. In the current extensive investigation of the uranium(iv) oxide-bismuth(iii) oxide system, this behaviour of the lattice parameter evolution with composition has been confirmed and its origin identified. Even under inert atmosphere at 800 °C, U(IV) oxidises to U(V)/U(VI) as a function of the substitution degree. Thus, using a combination of three methods (XRD, XANES and Raman) we have identified the formation of the BiU(V)O4 and Bi2U(VI)O6 compounds, within this series. Moreover, we present here the Rietveld refinement of BiU(V)O4 at room temperature and we report the thermal expansion of both BiU(V)O4 and Bi2U(VI)O6 compounds.

The charge distributions in α-Na2UO4, Na3NpO4, α-Na2NpO4, Na4NpO5, Na5NpO6, Na2PuO3, Na4PuO5, and Na5PuO6 are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L3, Np-L3, and Pu-L3 edges. In addition, a Rietveld refinement of monoclinic Na2PuO3, in space group C2/c, is reported for the first time, and the existence of the isostructural Na2NpO3 phase is revealed. In contrast to measurements in solution, the number of published XANES data for neptunium and plutonium solid phases with a valence state higher than IV is very limited. The present results cover a wide range of oxidation states, namely, IV to VII, and can serve as reference for future investigations. The sodium actinide series show a variety of local coordination geometries, and correlations between the shape of the XANES spectra and the local structural environments are discussed herein.

In order to fulfil the requirements as an actinide waste form, cheralite (CaTh(PO4)2) needs to be stabilised into a dense material. Thermal stability studies indicated that heat treatment causes gradual decomposition of CaTh(PO4)2 into the corresponding oxides above 1550K. Thus, conventional sintering of this material cannot be applied. However, we demonstrate that rapid densification without decomposition can be achieved by using spark plasma sintering. High density pellets of pure cheralite were produced and the thermal diffusivity and mechanical properties (hardness and elastic modulus) were measured. Within this study, 31P MAS NMR was used to complete the structural characterisation of the synthetic cheralite.

The physical and chemical properties at low temperatures of hexavalent disodium neptunate alpha-Na2NpO4 are investigated for the first time in this work using Mossbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mossbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of lambda-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm(-1) above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of alpha-Na2NpO4.

Investigations on the phase relation in the Pb-U-O ternary system have shown the formation of three lead uranium(VI) oxides: PbUO4, Pb3UO6, and Pb3U11O36. We present here the synthesis, thermal expansion, low- and high-temperature heat capacity, and thermal diffusivity of two representative phases (PbUO4 and Pb3UO6 uranates). Combining these results, a value for their thermal conductivity was derived and reported here for the first time.