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Neutron diffraction measurements of the double molybdate Cs3Na(MoO4)2 have been performed for the first time in this work and the crystal structure refined using the Rietveld method. The thermal expansion of this trigonal phase, in space group P3¯m1, measured using high temperature X-ray diffraction (XRD), remains moderate: αa=31·10−6K−1 and αc=24·10−6K−1 in the temperature range T = (298−723) K. The melting temperature of this compound has been determined at Tfus= (777 ± 5) K using Differential Scanning Calorimetry (DSC). No phase transition was detected, neither by DSC, nor by high temperature XRD or high temperature Raman spectroscopy, which disagrees with the literature data of Zolotova et al. (2016), who reported a reversible phase transition around 663 K. Finally, thermodynamic equilibrium calculations have been performed to assess the probability of formation of Cs3Na(MoO4)2 inside the fuel pin of a Sodium cooled Fast Reactor by reaction between the cesium molybdate phase Cs2MoO4, which forms at the pellet rim at high burnup, the fission product molybdenum (either as metallic or oxide phase), and the liquid sodium coolant in the accidental event of a breach of the stainless steel cladding and sodium ingress in the failed pin., Accepted Author Manuscript, RST/Reactor Physics and Nuclear Materials, RST/Neutron and Positron Methods in Materials, RST/Technici Pool

In the event of a clad breach in Sodium-cooled Fast Reactors (SFRs), the metallic sodium coolant will enter the pin and react with the (U, Pu, Np)O2 fuel. The reaction products are numerous, but there is still little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent Na3U1-xPuxO4 is expected to form, but its structure was the subject of controversy until now. This compound was shown in the 1980s to be of lower density and lower thermal conductivity than the fuel, leading to local swelling and creation of hot spots. Such a situation can induce further cladding failure and result in a contamination of the primary coolant with highly radioactive fission products. In this work, we have revisited the crystal structure of the Na3MO4 (M=U,Np,Pu) reaction product. Surprisingly, the structure of the sodium uranate differs from the one observed with neptunium and plutonium. In addition, this phase can accommodate excess sodium on the uranium site, with subsequent charge compensation from U(V) to U(VI), which was not previously foreseen. By contrast, the sodium neptunate and plutonate remain pentavalent. Temperature and oxygen potential are the two fundamental parameters that control the chemistry of the interaction. Assessing the margin to the safe operation of SFRs requires a thorough knowledge of the actinide cation valence state in the reaction products, and a complete thermodynamic description of the system. The valence state was determined in the Na-M-O (M=U,Np,Pu) ternary phases using XANES and Mössbauer spectroscopy, covering a wide range of oxidation states, namely (IV) to (VII). Coupling experimental thermodynamic investigations with thermodynamic modelling assessments using the CALPHAD method, we have calculated the phase equilibria in the Na-U-O system, and derived the oxygen potential threshold required within the fuel (and sodium coolant) to form the sodium uranate ternary phases.