Structural and thermodynamic study of Cs₃Na(MoO₄)₂: Margin to the safe operation of sodium cooled fast reactors

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: α_a=31·10⁻⁶K⁻¹ and α_c=24·10⁻⁶K⁻¹ in the temperature range T = (298−723) K. The melting temperature of this compound has been determined at T_fus= (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.