High-temperature behavior of dicesium molybdate Cs2MoO4: Implications for fast neutron reactors

Structural Studies and Thermal Analysis in the Cs2MoO4-PbMoO4 System with Elucidation of β-Cs2Pb(MoO4)2

The quaternary compound Cs₂Pb(MoO₄)₂ was synthesized and its structure was characterized using X-ray and neutron diffraction from 298 to 773 K, while thermal expansion was studied from 298 to 723 K. The crystal structure of the high-temperature phase β-Cs₂Pb(MoO₄)₂ was elucidated, and it was found to crystallize in the space group R3̅m (No. 166), i.e., with a palmierite structure. In addition, the oxidation state of Mo in the low-temperature phase α-Cs₂Pb(MoO₄)₂ was studied using X-ray absorption near-edge structure spectroscopy. Phase diagram equilibrium measurements in the Cs₂MoO₄-PbMoO₄ system were performed, revisiting a previously reported phase diagram. The equilibrium phase diagram proposed here includes a different composition of the intermediate compound in this system. The obtained data can serve as relevant information for thermodynamic modeling in view of the safety assessment of next-generation lead-cooled fast reactors.

Reactivity of FeMoO4 in CsCl Fluxes and Formation of the Salt-Inclusion Type of Compounds

The reactivity of FeMoO₄ in CsCl fluxes has been investigated by thermal analysis and chemical reactions in evacuated silica ampules. The products have been characterized by ex situ X-ray diffraction methods. Metathesis reactions involving CsCl lead to the formation of Cs₂Fe₂(MoO₄)₃ and the salt adduct Cs₂FeCl₄·CsCl. A side reaction has been observed, which is associated with a decomposition of [MoO₄]^2- in CsCl fluxes yielding Cs₂Mo₂O₇·CsCl, which contains the rare pyromolybdate anion, [Mo₂O₇]^2-, located in the center of a _∞²[CsCl] hetero-honeycomb arrangement. This salt-inclusion type of compound has been studied further in terms of its formation starting from Cs₂MoO₄, MoO₃, and CsCl. The intermediate adduct phase, Cs₂MoO₄·MoO₃, contains uncharged _∞¹[MoO₂O_2/2] chains that react with CsCl at elevated temperatures to Cs₂Mo₂O₇·CsCl. Furthermore, the site preference for alkaline-metal cations (K⁺, Rb⁺, and Cs⁺) has been evaluated for a mixed substitution series. In accordance with the Pearson concept, the polarizability of the respect cation outweighs any size differences for the occupancy of the salt-intergrowth motif, the honeycomb part of the structure.

Radiochemical analysis of the drain water sampled at the exhaust stack shared by Units 1 and 2 of the Fukushima Daiichi Nuclear Power Station

Radioactive gas of Unit 1 of the Fukushima Daiichi Nuclear Power Station was released from the exhaust stack shared by Units 1 and 2 through the venting line on March 12th, 2011. In the present study, radiochemical analysis of drain water sampled at the drain pit of the exhaust stack was conducted to study radionuclides released during venting of the Unit 1. Not only volatile ¹²⁹I, ¹³⁴Cs and ¹³⁷Cs but also ⁶⁰Co, ⁹⁰Sr, ¹²⁵Sb and Unit 1-originated stable Mo isotopes were detected. Although Unit 1-originated stable Mo isotopes were clearly detected, their amounts were quite low compared to Cs, suggesting that the formation of Cs₂MoO₄ was suppressed under the accident condition. Approximately 90% of iodine existed as I⁻ and 10% as IO₃⁻ in November 2020. Furthermore, larger amount of ¹²⁹I than ¹³⁷Cs was observed, suggesting major chemical form of ¹³¹I was molecular iodine rather than CsI at the accident time. The ¹³⁴Cs/¹³⁷Cs radioactivity ratio decay-corrected to March 11th, 2011 was 0.86, supported the results that Unit 1 originated radiocesium in environment has smaller ¹³⁴Cs/¹³⁷Cs radioactivity ratio than Unit 2 and 3 originated radiocesium.

Report of the Double-Molybdate Phase Cs2Ba(MoO4)2 with a Palmierite Structure and Its Thermodynamic Characterization

The existence of a novel double-molybdate phase with a palmierite-type structure, Cs₂Ba(MoO₄)₂, is revealed in this work, and its structural properties at room temperature have been characterized in detail using X-ray and neutron diffraction measurements. In addition, its thermal stability and thermal expansion are investigated in the temperature range 298–673 K using high-temperature X-ray diffraction, leading to the volumetric thermal expansion coefficient α_V ≈ 43.0 × 10^–6 K^–1. The compound’s standard enthalpy of formation at 298.15 K has been obtained using solution calorimetry, which yielded Δ_fH_m°(Cs₂Ba(MoO₄)₂, cr, 298.15 K) = −3066.6 ± 3.1 kJ· mol^–1, and its standard entropy at 298.15 K has been derived from low-temperature (2.1–294.3 K) thermal-relaxation calorimetry as S_m°(Cs₂Ba(MoO₄)₂, cr, 298.15 K) = 381.2 ± 11.8 J K^–1 mol^–1.

The existence of Cs₂Ba(MoO₄)₂ with a palmierite structure is reported for the first time. The structure has been characterized by X-ray and neutron diffraction, and the thermal expansion has been measured by high-temperature XRD. Moreover, key thermodynamic properties (standard enthalpy of formation and standard entropy) have been derived from solution calorimetry and low-temperature thermal relaxation calorimetry.

Investigation of the Cs2(Mo,Te)O4 Solid Solution and Implications on the Joint Oxyde-Gaine System in Fast Neutron Reactors

The formation of a thin layer, the so-called Joint Oxyde-Gaine (JOG), between the (U,Pu)O₂ fuel pellets and the cladding has been observed in fast neutron reactors, due to the accumulation of volatile fission products. Cs₂MoO₄ is known to be one of the major components of the JOG, but other elements are also present, in particular tellurium and palladium. In this work, an investigation of the structural and thermodynamic properties of Cs₂TeO₄ and Cs₂Mo_1–xTe_xO₄ solid solution is reported. The existence of a complete solubility between Cs₂MoO₄ and Cs₂TeO₄ is demonstrated, combining X-ray diffraction (XRD), neutron diffraction (ND), and X-ray absorption spectroscopy (XAS) results. High-temperature XRD measurements were moreover performed on Cs₂TeO₄, which revealed the existence of a α–β phase transition around 712 K. Thermal expansion coefficients were also obtained from these data. Finally, phase equilibra points in the Cs₂MoO₄–Cs₂TeO₄ pseudobinary phase diagram were collected using differential scanning calorimetry and used to develop a thermodynamic model for this system using a regular solution formalism.

The existence of a complete solid solution between Cs₂MoO₄ and Cs₂TeO₄ is demonstrated combining XRD, ND, and EXAFS data. The hexavalent states of Mo and Te in the Cs₂Mo_1−xTe_xO₄ solid solution have been confirmed by XANES. The existence of a phase transition at (712 ± 5 K) in Cs₂TeO₄ (from an orthorhomic to an hexagonal structure) is revealed. Phase equilibria measurements in the Cs₂MoO₄−Cs₂TeO₄ phase diagram are reported, and a regular solution model has been developed.

Structural and Thermodynamic Investigation of the Perovskite Ba2NaMoO5.5

Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba₂NaMoO_5.5 have been performed in this work. The cubic crystal structure in space group Fm3̅m has been refined using the Rietveld method. X-ray absorption near-edge structure spectroscopy (XANES) measurements at the Mo K-edge have confirmed the hexavalent state of molybdenum. The local structure of the molybdenum octahedra has been studied in detail using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mo–O and Mo–Ba distances have been compared to the neutron diffraction data with good agreement. The coefficient of thermal expansion measured in the temperature range of 303–923 K, using high temperature X-ray diffraction (HT-XRD) (α_V = 55.8 × 10^–6 K), has been determined to be ∼2 times higher than that of the barium molybdates BaMoO₃ and BaMoO₄. Moreover, no phase transition nor melting have been observed, neither by HT-XRD nor Raman spectroscopy nor differential scanning calorimetry, up to 1473 K. Furthermore, the standard enthalpy of formation (Δ_fH_m°) for Ba₂NaMoO_5.5(cr) has been determined to be −(2524.75 ± 4.15) kJ mol⁻¹ at 298.15 K, using solution calorimetry. Finally, the margin for safe operation of sodium-cooled fast reactors (SFRs) has been assessed by calculating the threshold oxygen potential needed, in liquid sodium, to form the quaternary compound, following an interaction between irradiated mixed oxide (U,Pu)O₂ fuel and sodium coolant.

The structure of the cubic perovskite Ba₂NaMoO_5.5 has been refined from neutron diffraction data and EXAFS measurements. The thermal expansion of Ba₂NaMoO_5.5 has been measured using high-temperature XRD. The stability of the compound has been investigated by DSC, XRD, and Raman data, up to 1473 K. The enthalpy of formation has been derived from solution calorimetry measurements as −(2524.75 ± 4.15) kJ mol⁻¹. The oxygen potential threshold to form Ba₂NaMoO_5.5 in liquid sodium has been evaluated.

Joint Raman spectroscopic and quantum chemical analysis of the vibrational features of Cs2RuO4

The Raman spectroscopic characterization of the orthorhombic phase of Cs2RuO4 was carried out by means of group theory and quantum chemical analysis. Multiple models based on ruthenate (VI+) tetrahedra were tested, and characterization of all the active Raman modes was achieved. A comparison of Raman spectra of Cs2RuO4, Cs2MoO4, and Cs2WO4 was also performed. Raman laser heating induced a phase transition from an ordered to a disordered structure. The temperature-phase transition was calculated from the anti-Stokes/Stokes ratio and compared with the ones measured at macroscopic scale. The phase transition is connected with tilting and/or rotations of RuO4 tetrahedra, which lead to a disorder at the RuO4 sites. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd.