Title: Interaction of advanced oxide fuels with sodium for Generation IV Sodium cooled Fast Reactors
Citation: Proceedings of the conference Plutonium Futures 2012
Publisher: Royal Society of Chemistry
Publication Year: 2012
JRC N°: JRC68921
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC68921
Type: Articles in periodicals and books
Abstract: The present study focuses on safety aspects of Sodium cooled Fast Reactors (SFR) and the potential interaction of advanced-oxide fuels with the metallic coolant in case of a clad breach. Transuranium elements (Np, Pu, Am, Cm) are generated during the irradiation process in a reactor and constitute a major issue for the nuclear industry because of their toxicity and very long half life (2.14*106, 2.41*104 and 432.2 years for 237Np, 239Pu and 241Am respectively). One main objective of the international GEN-IV program is to operate in a so called closed fuel recycling cycle by incorporating the separated minor actinides (Np, Am, Cm) into the fresh Mixed OXide fuel (MOX) (U, Pu)O2 and reusing them for energy generation. Such incorporation will however introduce a much more complex chemistry for which many data are still missing. Studies carried out in the past have revealed the formation close to the pellet rim (~923 K) of sodium urano-plutonate of formula Na3MO4 (M=U, Pu, U1-xPux) upon the reaction between sodium and (U,Pu)O2 fuels1. This compound was shown to be of lower density and thermal conductivity than the fuel leading to local swelling and temperature increase, potentially enlarging the breach up to pin failure. Moreover the authors measured the thermodynamic potential or partial molar Gibbs energies of oxygen (∆GO2eq) of some urania and urania-plutonia solid solutions. They showed that the metallic coolant coming into contact with the urania-plutonia solid solution could lead to an oxygen concentration increase in the liquid sodium in conjunction with the reduction to a lower valency of the plutonium in the oxide phase.. Other authors, Pillon2 and Kleykamp3 have contributed in the 1990’s to the investigation of the Na-U-O, Na-Pu-O and Na-UxPu1-x-O ternary and quaternary systems. The purpose of the present study is hence to assess the aftermaths on safety when minor-actinides-bearing-fuels come into contact with the metallic coolant. The work reported focuses on the chemical reactions between neptunium dioxide and sodium oxide in the presence of oxygen gas, leading to compounds with hexavalent and heptavalent neptunium. Solid state synthesis with different NpO2/Na2O ratios led to the following polycrystalline compounds: Na2Np2O7 monoclinic (P1211), α-Na2NpO4 orthorhombic (Pbam), β-Na4NpO5 tetragonal (I4/mmm) and Na5NpO6 monoclinic (C2/m)4. The crystal structures of the α-Na2NpO4 and Na2Np2O7 compounds were refined by Rietveld analysis. Evolution of the cell parameters of α-Na2NpO4 was also followed as a function of temperature up to 1273 K by X-ray diffraction. The corresponding linear thermal expansion coefficients along the different axis were determined: αa = 41.3 10-6 K-1, αb = 35.0 10-6 K-1, αc ~ 0 K-1. The high temperature X-ray diffraction experiment also revealed the formation of a high temperature γ phase tetragonal of Na2NpO4 between 663 and 763 K and the transition to a β orthorhombic (Pbca) phase upon cooling to room temperature4. Finally, thermodynamic measurements in a Knudsen effusion cell have given preliminary results on the vaporization behaviour of α-Na2NpO4 when heated up to 2500 K under vacuum conditions. The latter experiment also stands as a promising method for the determination of the Gibbs energy of formation and enthalpy of formation of Na2NpO4 at 298.15 K. 1. M.A. Mignanelli and P. E. Potter (1985) J. Nucl. Mat., 130, 289-297 2. S. Pillon, Study of the phase diagrams U-O-Na, Pu-O-Na, U,Pu-O-Na, Thesis, 1989 3. H. Kleykamp (1990), Kfk, Assessment of the Physico-Chemical Properties of Phases in the Na-U-P-O, 31 4. A.L. Smith, P.E. Raison, R.J.M. Konings (2011) J. Nucl. Mat., 413, 114-121
JRC Directorate:Nuclear Safety and Security

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