Thermal expansion of the nuclear fuel-sodium reaction product Na3(U0.84(2),Na0.16(2))O4 - Structural mechanism and comparison with related sodium-metal ternary oxides

Abstract Na 3.16(2) U V,VI 0.84(2) O 4 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)O 2 fuel pellets. High-temperature X-ray diffraction measurements show that a distortion of the uranium environment in Na 3.16(2) U V,VI 0.84(2) O 4 results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Na n -2 M n + O n -1 - including Na 3 SbO 4 and Na 3 TaO 4 , whose crystal structures are reported for the first time - has allowed us to assess the role played in the lattice expansion by the M n + 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.