A review of the thermochemical and thermo-physical properties of phases that can form following the chemical interaction between sodium coolant and irradiated (U,Pu)O2 fuel in a Sodium-cooled Fast Reactor is reported in the present chapter. This critical assessment is directed towards the assessment of a particular type of accidental scenario, namely that of a cladding failure and subsequent sodium ingress into the irradiated fuel pin, and focusses on the aspects of the (U,Pu,Np,Am)O2 fuel-sodium and fission products-sodium interactions.
The structural and thermodynamic properties of the sodium-oxygen system are first addressed. The latter system is particularly relevant since the levels of oxygen impurities in the liquid sodium coolant are directly related to the chemistry of the interaction. The Na-O system is still poorly known, especially above 50% at O. Discrepancies remain on the thermodynamic functions of sodium oxide Na2O, the compound forming when the oxygen solubility limit in liquid sodium is reached.
Then the sodium actinide ternary oxides are discussed as well as the quaternary phases with uranium and plutonium, including structural and thermodynamic properties, thermal expansion data and valence state determinations using X-ray Absorption Near Edge Structure (XANES) spectroscopy and Mössbauer spectroscopy. Past studies in the 1980s have shown that the main product of reaction between liquid sodium and urania-plutonia solid solution is Na3U1-yPuyO4, a compound of lower density and thermal conductivity relative to the mixed oxide fuel. But in light of recent findings on the Na3UO4 and Na3PuO4 end-member phases, it is clear that the exact structure and the valence state of the actinide elements in the sodium urano-plutonate still need further investigations. A thermodynamic model has only been reported for the Na-U-O system. The available thermodynamic data on Na-Np-O, Na-Pu-O, Na-Am-O and Na-U-Pu-O are to this date either very scarce or inexistent.
The potential products of reaction between liquid sodium and key fission product phases are furthermore treated. This review considers the fission product elements expected at high burn-up in the JOG (Joint Oxide Gaine) layer between fuel and cladding, i.e. phases of the Na-(Cs,Te,I-Mo)-O system. In addition, the properties of the Na-(Ba,Mo,U)-O system are described since the formation of the gray phase, a perovskite of general formula Ba(Zr,U,Pu,Mo,RE)O3 (RE=rare earths) is expected in the fuel pin at low oxygen potentials, while that of the scheelite Ba(Mo,U)O4 is expected at high oxygen potentials. Lastly, the data available on Na-(Sr,U,Mo)-O are discussed as strontium is for the main part soluble in the fuel matrix, while Sr(Mo,U)O4 could form at high oxygen potentials.
Finally, thermodynamic equilibrium calculations are described that allow to assess the likelihood of formation of a given product. Temperature, oxygen potential and burn-up are the three key parameters that determine the chemistry of the irradiated fuel-sodium interaction. The oxygen threshold required for the formation of sodium actinide oxides depends in particular on the valence state of the actinide elements, while the burn-up level dictates the amount, form and valence state of the fission products, which again has a direct influence on the chemical interaction with sodium. Under reactor conditions, the formation of α-Na3UO4, α-Na3.16U0.84O4, Na4UO5, Na2MoO4, Cs3Na(MoO4)2 and NaBa2MoO5.5 is to be expected. These all have a higher thermal expansion than the fuel, hence fuel swelling could occur upon formation. The oxygen potential threshold of formation of the sodium urano-plutonate was reported to be very similar to that of the sodium uranate. However, considering the uncertainties on the crystal structure of Na3(U,Pu)O4 solid solution as a function of plutonium content and temperature, and the lack of thermodynamic data, it is clear that complementary investigations would be very valuable.
