Chemistry of the nuclear fuel-coolant interaction

One major question for the safety assessment of the SFR concerns the potential interaction of the sodium metallic coolant with the nuclear fuel in the event of a breach of the stainless steel cladding, although extremely rare under normal operating conditions.

Although various types of fuels, i.e. nitrides, carbides, and metals, are potential candidates, (U,Pu)O₂ mixed oxide (MOX) fuels are currently the preferred option for SFRs as substantial experience has already been gained in terms of fabrication, reactor operation, reprocessing, and risk assessment. The plutonium concentration in the (U,Pu)O₂ fast reactor fuel is typically of the order of 20wt%.

The prediction of the phases that can form following the fuel-coolant chemical interaction as a function of temperature, oxygen potential and burn up is a crucial issue. The potential products in the Na-(U,Pu,FP)-O system (FP=fission products) are numerous. Most of them show a lower density and thermal conductivity than the fuel, leading to swelling and temperature increase in the fuel pin. Such a situation can induce further cladding failure, restrain the flow of coolant within a sub-assembly of fuel pins, or result in a contamination of the primary coolant with plutonium, minor actinides, or highly radioactive fission products. The study of these systems has attracted considerable interest since the 1950s and 1960s because of their technological importance. However, many discrepancies and uncertainties remained because of the difficulties to work with radioactive materials and the complexity of the involved chemistry. At TU Delft we investigate in the radioactive laboratories of the Reactor Institute Delft the structural and thermodynamic properties of such phases using a multidisciplinary approach combining advanced techniques such as X-ray and neutron diffraction, X-ray Absorption Spectroscopy (XAS), Differential Scanning Calorimetry (DSC) and solution calorimetry. Moreover, we develop thermodynamics models for these systems (for instance Na-U-O) using the CALPHAD method, which can feed computer simulation codes that model accidental events from clad breach to potential release of radioactive elements into the environment.

(a) Sketch of the structure of Na₃UO₄
(b) Na-U-O ternary phase diagram at T=900 K modelled using the CALPHAD method

References

A.L. Smith, C. Guéneau, J.-L. Flèche, S. Chatain, O. Beneš, R.J.M. Konings, Thermodynamic assessment of the Na-O and Na-U-O systems : Margin to the safe operation of SFRs, Journal of Chemical Thermodynamics, 114 (2017) 93-115, doi.org/10.1016/j.jct.2017.04.003

A.L. Smith, P. Martin, D. Prieur, A.C. Scheinost, P.E. Raison, A.K. Cheetham, R.J.M. Konings, Structural Properties and Charge Distribution of the Sodium Uranium, Neptunium, and Plutonium Ternary Oxides: A combined X-ray Diffraction and XANES Study, Inorganic Chemistry, 55 (2016) 1569-1579, doi.org/10.1021/acs.inorgchem.5b02476