Study of the oxidation mechanisms of 316LN steel in liquid sodium - 15407

The sodium cooled fast reactor is selected in France as the 4. generation of nuclear power plant. Reactors vessel, primary loop structures and heat exchangers will be made of austenitic stainless steels: 316LN is the reference material. To assess reactor service life time, corrosion of austenitic stainless steel by liquid sodium is studied in normal operating conditions as well as in transient conditions either expected or not. Oxygen is one of the impurities present in liquid sodium at the level of a few parts per million in weight. It presents a major effect on corrosion phenomena of austenitic stainless steel, although not totally understood yet, in increasing the iron solubility as well as its dissolution rate. Oxygen is also known to form sodium chromite scale (NaCrO₂), those behavior is little documented in the literature. Based on corrosion tests performed in the static sodium test device ('CorroNa') at 550 and 650 Celsius degrees, sodium chromite scale (NaCrO₂) is observed in slightly oxidizing conditions. Below the oxide scale, a chromium depleted layer is characterized. Some aspects of the morphology of this oxide scale and its properties are presented. To predict long term corrosion effects on materials (40-60 years), the oxidation mechanisms are investigated in order to identify the limiting step of the kinetics, as well as to understand the basic phenomenon at play in this complex corrosion phenomenon. This paper proposes that sodium oxide diffuses through the oxide scale and that the oxide scale grows inwards thanks to test achieved with inert corrosion marker and isotopic tracers (¹⁶O-¹⁸O). Tests are characterized by Rutherford Backscattered Spectroscopy (RBS), Nuclear Reaction Analysis (NRA), Glow Discharge Optical Emission Spectroscopy (GD-OES), and Secondary Ion Mass Spectroscopy (SIMS). In addition, these results are used to assess the sodium diffusion coefficient in the austenitic stainless steel. Moreover, in knowing chromium diffusion coefficient in stainless steel, kinetics assessment of oxidation is then made possible. (authors)