Characterization of Sodium Thermal Insulation Reaction Product and its Effect on Fatigue Crack Growth in 316 LN Austenitic Steel

Abstract Though sodium is the most preferred coolant for fast breeder reactors due to excellent heat transfer capabilities, its high chemical reactivity poses a concern during accidental leaks. Despite defense in depth in various levels for protecting the sodium leaks in the heat transport circuits, small undetected leaks may lead to localized degradation of structural material due to deposition of reaction products in the leak zone. Experiments have been conducted by exposing SS-316 LN specimens to sodium–glass wool interaction zone at 300 °C in air to study the influence of reaction products on chemical composition and crack growth properties. Post-exposure X-ray diffraction analysis of the samples revealed presence of silicates and aluminates of sodium in the outer layers, and complex ternary and quaternary oxides of sodium, iron, chromium and nickel in the inner layers. Scanning electron microscopy analysis revealed deposition of oxides on the exposed surface. Elemental analysis of the samples carried out by energy-dispersive spectroscopy indicated leaching of major alloying elements like chromium, nickel and iron to a depth of about ~ 25 µm from the exposed surface. Room-temperature fatigue crack growth studies on the exposed specimen shown considerable reduction in crack initiation (threshold) fracture toughness range Δ K th .