Influence of sodium on the mechanical properties of fuel element cladding

The creep and corrosion behaviour of cladding tubes of austenitic stainless steels has been tested in a test series up to 10,000 hours. The test temperature has been 873 resp. 9730K, the flow velocity 5 m/s. The purity of the sodium has been varied within the range 3.5 to 9 w-ppM oxygen. The weight losses depend linearly on the oxygen content of the sodium, and the temperature dependence follows an ARRHENIUS law. Metallurgical and analytical examinations show that more than a third of the wall thickness suffers changes of composition and microstructure at 9730K. The observed phenomena are ferritic surface layers, grain boundary ferritization, carburization, increases of hardness, and finally weak carburization. In spite of the changes due to the sodium corrosion the largest part of the specimens does not show losses of creep strength. However, cold worked specimens of 1616CrNiMoNb creep with accelerated rates. The examinations lead to the conclusions that changes in microstructure may have caused the loss of creep strength of this material