Influence of sodium on creep-rupture behaviour of type 304 stainless steel

The influence of flowing liquid sodium at 550deg C on the creep-rupture behaviour of the structural material of the SNR 300 reactor, X6CrNi18 11 (DIN 1.4948, equivalent to Type 304 SS) was studied in two non-isothermal sodium loops. It was shown that the effects of sodium are dependent on the carbon activity of the sodium. Under normal (non-decarburizing) sodium conditions a limited reduction of times-to-rupture occurs. This reduction is due to a reduced ductility within the tertiary creep range. The minimum creep rate and the onset of tertiary creep are not influenced. Under decarburizing conditions, which are not expected to occur in the sodium of LMFBRS, an additional loss of creep strength was observed. The steel showed higher creep rates and an earlier onset of tertiary creep. This additional effect seems to be caused by sodium corrosion of surface-near layers reducing the unaffected cross section. It depends on the surface-to-volume ratio, and it was nearly suppressed, when thicker specimens (6 mm diameter) were used. (orig.)