Transport and Deposition of Metals in Sodium-Stainless Steel Systems, (VI)

The diffusion of radioactive manganese, zinc and cesium into stainless steel tubes exposed during 7001, 500 hr to liquid sodium containing the radioisotopes was studied by measuring the residual radioactivity on samples with their surface layers successively removed by electropolishing. Plots of the logarithm of the residual radioactivity in the tube wall drawn against the square of the thickness polished off from the surface were found to fall along straight lines, which changed their slopes in three steps. That of outermost step was most steep, with the residual radioactivity indicating a drop of about one order of magnitude within a layer of a few microns from the surface. The ensuing step presented a much gentler slope, from which the diffusion coefficients were determined. The ratio obtained between the diffusion coefficient and the approximate jump frequency of the diffusing radioisotope was correlated with the reciprocal of absolute temperature, which indicated that the diffusion mechanism of radioactive manganese and zinc differs from that of radioactive cesium.