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Distribution of Tritium in the Near Surface of Type 316 Stainless Steel

M. Sharpe, C. Fagan, W. T. Shmayda

Fusion Science and Technology / Volume 75 / Number 8 / November 2019 / Pages 1053-1057

Technical Paper / dx.doi.org/10.1080/15361055.2019.1644136

Received:August 13, 2018
Accepted:July 11, 2019
Published:November 11, 2019

The distribution of tritium in the near surface of Type 316 stainless steel has been measured using a combination of a zinc chloride (ZnCl2) wash and acid etching with diluted aqua regia. This method improves upon etching measurements reported in the literature: Results show depth resolutions of ~10 nm using the diluted aqua regia. The ZnCl2 wash results show very high surface concentration (~1.5 × 1013 Bq/cm3), which decreases by a factor of 106 after etching to a depth of ~10 μm. Further, the tritium concentrations in the near surface (<10 μm) of unmodified stainless steel samples do not change significantly over the course of 233 days, which indicates a quasi-equilibrium state has been reached. Tritium migration to the surface from the subsurface region was measured by etching a sample and then storing it in air for 2 to 4 days. After storing in air, the surface concentrations increased a thousandfold and rapidly decreased to base levels after etching an additional ~2 μm. These measurements indicate that perturbing the quasi-equilibrium concentration profile results in tritium migration to the surface in order to reestablish the prior equilibrium state.