Tritium permeation from tritiated water to water through Inconel

From a safety standpoint, it is important to understand the behavior of tritium in the coolant of DT fusion reactors. The high-temperature and high-pressure water is expected to be used as a coolant in JA DEMO. Therefore, it is necessary to deepen the understanding of the tritium transfer from the primary cooling water to the secondary cooling water in a heat exchanger. In this study, a double-tube permeation device consisting of Inconel 600 tube, which is a heat exchanger material, and SS316 tube, was assembled and the tritium permeation from tritiated water to water at 300 °C and 17 MPa was observed. Remarkable tritium permeation was observed after around 17 days as the cumulative heating time. Gaseous tritium (HT and T₂) was detected in the gas phase of the tritiated water side after the permeation experiments. This suggests that a part of tritium generated in the metal oxidation reaction dissolved in the Inconel and diffused and permeated. With assuming that the diffusion process of tritium through the Inconel was the rate controlling step of permeation, the permeability of tritium through the Inconel tube from the tritiated water derived from the experimental data was 3 orders of magnitude smaller than that obtained from the hydrogen isotope permeation experiments from gas phase to gas phase.