From a safety viewpoint, it is important to understand the behavior of tritium in the coolant of deuterium-tritium 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 tritium permeation behavior from the high-temperature and high-pressure water, which is assumed in a heat exchanger. In this study, hydrogen isotopes permeation experiments from high-temperature and high-pressure water were conducted with double-tube permeation devices consisting of Inconel 600 tube, which is a heat exchanger material, and SS316 tube.
First, in tritium permeation experiment, tritium permeation from tritiated water to water at 300 °C and 17 MPa was measured. Remarkable tritium permeation was observed after approximately 17 days as the cumulative heating time. Gaseous tritium was detected in the gas phase of the tritiated water side after the permeation experiments. This suggests that a part of tritium was generated through oxidation-reduction reaction on the metal surface, dissolved in the Inconel and permeated. With assuming that the diffusion process was the rate-controlling step in the permeation and primary partial pressure was equal to the saturated vapor pressure of tritiated water, tritium permeation fluxes through the Inconel tube from the tritiated water derived from the experimental data was 5 orders of magnitude smaller than that obtained from the hydrogen isotope gas permeation experiments.
Second, in hydrogen permeation experiment, hydrogen permeation from water to gas phase at 300 °C and 17 MPa was measured. With assuming that the diffusion process of hydrogen through the Inconel was the rate-controlling step in permeation and primary partial pressure was equal to the saturated vapor pressure, hydrogen permeation fluxes through the Inconel tube from water derived from the experimental data was about 3 orders of magnitude smaller than that obtained from the hydrogen gas permeation experiments. This implied that the hydrogen permeation rate from high-temperature and high-pressure water approximated by setting the partial pressure to the saturated vapor pressure corresponding to the temperature of the primary side of permeation multiplied by 10^-6 and using the equation for permeation between gas phases.
Finally, two experiments were compared to investigate the relationship between concentration of permeable material in liquid phase and permeation flux. As the result, it was suggested hydrogen isotope permeation between liquid phases on may be proportional to the 0.5th ~ 1st power of the concentration in water. Using this result, tritium permeation rate from the primary cooling water to the secondary cooling water in JA DEMO was estimated to be 1.91×10^(-4) ~ 6.50×10^(-2) g-T_2/day. This result implied that the tritium concentration in the secondary cooling water exceeds the wastewater regulation value as soon as the reactor starts operation. Therefore, tritium concentration monitoring and water detritiation system (WDS) are required for the secondary cooling water as well as the primary cooling water.

[1] J. Yagi, J. Plasma Fusion Res., Vol.93, No.4 (2017), 196-199, in Japanese.
[2] S. Konishi and M. Enoeda, "Intelligible Seminar on Fusion Reactors – (6) Blanket that converts energy, and produces fuels", Journal of the Atomic Energy Society of Japan, Vol.47, No.7, (2005), 488-494, in Japanese.
[3] Y. Ueda, et al., " Intelligible Seminar on Fusion Reactors – (1) Introduction to Fusion Reactors", Journal of the Atomic Energy Society of Japan, Vol.46, No.12, (2004), 845-852, in Japanese.
[4] S. Uchida, "Water Chemistry of Light Water Cooled Reactor Plants (1)", Journal of the Atomic Energy Society of Japan, Vol.51, No.2, (2009), 106-111, in Japanese
[5] K. Tobita, et al., Fusion Eng. Des., 136 (2018), 1024-1031.
[6] Y. Someya, et al., Fusion Eng. Des., 146 (2019), 894-897.
[7] H. Nakamura and M. Nishi, J. Nucl. Mater., 329–333 (2004), 183-187.
[8] T. Hayashi, et al., Fusion Eng. Des., 87 (2012), 1333-1337.
[9] T. Hayashi, et al., Fusion Eng. Des., 89 (2014), 1520-1523.
[10] T. Chikada, "Research frontier of tritium for fusion reactor - toward the DEMO reactor (6); Tritium permeation study and technology development for mitigation of tritium permeation", Journal of the Atomic Energy Society of Japan, Vol.60, No.12, (2018), 759-763, in Japanese.
[11] S. Tosti, et al., Fusion Eng. Des., 43 (1998), 29-35.
[12] A. Suzuki, Establishment of new paradigm for hydrogen permeable metal membrane and its application to optimal design of alloy membrane for low operative temperature, Nagoya, Nagoya University, 2016, Ph. D. thesis, in Japanese.
[13] D. Henzan, Study on Tritium Confinement with Zr and Al2O3 for Tritium Production System for Fusion Reactors with a High-Temperature Gas Cooled Reactor, Fukuoka, Kyushu University, 2020, Master's thesis.
[14] S. Fukada, Suisokyuzogokin niyoru Suisodouitai no Bunrigijyutsu : Jisedai Enerugi heno Nenryou (Hydrogen Isotope Separation Technology Using Hydrogen Storage Alloys : Fuel for Next Generation Energy), NTS Inc., 2000, 139, in Japanese.
[15] R. G. Hickman, J. Less-Comm. Met.,19 (1969), 369-383.
[16] T. Shiraishi, et al., J. Nucl. Mater., Vol. 254, Issues 2-3 (1998), 205-214.
[17] J. T. Bell, et al., Fusion and Isotopic Applications,1980a (1978), 48-53.
[18] E. M. Field and D. R. Holmes, Corrosion Sci., 5 (1965), 361-370.
[19] T. Tanabe, et al., J. Nucl. Mater., Vol. 122-123, Issues 1-3 (1984), 1568-1572.
[20] T. Takeda and J. Iwatsuki, Nucl. Technol., Vol. 146, Issue 1 (2004), 83-95.
[21] N.S. Mclntyre, et al., J. Electrochem. Soc., 126 (1979), 750–760.
[22] J.B. Ferguson and Hugo F. Lopez., Metall. Mater. Trans. A, 37A (2006), 2471–2479.
[23] D.H. Lister, et al., Corros. Sci, 27 (1987), 113–140.
[24] H. D. Rohring, et al., Nucl. Eng. Des., Vol. 34, Issue 1 (1975), 157-167.
[25] R. Hiwatari, et al., Fusion Eng. Des., 143 (2019), 259-266.
[26] Y. Miyoshi, et al., Fusion Eng. Des., 136 (2018) 1577-1580.