Several fusion plants plan to utilize two high-temperature and high-pressurized water coolant systems. Because of the high hydrogen-isotope mobility in high-temperature metal, tritium will inevitably transfer from the plasma side to the secondary coolant through the primary coolant. From the viewpoints of fuel control, tritium safety, and social acceptance, it is compulsory to investigate the tritium concentration dependence of permeation phenomena experimentally. Therefore, this study conducted a protium permeation experiment instead of tritium, which mocked the situation where the tritium concentration in the primary loop was extremely high. Considering the results in the previous tritium permeation research by the present authors, the tritium permeation behavior was likely proportional to the first power of the tritium concentration. Then, based on these experiments and references regarding the tritium permeation rate and water detritiation system (WDS) design, tritium concentration was computed in both loops. In this calculation condition, the primary and secondary loops reached about 0.4 TBq/kg and 167 MBq/kg during 3-year operations, respectively. Also, it was found that the required feed rate to keep the tritium concentration at 1 TBq/kg was 46.5 kg/h, which is less than the existing WDS specification.