The very-high-temperature reactor (VHTR) has been included in the conception of the Generation IV nuclear energy system proposed by the United States Department of Energy Nuclear Energy Research Advisory Committee and the Generation IV International Forum. After the Fukushima accident, concern has grown regarding the safety of nuclear power plants. For its inherent safety, superior proliferation resistance, and high thermal efficiency, the VHTR and its prototype, the high-temperature gas-cooled reactor (HTGR), attracted worldwide interest. Tritium exists in the fuel element and graphite reflectors in the core, primary coolant, some liquid and solid wastes, and gaseous effluent of HTGRs. Due to its permeation behavior at high temperatures, tritium is the most important nuclide in the secondary loop of HTGRs. Considering the future application of HTGRs in hydrogen production, tritium will be non-negligible and could be difficult to separate from hydrogen. In addition, the interaction between tritium and graphite is expected to be very strong. The absorption, desorption, and diffusion of tritium at the interface between the primary coolant and graphite are of great interest. Experimental measurements of the diffusion coefficients of tritium and its isotopes do not agree well with theoretical predictions based on the consideration of isotope effects. In this paper, research progress on the permeation, absorption, desorption, and diffusion behaviors of tritium in HTGRs is summarized and discussed. The chemical form of tritium in the primary coolant, and its spectroscopic characteristics, internal structures, and transport behaviors are organized and analyzed. Basic properties related to tritium, such as isotope effect, nuclear spin, radioactivity, etc. are also discussed.

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