Summary of Tritium Source Term Study in 10 MW High Temperature Gas-Cooled Test Reactor

Tritium (³H) has been increasingly researched when assessing the environmental impact of nuclear reactors and other nuclear facilities because it is widely present in nuclear systems and can easily enter the environment. The first pebble-bed gas-cooled test reactor in China, the 10 MW high temperature gas-cooled test reactor (HTR-10), uses helium, graphite, and graphite spheres containing embedded tristructural-isotropic–coated particles as primary coolant, reflectors, and fuel elements, respectively. Several experiments that involved the ³H source term in HTR-10 were performed, and they measured the ³H specific activity and its distribution in the irradiated graphite spheres from the core, ³H activity concentration in the primary helium, ³H activity concentration during the regeneration of the molecular sieve adsorber in the helium purification system, and ³H amount in the gaseous effluent discharge from the stack. The experimental data were summarized and compared with the theoretical predictions. The balance diagram of the ³H source term in HTR-10 is introduced in this paper. Sensitivity analysis was performed to illustrate the effect of the ³He abundance in the primary helium and Li content in the graphite reflectors on the ³H activity concentration in the primary coolant of HTR-10. The interactions between graphite and different hydrogen isotopes (¹H, ³H, ¹H₂, ¹H³H, and ³H₂) were investigated using first-principles calculations and the diffusion theory. The results indicated that molecular ³H tended to diffuse in graphite.