Generation and Distribution of Tritium in HTGRs and Review on the Tritiated Water Treatment Technologies

Tritium (H-3) can discharge mainly in the form of HT and/or HTO as gaseous and/or liquid waste into the environment from the nuclear power plant, and participate in the cycle among hydrosphere, atmosphere and biosphere, which would lead to the long-term radiological effects on organisms. Thus, in the daily operation of the nuclear power station, tritium became one of the most concerned nuclides in the source term analysis. In high temperature gas-cooled reactors (HTGRs), tritium was mainly generated by the ternary fission reaction of heavy nuclei in the fuel and neutron activation reaction of impurities like lithium-6 (Li-6), lithium-7 (Li-7) and boron-10 (B-10) in the graphite matrix, carbon bricks, etc. Tritium would be resorted completely in the intact tristructural-isotropic (TRISO) coated particles, while tritium in the graphite can diffuse into the primary loop depending on the local temperature. In the helium purification system of a typical HTGR, the molecular adsorber can adsorb the tritium in the primary coolant, and then the tritiated water was formed from the regeneration and desorption process of the molecular adsorber. Meanwhile, since the high permeability of tritium at a high temperature, it can permeate into the secondary loop through stainless steel heat exchange tube from the primary loop, and entered into the environment with leakage of the secondary water. Therefore, it was very important to analyze the production, transport and release mechanism of tritium for the estimation of the inventory and distribution of tritium in a nuclear power plant. With the rapid development of nuclear energy and the commercial application of HTGRs, tritiated water treatment technologies attracted more attention in the field of radioactive nuclear waste. Current paper will introduce and summarize general tritiated water treatment technologies, including water distillation, tritium sorbent process, palladium membrane reaction (PMR), and combined electrolysis catalytic exchange (CECE).