Studies on helium-3 separation at Cernavoda nuclear power plant

•The quantity of helium-3 produced so far in the moderator circuit from Cernavoda NPP was assessed.•Multiple methods of separation of 3He from 4He were investigated.•Cryogenic distillation is the most suitable method for the final stage of separation of 3He from 4He.•The parameters of the cryogenic distillation column were determined. Helium-3 is a rare isotope of helium (1.37 ppm as fraction of total helium – natural abundance), with applications in medicine, industry, security, and science. The world is experiencing nowadays a shortage of helium-3, related to its high request and to the scarce availability of tritium, which is the most common source of helium-3. Tritium is an unstable isotope of hydrogen because it undergoes a beta-decay with a half-life of 12.3 years, and is a by-product in CANDU type nuclear reactors. Tritium is found mostly in the heavy water from the moderator circuit. It is produced by the thermal neutron activation of deuterium atoms from the heavy water molecules. It can be also found in the cover gas used to control the pressure in the calandria and to provide a non-corrosive, non-radioactive atmosphere in parts of the system not filled with water. In the first part, the paper presents an analysis regarding the feasibility of helium-3 extraction from the cover gas used in Cernavoda Nuclear Power Plant. A theoretical calculation of the amount of helium-3 produced so far is presented. The production rate of helium-3 is related to the concentration of tritium in the heavy water from the moderator circuit. The solubility of helium-3 in water is very low, therefore almost all of it will be found in the moderator cover gas. The second part of the paper presents a technological solution for separation of helium-3 from helium gas by cryogenic distillation that permits obtaining a purity of the distillate of 99.1%, while the raffinate contains only 0.03% helium-3.