Impact of atmosphere on the transport of Ruthenium in the primary circuit of nuclear power plant

Ruthenium is a semi-volatile element existing as a fission product in nuclear reactor fuel that can be released in case of a severe nuclear accident. This release is promoted by air ingress, high humidity, high temperature and oxidative conditions in the reactor containment when the consistency of primary circuit is lost. In the severe accident conditions, ruthenium will be released from the fuel in form of volatile oxides RuO₃ and RuO₄. According to thermodynamic equilibrium calculations, 91% of ruthenium will be released in form of RuO₃ and 9% in form of RuO4 in dry air at 1500 K. When the temperature in primary circuit drops under 1000 K, RuO₃ readily decomposes to solid RuO₂. Therefore, ruthenium will be transported to the containment mainly in form of RuO₂ aerosols and gaseous RuO₄. In this work, the impact of gaseous atmosphere composition on the transport of ruthenium through a model primary circuit was examined. The first experiments were conducted in an air atmosphere at 1500 K. In these experiments impact of humidity on the release and transport of ruthenium species was examined. In the following experiments, additional silver nanoparticles were used to simulate aerosols transporting in the primary circuit. Furthermore, the impact of NO₂ gas, as a product of air radiolysis, was also examined. The amount of ruthenium transported both as gas (RuO₄) and aerosol was quantified with use of neutron activation analysis method. The number size distribution and total number and mass concentrations of the formed aerosol particles were monitored online during experiments. Chemical composition of ruthenium species was evaluated by XPS and Raman spectroscopy techniques. Chemical speciation of the transported aerosols was concluded to be RuO2 in all performed experiments. Transported gaseous ruthenium was trapped in sodium hydroxide solution. The solution was then analysed with use of UV-VIS spectroscopy. Obtained absorbance spectra showed peaks typical for ruthenium in oxidation state +VII. This is a strong indication that the transported gaseous ruthenium was in form of RuO4, which got reduced in the hydroxide solution. Tests of humidity impact on the ruthenium transport at 1500 K showed lower amount of RuO4 transported through the circuit when compared with dry conditions. Introduction of silver particles into the model primary circuit led to a decreased transport of gaseous RuO₄ but to an increased amount of aerosols (RuO₂). The transport of gaseous ruthenium through the circuit increased significantly, when NO₂ gas was mixed with the airflow. Addition of both silver particles and NO₂ to the airflow promoted also the transport of ruthenium. It was concluded that the composition of gaseous atmosphere in the primary circuit has a significant effect on the amount and chemical form of ruthenium transported to the containment during a severe accident.