Effect of air radiolysis products on the chemistry of ruthenium during a severe nuclear accident

During a nuclear accident the most important chemical elements are those forming volatile compounds that can be released from the irradiated fuel into the environment. For the proper evaluation of a possible source term during an accident, the quantity of the released radioactive nuclides, their transport and further interactions with the structural materials in the nuclear power plant are necessary to be known. Among others ruthenium is one of the critical radiotoxic elements in the case of an accident due to its ability to form volatile oxides. This work was focused on the transport of ruthenium through the reactor cooling system in case of a nuclear accident connected with air ingress. Main objective was to evaluate the effect of air radiolysis products that will be unavoidably formed during an accident and their impact on the quantities and chemical speciation of transported ruthenium. Separate effects of NO2, N2O and HNO3 on the ruthenium transport were studied in the primary circuit simulating facility. Gaseous and aerosol forms of ruthenium were collected at the outlet of the facility and quantified with use of neutron activation analysis. Chemical characterization of the collected samples was performed with use of SEM, XPS, XRD and photospectroscopy. In the experiments nitrogen oxides as well as nitric acid had significant effect on the ruthenium chemistry in the model primary circuit. The fraction of transported gaseous ruthenium was increased when NO2 or HNO3 were injected into the airflow with volatile ruthenium oxides. This effect was most prominent in case of NO2 precursor at temperature of 1300 K. The overall transport of ruthenium was strongly increased at 1500 when N2 was injected into the gas phase, when compared to the pure humid air atmosphere. The obtained results indicate a strong effect of air radiolysis products on the quantity of transported ruthenium and its partition to gaseous and aerosol compounds.