Summary of the Nordic collaboration: Impact of air radiolysis, fission product and control rod species on the transport of ruthenium in the primary circuit of NPP in a severe accident

During the operation of a nuclear power plant (NPP), a significant amount of ruthenium is built up in the fuel as a product of the nuclear fission. The importance of ruthenium from the radiological point of view is mainly due to the isotopes 103Ru and 106Ru with half-lives of 39.35 days and 373.5 days, respectively. When ruthenium is released from the fuel to the environment in a severe NPP accident, these ruthenium isotopes cause a radiotoxic risk to the population both in a short and long term by building-up to the human body and external exposure to the radiation, thus possibly leading to a development of cancer. The transport of ruthenium through a reactor coolant system, after being released from the fuel, has been investigated in several experimental programmes recently. The VTT Ru transport programme has shown that the release rate of Ru from RuO2 powder was dependent on the oxygen partial pressure, as well as temperature, in air-steam atmospheres at 827, 1027, 1227 and 1427 °C. The highest fraction of gaseous RuO4 at the outlet of the model primary circuit was observed at 1027 °C oxidation temperature. At higher temperatures, ruthenium was transported mainly as RuO2 aerosol. In the experiments of RUSET programme (KFKI AEKI), the presence of other FPs, e.g. BaO and CeO2, mixed with the metallic Ru precursor when the sample was oxidized at 1100 °C, decreased the fraction of gaseous RuO4 in the outlet air over the stainless steel surface compared to the pure Ru oxidation. Also, the transport of RuO4 was dependent on the surface material in the coolant circuit. In both VTT and RUSET programmes it was noticed, that the partial pressure of RuO4 reaching the outlet of model primary circuit was in the range of 10-7 to 10-6 bar, which is significantly higher than what is expected based on thermodynamic equilibrium calculations. As the previous studies have mainly been conducted in pure air-steam atmospheres, the current Nordic study was dedicated to air ingress conditions with representative airborne air radiolysis (N2O, NO2, HNO3), control rod (Ag) and fission product (CsI) species which were mixed with vaporized Ru oxides. The aim was to study the impact of these additives on the transport of ruthenium as gas and aerosols through the primary circuit of nuclear power plant in a severe accident (SA). As a main outcome, the transport of gaseous ruthenium compound through the facility (heated up to 1027, 1227 and 1427 °C; outlet at 30 °C) increased significantly when the oxidizing NO2 gas was fed into the atmosphere when compared to the pure air-steam atmosphere. A notable increase in the transport of gaseous compound was also observed with the HNO3 feed even at the highest temperature. Introduction of N2O into the atmosphere led to a decrease of gaseous ruthenium transport through the facility as well as to an increased fraction of ruthenium transported in the form of aerosols at 1027 °C and 1227 °C. Similarly, the feed of pure silver particles into the gas flow showed an immediate decrease in gaseous RuO4 reaching the outlet of the facility. Simultaneously, an intense increase of ruthenium in form of RuO2 trapped on the filter was observed. The feed of CsI into the flow of ruthenium oxides had a strong effect on the thermodynamic equilibrium of Ru species. The transport of gaseous ruthenium compound (10-5 bar) was the highest ever observed with this facility, whereas the aerosol transport of ruthenium decreased significantly. Based on experiments it was concluded that the composition of atmosphere in the primary circuit will have a notable effect on the speciation of ruthenium transported into the containment building during a severe accident. These experimental observations should be considered when developing the SA analysis codes.