Thermodynamics of aerosols during a molten core-concrete interaction at Fukushima Daiichi Unit 2 estimated conditions

•Molten core-concrete interaction is likely to generate Fukushima Cs-particles.•Fukushima Cs-particles result from a low oxidised corium rich in Zr.•A temperature of interaction of 2000–2200 °C favours similar particles. Several radioactive releases from the containment took place during all the events leading up to Fukushima Daiichi endstate. Radionuclides released into the environment may differ in their composition or chemical form depending on the unit involved in the discharge and depending on the course of the accident. These differences may result from the path of the radionuclides from the core to the environment (e.g. through suppression pool water) and from the chemical and physical properties within the reactor core and the containment. On March 14th and 15th 2011, spherical glassy Cs-bearing microparticles (type-A CsMP) were collected for the first time. These microparticles might result from molten core-concrete interaction (MCCI) inside Unit 2, where the Zircaloy from the fuel cladding interacted with the SiO2 of the concrete pedestal at high temperature. The result of this interaction is a significant release of silica-rich gases or aerosols from the melt pool, which condense in colder locations. Calculations using ThermoCalc with OECD/NEA’s database TAF-ID version 11 were performed on the thermodynamics of vaporization during an MCCI to find correlations between the chemical and physical properties of the reactor atmosphere and the chemical composition of type-A CsMP. Data from ORIGEN2 and MELCOR calculations were used to estimate the core melt and atmosphere composition at the assumed time of the reactor pressure vessel failure. The vapour temperature at which type-A CsMP may have condensed in the containment has been evaluated. Using Fe/Si, U/Si and Cs/Si ratios in the observed CsMP as an indicator for vapour composition, this suggests that type-A CsMP originate from condensation of vapours around 2000–2200 °C from a low oxidised composition of corium, rich in zirconium and poor in stainless steel.