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