With regards to the safety of the Nuclear Power Plants (NPP) in case of
a severe nuclear accident, one of the main challenges associated is the
retention of the molten nuclear fuel and reactor internals, called corium,
within the Reactor Pressure Vessel (RPV). One of the ways of cooling
corium with in the RPV is by cooling the vessel from outside. This strategy is
termed as In-Vessel Retention (IVR). In case of the In-Vessel Retention (IVR)
strategy, it is expected that the corium pool will be surrounded by an oxide
crust, which will be in contact with molten steel from top of the pool as well
as from sides of the vessel. It has been observed in CORDEB experiments (funded
by IRSN, CEA and EDF), that this crust becomes permeable, which has an impact
on the thickness of molten steel layer, lying on top of it. With respect to the
IVR strategy, a thin molten steel layer on top of the crust may lead to an
excessive heat flux to the Reactor Pressure Vessel (RPV), resulting in a
possible rupture or melt-through. Such phenomenon is commonly known as focusing
effect.
The present work deals with the study of dissolution of such crust in
order to estimate the time for molten steel to flow through the crust.
Thermochemically, corium is a mixture of UO2, ZrO2 and Zr. The quaternary
chemical interaction between U, Zr, O species and Fe plays an important role in
the stability (or not) of the crust. However, a thermochemical study done
within the framework of the present thesis, shows that the quaternary system of
U, Zr, Fe and O atoms, can be reduced to a ternary in each phase: (U, Zr) + O
in the oxide phase and (U, Zr) + Fe in the metal phase, where Fe and O atoms
remain in metal and oxide phases respectively, and (U, Zr) atoms as a whole
participate in the mass transfer across the interface, having ratio of (U/Zr)
atoms always constant. Further, it has been shown in the present work that, in
this ternary-in-each-phase system, chemical interactions between the crust and
liquid (Fe) can be modeled as the dissolution of a binary two-phase porous
region by a liquid. Consequently, an up-scaled model for binary mixture has
been derived by volume averaging transport equations — Mass, Momentum, Species
and Energy transport — over a Representative Elementary Volume (REV).
The final
system of Partial Differential Equations (PDEs) has been closed by deriving
several empirical relations for effective species diffusivity, mass transfer
coefficients, permeability and effective conductivity. At first the model has
been solved numerically to study the progress of dissolution in a two-phase 2D
domain without convection in the liquid-metal phase (vm = 0) with several
closure relations of mass transfer coefficient. This study is done to identify
the closure relation capable of accurately describing the dissolution in the
crust by molten steel same as observed in the CORDEB experiments. This
sensitivity analysis revealed that in order to reproduce the microstructure
similar to the one observed in CORDEB experiment, the characteristic time of
effective diffusion has to be much less than that of dissolution. Further, the
model has been solved with convection in liquid-metal phase (vm = 0) for two
cases. In the first case the flow of metal within the crust is driven by the
pressure gradient once there is a breach in the crust after sufficient
manifestations of the percolations because of the dissolution. The second case
represents the study of the thermo-solutal convection occurring due to the
temperature gradient and concentration variations resulting from dissolution.
Both these cases are analyzed for their implications to the IVR strategy. At
the end general conclusions have been drawn regarding the issues related to the
modeling of the dissolution and the IVR strategy.
