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Evolution of corium configurations in the bottom of the reactor pressure vessel in case of severe accident in a nuclear reactor

Shambhavi Nandan ​has defended his thesis​ on 20th December 2019 in Cadarache (France).

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Research Unit > IRSN/PSN-RES/SAG/LEPC

Authors > NANDAN Shambhavi

Publication Date > 20/12/2019

Summary

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.

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