Experimental and Analytical Simulation of MFCI (Molten Fuel Coolant Interaction) during CDA (Core Disruptive Accident) in Sodium Cooled Fast Reactor

With increasing demand for understanding Severe Accident Scenario in Sodium Cooled Fast Reactors, there is an urgent need of enhancing numerical and experimental simulation techniques. Extensive literature survey indicates complete data but firm conclusions on molten coolant interaction and their consequent effects of fragmentation and settlement behaviour on core catcher is not available comprehensively. Hence the entire phenomena of settlement of molten core debris on the core catcher are described by defining individual phenomenon associated with this scenario. These phenomena include expulsion of molten fuel from the cladding and its interaction with sodium. These phenomena are important to understand MFCI effects on pin failures. This is the pre-disassembly phase in CDA. The order of pressure developed and the associated temperature would govern pin-pin interaction and ultimately leading to melting of sub-assembly phenomena. In this aspect it is highly important to know the phenomena of molten fuel in liquid sodium. In the post accident phase, entire fuel melts and its relocation occurs inside the core. During the relocation, the molten fuel comes into contact with the structures inside the core which leads to fragmentation. Due to this numerous small molten particles are created which interacts with sodium. The integrated effects of these particles are required to determine the loading on the vessel. The transport phenomena of the molten fuel in the core and its settlement behaviour are also analyzed in the post accident phase. In order to analyze these phenomena, sophisticated experimental facility is needed to validate the numerical simulation. This thesis deals with the analytical simulation of the pressure, temperature rise and the transport behaviour of a single particle. To understand the complexity involved in performing this experiment, a small scale experimental facility is constructed at IGCAR and preliminary data are generated. The input data from these basic phenomena can be used for developing and validating international codes like SAS4, SIMMER. A possible approach for addressing the re-criticality phenomenon is proposed in this thesis and a model calculation is also done. This model analysis yields a situation which seems conducive for re-criticality to occur. Even though our analysis takes into account the effect of a single particle reacting with sodium which might not lead to a potential consequence in the point of re-criticality, for asserting the safety of such situation, the approach proposed must be validated and developed further in order to accurately claim the probability of the situation in realistic condition with large amount of fuel