Investigation of turbulent natural convection in top shield enclosure of a fast breeder reactor

Following a Core Disruptive Accident (CDA), leak of 350 kg of sodium in the cylindrical enclosure between working platform and the top shield has been estimated. Such a scenario makes a strong possibility of sodium pool fire within the enclosure. In order to keep the temperature and pressure within the permissible limits in the enclosure, lateral opening area has been provided which will facilitate the natural circulation cooling by the ambient air. In the present study the cylindrical enclosure has been simulated by an axisymmetric cylindrical model with lateral opening. The lower wall has been subjected to constant temperature simulating the flame temperature of the burning sodium. The upper wall along with lateral wall (excluding the lateral opening) has been made adiabatic. A constant heat generation by the enclosure air has been considered simulating the burning of the sodium. Numerical studies have been carried out for the temperature distribution along with flow patterns using a CFD solver. Various cases with different boundary conditions have been analyzed and comparative study has been made to depict the percentage increase and decrease in the physical parameters such as temperature, velocity and mass flow rate. It has been seen that with the addition of the heat source, there is an enhancement in natural circulation within the enclosure, resulting in increased mass flow rate and velocity. Effect of lateral opening has also been investigated to show how the decrease in opening area results in rise in the axial temperature and reduction in mass flow rate. (author)