Tilting of reactor vessel and other components due to circumferential temperature gradient is a critical safety and operational issue in loop type Fast Reactors. Natural convection of reactor cover gas developed in the narrow component penetrations is the main cause for such a phenomenon. Numerical studies have been carried out using CFD code to investigate the possibility for development of circumferential temperature gradient in the reactor vessel of Fast Breeder Test Reactor (FBTR). The computational model which accounts for all the three modes of heat transfer has been validated against experimental data from a mock-up experimental study as well as measured data from the plant. The analysis of natural convection phenomenon in the annular region of cover gas space reveals the formation of cellular convection pattern with single convective cell. This causes circumferential temperature gradient along the reactor vessel. The temperature asymmetry is also found to vary with elevation, with a maximum value at the plug bottom. With respect to the computational model adopted, parametric studies have been carried out using various turbulence models and by varying the radiation emissivity of different surfaces present in the domain. With respect to the system design and operating conditions, parametric studies have been carried out to investigate the influence of anti-convection barrier incorporated in the design and the operating status of plug cooling and biological shield cooling systems. It is seen that the anti convection barrier and labyrinth help in significant reduction of temperature asymmetry in the reactor vessel. The operation of reactor plug cooling has been found to be very effective in reducing the temperature asymmetry as well as the average temperature of reactor vessel in the cover gas space, while the biological shield cooling system is not found to provide significant effects. Another important study that is carried out is the analysis of the natural convection behaviour as a function of the thermo-physical properties of the cover gas medium. The relative performance of Helium and Argon as cover gas medium has also been investigated. It is seen that when helium is selected as the cover gas medium, it causes lower temperature asymmetry in the system as compared to argon. This is attributed to higher thermal diffusivity of helium compared to that of argon. Hence, Helium is desirable as a cover gas medium from cellular convection considerations.
Key Words: Circumferential Temperature Gradient, Fast Reactors, Reactor vessel, CFD, Turbulence Modeling
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|Affiliation/Organization||Dept. of Atomic Energy|