Investigation of heat transfer from a totally blocked fuel subassembly of fast breeder reactor with 7 and 19 pin bundles

Liquid metal cooled fast breeder reactor is emerging as an important future power source. It uses sodium as the primary coolant, due to its favourable neutronic properties, high boiling point and large heat transfer coefficient. It is essential to continuously monitor adequacy of fuel pin/fuel subassembly cooling, so that sodium does not reach its boiling point. Sodium boiling within the fuel subassembly leads to large coolant flow reduction (as the density ratio of sodium liquid to sodium vapour is very high), under-cooling of fuel pins and hence damage to fuel subassembly. Development of partial flow blockage in the fuel pin bundle is possible due to the compact nature of fuel pin arrangement within the fuel subassembly, with small hydraulic diameter (typically, ∼3 mm), despite the elaborate design provisions and coolant chemistry control. In the safety analysis, a total flow blockage is usually considered as an upper bound of all the partial blockages. The present work investigates the power level at which liquid sodium reaches its boiling point for different sizes of fuel subassembly. A porous body model has been developed based on local pressure-drop correlations for the axial/cross flow through fuel pin bundles. The model considers the total flow blockage situation in the fuel subassembly wherein the fuel pins and sodium (within the blocked subassembly) are considered as heat generating orthotropic porous medium. The proposed porous body model has been compared with a Computational Fluid Dynamics (CFD) model describing the same blockage condition. The commercial CFD code ANSYS FLUENT 15.0 has been used for the computational purpose. It has been found that the trapped liquid sodium coolant inside the blocked subassembly reaches its boiling point (1153 K at atmospheric pressure) at lower power input per pin in a 19 pin bundle model than that in a 7 pin bundle. This is traced mainly due to the fact that as the number of pins in the subassembly increases, the surface area of the hexcan wall (which is the ultimate heat sink) increases leading to enhanced heat dissipation in large size subassemblies. However, as the number of pins increases, the internal resistance for radial heat transfer increases, leading to increased sodium temperature inside the subassembly. These two competing effects determine the peak sodium temperature in the subassembly. Since the later effect is dominant, the permissible heat generation per pin decreases as the number of fuel pins in the bundle increases.