Thermal hydraulic investigation of heat transfer from a completely blocked fuel subassembly of SFR

Abstract

A non-classical porous body model has been adopted to predict the thermal hydraulics within a totally blocked prototype spent fuel subassembly of a sodium cooled fast reactor. The heat transfer within the subassembly is by natural convection of sodium and the ultimate heat sink is inter-wrapper flow. The decay heat is considered as a source term in the energy equation. Governing equations for 3-D, steady state, porous-body based natural convection have been solved by using the commercial CFD code ANSYS WORKBENCH 15.0. The present study considers a 217 pin wire wrap bundle of spent fuel subassembly, for an axial length of five helical pitches. The magnitudes of maximum sodium temperature and natural convection velocity inside the blocked subassembly have been obtained from the model. Through parametric studies, the permissible decay power (i) that avoids boiling of sodium trapped inside the subassembly (450 kW) and (ii) that restricts the peak clad temperature less than 823 K (100 kW), have been obtained. Further, an equivalent conduction model for the natural convection has been developed which can be used for quick estimate of sodium temperature at various values of decay power. It is seen that the conductivity value of sodium in the equivalent conduction model varies as keq(q′′′) = kNa + C×(q′′′)n where the values of C and n are found to be 1 and 0.24. From the point of view of safety, subassembly outlet sodium temperature monitoring has to be initiated at this power level itself, during power raising operation in a fast reactor. The permissible decay power from clad safety point of view is reached 6 h after reactor shut-down. This suggests that fuel handling operation can be initiated ideally after this time.