Heat Transfer in a Blocked Spent Fuel Subassembly of a Sodium cooled Fast Reactor
Sodium cooled Fast breeder reactors (SFRs) pave an effective way for the utilization of uranium resources due to their breeding capability. SFR core is composed of a large array of hexagonal fuel subassemblies (FSAs) held by a grid plate. FSAs which have undergone targeted burnup (spent FSAs) are relocated from the primary core region to in-core storage locations during every fuel handling campaign. During fuel handling operation, reactor will be at shut down condition. Because of the presence of fission products, the spent FSA will continue to generate decay heat even after removal from active core. Decay heat generation progressively decreases with time. Decay heat generated from a spent FSA while reaching the storage location is of order 100 kW. Due to the compact nature, any spent FSA in storage location may encounter local or partial flow blockage. Sodium outlet temperatures of storage location FSAs are usually not monitored by thermocouples owing to spatial constraint. Therefore detecting sodium boiling incipience is not possible at these locations. Total instantaneous flow blockage (TIB) at the inlet of a FSA has been identified as an extreme situation which encompasses all the local blockages. Postulating a TIB in the spent FSA present in storage location will yield conservative predictions of peak clad temperature for reactor safety purpose. In this case, the dominant mode of heat transfer will be natural convection. Difficulties in performing heat transfer experiments in sodium calls for the aid of 3D Computational fluid dynamic investigations to predict flow and temperature fields. But for SFR subassembly geometry such investigation would require a huge mesh count. This is due to the facts (i) FSA has 217 fuel pins, (ii) each pin has a helically wound spacer wire and (iii) pin length is more than 1.6 m. Under these circumstances, modelling the FSA as a heat generating porous medium offers great advantage regarding computational expense. Therefore the main objective of this study is to develop an axisymmetric porous body code with the assistance of 3d CFD simulations performed on a limited region of the actual subassembly geometry and use it to predict the peak clad temperature for SFR spent FSA kept at an internal storage location. For this purpose, the finite volume formulation for axisymmetric natural convection in a cylindrical cavity was modified to include the effects of the presence of fuel pins. The safety limit of decay power levels is also assessed by varying the heat generation rates.
The results of the porous model reveal significant peak clad temperatures only during fuel handling operation. Low power levels of spent FSA makes peak clad temperature at reactor restarting conditions safe. The temperature contours for spent FSA under blockage conditions during fuel handling campaign is presented in figure 1. The bottom blanket has little effect on
the temperature field. Peak clad temperature as a function of spent FSA power under same conditions is shown in figure 2 which indicates a linear trend. The safety limit of decay power for the reactor is found to be 162 kW.
Full paper will provide details of (i) porous body model, (ii) validation of computer code, (iii) parametric studies and (iv) recommendations for fuel handling of SFR.