High Resolution Temperature and Flow Measurements in Wire-Wrapped Fuel Assemblies

Fuel rod assemblies of fast reactors are characterized by a high power density and a tight lattice of the rods in the fuel element with narrow gaps between adjacent rods. A high coolant mixing rate is necessary for these configurations to reduce the local maximum temperatures during normal operation of the reactor. Helically wound spacer wires are provided over each fuel pin to avoid pin-to-pin contact and to guard the pin bundle against flow induced vibration. The helical spacer wires also promote mixing of coolant among various sub-channels. In view of thermal and hydraulic design of liquid metal fast reactor cores, it is of paramount importance to investigate the coolant mixing and the heat transfer behavior due to wire spacers. In this type of fuel bundles, rods are tightly disposed with helically wound spacer wire, forming narrow gaps among adjacent rods and between an outer rods and containing walls. Change in the fuel bundle geometry may also occur during the reactor operation due thermal-mechanical stresses. Wire wrapped rod bundles present challenges to advanced modelling and simulations tools (such Nek5000, currently in development under NEAMS program) as little available high resolution experimental data is suitable for the validation of such advanced computer codes.