Thermal-hydraulic and structural considerations on the gap variation between fuel bundle and hexcan

One of the safety concerns in reactor operation is the temperature of the fuel elements, which are contained in a hexagonal duct, referred to as the hexcan. There are gaps between the fuel bundle and the hexcan, and the gaps somewhat affect thermal-hydraulic behavior of the fuel element. This paper investigates the impact of gap variations on the thermal-hydraulic responses of the fuel element as well as the possibility of fuel bundle and hexcan interaction. The gap variation between hex duct and fuel bundle is caused by differential thermal expansion, creep strain, and irradiation-induced swelling of fuel-cladding and hex duct, and it is a function of fuel burnup. The effects of gap variations on the thermal-hydraulic responses of typical driver subassemblies in the Experimental Breeder Reactor (EBR-II) are investigated. A structural analysis was performed to predict the in-reactor deformation behavior of fuel bundle and hex duct, and followed by a thermal-hydraulic analysis to determine the flow and temperature response due to gap changes. The effects of geometry variation on the thermal-hydraulic response are discussed. The study indicates that the effect of gap variation should not be ignored for subassembly designs