Influence of design and operating parameters on mixed-oxide fuel power-to-melt

Eighteen (U,Pu) mixed-oxide fuel rods have been destructively examined after EBR-II irradiation at peak liner powers of 50 to 70 kW/m (15 to 21 kW/ft) in the GE-F20 experiment. This power was sufficient to cause a limited amount of fuel melting. Twelve of these rods had been pre-irradiated at 40 to 50 kW/m (12 to 16 kW/ft) to produce some fuel restructuring and to affect thermal performance during the subsequent high-power exposure. Three of the twelve were again irradiated at 40 to 50 kW/m following operation at 50 to 70 kW/m. The intent of these three rods was to explore restructuring effects in axially relocated once-molten fuel. Nine sibling rods were irradiated at 40 to 50 kW/m, without any high-power exposure, to assess the fuel structure prior to the high-power operation of the partially melted fuels. Fuel density, fuel-cladding gap thickness, and burnup were major variables investigated. This paper describes some of the fuel microstructural observations from the F20 experiment. The effects of prior irradiation and fuel variables are outlined