Residual Heat and Corrosion Processes in the ELECTRA-Reactor Concept

Abstract: ELECTRA is a Swedish training reactor concept that will use fourth generation nuclear reactor technology, and is cooled by natural circulation of liquid lead. During a fuel inspection the reactor vessel has to be emptied on metal coolant and the power must be shutoff, this can lead to a meltdown. By calculating the produced residual heat in the fuel pins during the power shutdown and coolant drainage, the cladding temperature could be calculated. The potential usage of argon, helium or nitrogen gas as an emergency coolant during loss of lead was investigated. The gas coolant will unavoidably contain small amounts of water vapour which can have a negative impact on the core. This matter was examined with a literature study on the effect of water vapour on the cladding surface; FeCrAl, an iron- base alloy with high aluminium and chromium content.  The burnup simulation code Serpent was used for calculating the residual heat. The results indicated that just after five days of shutdown the residual heat decreased to 31 W/pin and continued to progressively decrease in the coming days. It was evaluated that the residual heat developed in the core would not exceed 600°C.  Calculations on the emergency gas coolant showed that nitrogen gas offered the best solution on terms of thermal decrease and cost-effectiveness. While helium cooling was the recommended option due to its inertness.  The literature study done on the oxidizing effect of water vapour on FeCrAl, shows little impact due to the high oxidation resistance of this alloy as a result of the high aluminium and chromium percentage.