Oxygen for protective oxide scale formation on pins and structural material surfaces in lead-alloy cooled reactors

• Pb alloy coolants require oxygen for in situ oxide scale formation.
• Initial scale growth requires large amounts of oxygen.
• Pre-oxidation is needed to assure oxygen delivery.
• Surface aluminization slows down oxygen consumption by half.
• PbO based oxygen transfer needed for phases of high consumption.

Oxygen dissolved in liquid Pb alloys prevents dissolution attack on structural steels by formation of oxide scales on the surface. The required amount of oxygen for continuous scale formation depends on temperature on the kind of steel and its oxidation behavior. Compatibility of the steels in reactor systems with liquid lead alloys can only be maintained if the oxygen consumed by the growing oxide scales is replaced. Calculation of the oxygen consumed is performed on the basis of experiments on the oxide scale growth on steels in liquid Pb alloys. These calculations consider also the change in surface temperature caused by the growth of the oxide scales. The design data of EFIT (European Facility for Industrial Transmutation) and a realistic start-up scenario for such a system was used in the investigation. Also the impact of fuel element exchange and the replacement of heat exchangers were considered.The oxygen consumption is very high in the beginning, when the metals get into contact with the lead alloy. It is a great advantage to employ structures that have already protective oxide scales or can form those during a commissioning phase at 400 °C inside the reactor. Otherwise, the oxygen consumption rate would be difficult to handle. A further great improvement would be to alloy the surface of heavy loaded structure parts like the high temperature parts of fuel pins and the heat exchanger tubes with Al to form thin stable oxide films, which, furthermore, have the advantage of a low thermal resistance. This surface treatment would cut down the overall oxygen consumption to less than the half of it.It may be not possible to provide the reactor completely with oxygen via the gas phase because even the surface of the whole pool may not be large enough to transfer the required oxygen. An additional transfer of oxygen by solution of PbO could support the gas phase transport during the peak times of the consumption rate.