Effect of Pb chemistry on hydraulics in lead-cooled reactors

Belgium is actively considering small modular reactors (SMRs) as a solution to establish a continuously available, affordable and sustainable energy system, with the goal of net-zero emissions by 2050. SMRs are currently in development worldwide, with water-cooled reactors foreseen to be deployed first. However, for the Belgian SMR, liquid Lead (Pb) is the coolant of choice, based on the extensive know-how gained with lead-bismuth eutectic (LBE), the heavy liquid metal (HLM) coolant foreseen for the MYRRHA Accelerator Driven System.

In any HLM cooled system, the impurity content of the coolant must be kept within certain boundaries to ensure long-term operations. Impurities can be added from various sources, such as  corrosion of structural steel in contact with HLM.  Dissolved oxygen (DO) plays a key part as it can react with dissolved impurities to form solid oxides.  Nucleation and/or dissolution of these oxides influences the local DO content, which in turn can affect corrosion of structural steels.

Oxidation of the coolant itself, resulting in the nucleation and growth of PbO particles, has been observed to impact hydraulics with mechanisms involved that are not well understood. Coolant oxidation has been potentially correlated with increases in liquid flow resistance in non-isothermal loops and with changes in apparent viscosity. By which mechanisms and under which conditions these hydraulic changes are generated remains unclear, as the oxide particle fractions involved are too low to induce the observed effects, according to current theories.

Coolant oxidation is not a rare event as it will occur during reactor maintenance and all other events where the DO addition rate exceeds the DO removal rate. As these observed thermal-hydraulic effects can significantly affect operational procedures in a nuclear system, a more fundamental study into this topic is warranted. 

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