Formation, Transport and Deposition of Lead Oxide in Lead-Bismuth Cooled Nuclear Systems

A unique Multi-purpose hYbrid Research Reactor for High-tech Applications, MYRRHA, is under development at the Belgian Nuclear Research Centre (SCK•CEN). MYRRHA is conceived as an Accelerator Driven System (ADS) and operates by injecting a proton beam inside the nuclear reactor. The primary coolant is chosen to be a eutectic mixture of lead and bismuth (LBE). The dissolved oxygen concentration in LBE is important for a reliable operation of MYRRHA. A minimal dissolved oxygen concentration is needed to form a protective oxide layer on stainless steel surfaces of interest such as SS316L. An upper limit on the dissolved oxygen concentration is defined by the process of coolant oxidation. The concentration of dissolved oxygen in LBE is limited by the formation of lead oxide (PbO). Even though various methods for oxygen measurement and control have been developed in the past, research on LBE filtration and purification has been lagging. The process of coolant oxidation following accidents with oxygen ingress is not suficiently understood to calculate and quantify the safety measures that need to be taken. This thesis therefore aims at enhancing the knowledge regarding coolant oxidation, which will contribute to the development of efficient filtration methods for LBE. Nucleation, growth and dissolution of PbO in LBE has been studied both experimentally and theoretically. An empirical onset of PbO nucleation, a so-called metastable limit for PbO formation, is defined and validated in various experimental set-ups. PbO crystallization fouling experiments combined with a theoretical model allowed the quantification of PbO deposition on non-isothermal surfaces. A proof-of-principle cold trap experiment is performed and the possibility of oxygen trapping in LBE via PbO formation is validated. The obtained results are directly applicable to the design of a PbO cold trap purification system for LBE in MYRRHA.