Coolant Chemistry Control in Heavy Liquid Metal Cooled Nuclear Systems

Heavy liquid metals (HLMs) such as lead and lead-bismuth eutectic are considered as primary coolants in Lead-cooled Fast Reactor and Accelerator Driven System. A major issue in HLM-cooled nuclear reactors concerns the HLM chemistry and the dissolved oxygen, which has to be balanced within an optimal concentration range to prevent HLM oxidation and minimize corrosion of steels in contact with the HLM via formation of a protective Fe-Cr oxide layer. In this framework, the present research focused on the development of oxygen sensors based on solid-electrolyte and the control of the oxygen concentration in HLM via gas phase. A baseline study about the performance of different sensors was carried out in laboratory scale. Solid electrolytes such as Yttria Partially and Totally Stabilized Zirconia were considered as well as internal references such as the Pt-air, Bi/Bi2O3 and Cu/Cu2O. The sensors were calibrated in HLM in a wide temperature range to assess the minimum reading temperature and the accuracy. In parallel, the development and the test of a sensor for large HLM pools was performed. The sensors so manufactured were used to study the HLM deoxygenation with Ar-H2 gas in different conditions of H2 concentration, HLM temperature and HLM fluid-dynamic regime. A gas control system was implemented to create safe gas mixtures with flexible H2 concentration and to obtain easy HLM deoxygenation in small steel capsules and HLM storage tanks. The HLM deoxygenation in dynamic condition was performed with success in a loop facility by injecting Ar-3%H2 and the effect of metal-oxygen interactions in the HLM was analysed. The results here obtained helped in identifying the factors influencing the process and led to a preliminary and qualitative interpretation of the deoxygenation mechanism.