Thermodynamic modelling of radionuclide release in lead-cooled reactors

To establish an energy system that is affordable, sustainable and aiming at net-zero emissions until 2050, Belgium is actively considering Small Modular Reactors (SMRs). SMRs are in development worldwide, with water-cooled reactors foreseen to be deployed in a first stage. An attractive alternative proposed in Belgium would be using heavy liquid metals, such as lead (Pb), as coolants. The main challenges related to the coolant chemistry of such a system focus on the corrosion of materials and release, precipitation, and filtration of highly radiotoxic radionuclides.

During the operation of Lead Fast Reactors (LFRs), the coolant produces various radionuclides as a consequence of nuclear reactions. Understanding the release of these radionuclides from the coolant and their interactions within other impurities, in Pb and/or cover gas and surfaces, are major challenges and are related to the chemistry of Pb-cooled reactors. The source term and the contribution of each radionuclide must be known to precisely predict the short and long-term health and environmental effects of the release of these radionuclides during relevant scenarios (cover gas conditioning, maintenance, accidents).

To characterize such a complex system, the main objective of this work is two-fold: (i) develop a thermochemical model able to predict the release of radionuclides from liquid Pb depending on temperature and the presence of various solid and gas phase impurities and (ii) validate this model by conducting transpiration experiments to determine the vapor pressure of elements of major interest (e.g. I, Te, Cs) above the lead.

A first step in this research project involves creating an equilibrium model in the HSC software considering the behavior of elements in Pb (e.g. activity coefficients) and main chemical interactions between them in Pb describing their release into the gas phase. The developed model is subject to  experimental validation based on a set of selected relevant scenarios using the experience accumulated in transpiration experiments at SCK CEN.

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