Heavy liquid metals are very promising media for innovative power systems. They have a wide application range: as coolant for Generation IV nuclear reactors , as heat transport fluid for thermal solar power applications  and as electrodes in high capacity batteries for energy storage . However, for all these systems there is an issue of compatibility with the structural and functional materials with which the liquid metal is in contact. Liquid metal corrosion (LMC) is one of the most challenging materials degradation phenomena that developers of power systems cooled with heavy liquid metals are facing. The incorporation of LMC for the selected candidate materials in the design criteria for the MYRRHA nuclear system  is presently the highest priority task in materials test program.
Despite significant efforts made in the last decade, there is still no consolidated theory explaining the variety of experimental observations of LMC on steels in liquid lead alloys. Such a theory is hard to obtain due to the occurrence of different physical-chemical phenomena from overlapping research areas:
- Chemistry of liquid metal and chemical processes at the liquid-solid interface;
- Mass-transport phenomena in liquid metal, in the oxide layer and the steel matrix;
- Materials microstructure;
- Deformation mechanisms;
Over the past seven years state-of-the-art experimental facilities to investigate LMC have been constructed at SCK•CEN in framework of the MYRRHA project. These facilities allowed a large corrosion database to be produced, which is used in the development of the corrosion mitigation strategy for MYRRHA, as well as for dedicated investigations on the effect of various parameters on development of corrosion.
To exploit this database to the fullest in macroscopic regression models, a better physical understanding of the following experimental observations on austenitic steels as candidate structural materials for MYRRHA is necessary:
- Penetration of LBE and development of corrosion damages along planar defects in steel matrix ;
- Oxidation reaction at the steel surface and formation of oxide scale;
- Mass transport of corrosion products in corroded areas;
- Effect of steel deformation on dissolution corrosion .
Since the corrosion phenomena occur on different length and time scales, ranging from the atomic oxidation reaction to oxide films on macroscopic components during their service lifetime, a multi-scale modelling approach is in place. In the past decade, SCK•CEN has successfully developed and applied multi-scale modelling techniques to complement experiments [8- 10]. The applied techniques range from electronic structure calculations (so-called density functional theory [11, 12]) and interatomic potential development [13, 14] to large scale atomic calculations using classical molecular dynamics [14, 15] and kinetic Monte Carlo models . In recent years this expertise was extended with the use of thermodynamic and kinetic packages, such as ThermoCalc and Prisma.
The mentioned modelling schemes and tools are deemed suitable also to gain a fundamental understanding of the mechanisms governing the above mentioned experimental observations. The ultimate goal is that, based on this insight and on the available corrosion database, a predictive modelling tool should be developed for the rationalization of the corrosion kinetics as a function of time, temperature, coolant chemistry and other selected parameters. Such a modelling tool, based on the use of physical laws, rather than simple empirical regressions, should allow extrapolations with a higher degree of confidence.
Objectives and Methodology
The main goal of the present PhD work proposal is to address the problem of modelling specific phenomena that underlie liquid metal corrosion, using tools suitable for the process of interest and its characteristic length and timescale. While pursuing the ultimate goal of integrating these different models into a single suite of tools to predict the development of corrosion as a function of time, temperature, coolant chemistry and other selected parameters, the attention will be mainly and more pragmatically focused on providing support to the correct interpretation of the experimental data gathered at SCK•CEN and by other partners within the H2020 GEMMA project. More precisely:
Density functional theory (DFT) calculations will be used to assess the thermodynamic stability of specific oxides observed in experiments, as well as to evaluate the affinity of lead and bismuth atoms for specific boundaries (twin, grains, …) where the liquid metal is experimentally observed to penetrate. These indications will be key to provide a physical explanation of the experimental data.
Furthermore, DFT will allow the characteristic formation and activation energy values to be precisely quantified: these will be the building blocks for the development of interatomic potentials capable of describing the atomic-level properties of the heavy liquid metal, the oxide film and the base steel in a simplified manner. Likewise, DFT will enable the calculations of thermo-dynamic and kinetic properties usable for the parameterization of existing thermodynamic and kinetic packages, such as Thermocalc and Prisma.
MD simulations with the interatomic potentials developed in 2 will be used to derive mass transport coefficients in the liquid phase and liquid/oxide or liquid/metal interface, as well as to assess the effect of applied stresses on the stability of the oxides, thereby providing quantitative indications on the severity of stress corrosion.
Since the diffusion in solids is significantly slower than in the liquid, KMC models will be parameterized on the above data to obtain mass transport coefficients in the oxide and oxide/metal interface, accounting also for the effect of nearby planar defects .
Finally, existing thermodynamic and kinetic packages parameterized using the transport coefficients of 3 and 4 and the DFT data of 2 will be used to assess the macroscopic growth rate and stability of various oxide films.
It is important to emphasize that the described modelling schemes will be applied in a pragmatic manner: as the PhD moves forwards and new insight is gained, unforeseen simulations schemes might become relevant or foreseen simulation schemes might become irrelevant.
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