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Radiation-Induced Damage Processes in Nuclear Reactor Environments

Time: Fri 2021-03-26 16.00

Location: Via Zoom, (English)

Subject area: Physics

Doctoral student: Elin Toijer , Fysik

Opponent: Associate Professor Julie Tucker, Oregon State University, Oregon, USA

Supervisor: Professor Pär Olsson, Kärnkraftssäkerhet

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It is essential that the materials used in nuclear reactor applications maintain their structural integrity. However, consequences of the strong radiation field in the reactor core include radiolysis of the water surrounding the materials, as well as direct defect generation in the material itself. This may result in the combined corrosive attacks from the environment, and the internal weakening due to processesin the bulk. The goal of this thesis is to extend the current knowledge of the many radiation-induced processes in the materials used in nuclear reactor applications, and to assess how such processes in turn can influence the materials’ stress tolerance. Radiation-induced surface effects have here been evaluated from experiments, whereas bulk processes are investigated in a theoretical perspective. This providesa broad picture of the many phenomena which lead to material degradation in an irradiated environment.

In the first part of this thesis, the reaction mechanisms of H2O2 , which is an important product of water radiolysis, and oxide surfaces of interest in nuclear reactorapplications, have been evaluated. As a first step, the impact of Br , Cl and HCO3 on the reaction mechanisms between ZrO2 and H2O2 are determined. This provides an increased understanding for how competing reactions between the anions and H2O2 for the oxide surface can influence the kinetics of the system. As a second step, reaction mechanisms and reaction rate constants between H2O2 and the 304L steel in solution have been determined. Oxidative dissolution of steel components are here assessed, and the reaction paths of H2O2 towards the steel are discussed.

In the second part of this work, the objective has been to evaluate how direct effects of radiation can influence the stress tolerance of the material. Radiation-induced segregation of chemical components in the model material fcc Ni have been quantified, and focus has been on the driving forces behind this segregation. Based on the segregation trends, the impact of solute enrichment on grain boundaries have been assessed. In this context, focus has been not only to quantify the effects of the solute segregation, but also to evaluate how common modeling techniques can influencethe results. Particularly, it is here shown that the applied model can have a decisive impact on observed trends. Moreover, it is shown that the common assumption that all solutes that are driven to sinks such as grain boundaries will end up in the grain boundary center, can also give very misleading results. The concentration-dependent effect of Si, the slightly weakening effect of P, and the particular effects of Cr are all quantified using state of the art atomistic modeling. As a final step, the impact of magnetic disorder on thermal expansion and defect formation energies have here been calculated in Ni. The model used in this part of the work very accurately predict the thermal expansion of the material, and shows that this is a majorcomponent in the temperature dependence of the defect formation enthalpies.