Effects of Gamma Radiation on Water Chemistry, Polymers and Fe-, Ni- and Zr-based alloys

Licentiate thesis, 2025

Due to the harsh conditions in a nuclear reactor, surface reactions such as metal corrosion, oxide deposition, and oxide release/dissolution are processes that may have a significant impact on the performance of and the occupational safety around nuclear facilities. To maximise the efficiency of the nuclear power plant and minimise the risks, a deep understanding of the mechanisms behind these phenomena is needed, as well as the possibility to test materials on a lab scale. The aim of the project, of which this thesis is a part, is to identify situations where a simplified approach can be sufficient to mimic the surface processes taking place in a nuclear reactor and situations where this is not sufficient.
The goal is pursued through the development and validation of reliable investigative methods and the exploration of oxidation (through SEM, EIS, and XPS), elemental dissolution (ICP-MS) and radiation chemistry (monitoring H2O2 concentration) under gamma irradiation. The results indicate that polysiloxane rubber septa can significantly reduce the radiolytically-produced H₂O₂ by up to 90% compared to polytetrafluoroethylene (PTFE) and aluminium septa,  The suppression of H₂O₂ was observed exclusively during irradiation and appears to be related to the availability of oxygen, possibly indicating the consumption of O2 by radicals in the polymer matrix formed by the radiation, which in turn suppress H2O2 formation as O2 is a precursor for hydrogen peroxide.
As for the investigated alloys, namely AISI 441, Inconel 718, Inconel X750 and Zircalloy-2 low alloy, collected data highlight that different alloy formulations cause different H2O2 steady-state in the system (i.e., glass vials containing the metallic coupon and water) and how this phenomenon is triggered by gamma radiations. Moreover, no detectable traces of oxide growth have been detected on the samples, ruling out surface oxidation as the root cause for H2O2 consumption. On the other hand, it seems to be possible to correlate the electrochemical response of the alloys under irradiation with their effect on the hydrogen peroxide steady-state concentration achieved during prolonged irradiation (up to 2.2MGy).