Activated corrosion products: experimental and numerical investigations for Tokamak-type fusion plants applications

In the context of advancing in nuclear fusion technology, ensuring the safety of a Fusion Power Plant (FPP) is crucial because of the inherent complexities of fusion reactions and the challenges posed by the demanding operational conditions. Nuclear fusion technology is still under development, and therefore, it is essential to adopt approaches, techniques, and strategies to address and overcome the technical and scientific challenges hindering progress toward a functional and safe fusion plant. The ultimate goal is to create a complex and integrated system capable of achieving the safety objective, which is to ensure that a nuclear facility does not pose a significant risk to people and the environment. This Ph.D. thesis investigates both experimentally and numerically the behavior of contaminants into a cooling loops since those contaminant affect Occupation Radiation Exposure (ORE) and the waste management and disposal. A portion of the mentioned contaminants in water loops of tokamak-type FPP, consists of Activated Corrosion Products (ACP). The study includes experimental investigations of materials selected for the EUropean DEMOnstrator (EU-DEMO) project, as well as numerical analyses for both the EU-DEMO and Divertor Tokamak Test (DTT) projects, with the goal of supporting the safety design of selected installations. Additionally, a physical model was developed to enhance the theoretical tools available, with the long-term objective of providing more reliable and realistic safety analyses. This dissertation has two separated parts dedicated to: experimental investigation and numerical analyses and modeling. During the experimental investigation a test apparatus - located in RINA - Consulting, Center for Materials Development - has been used to study the corrosion behaviour of iron based alloys in DEMO-like environments. A water solution has been used as working fluid, with variation in alkali concentration and dissolved oxygen concentration into the solution, with the goals of stabilize the pH trend of the solution during the testing time but also to find localised corrosion phenomena caused by oxygen contamination of the water solution. Part two is dedicated to numerical analyses and modeling. With the Outil de Simulation de la ContAmination in Reactors (OSCAR)-Fusion code the effect of water chemistry on ACP buildup has been assessed for the Primary Heat Transfer System (PHTS) of the DEMO Plasma Facing Unit (PFU), and the Water Cooling System (WCS) of the DTT Vacuum Vessel (VV) support safety oriented design. The analyses conducted for the DEMO PFU were based on the OSCAR-Fusion V1.3 code, with two key parameters being perturbed: the Lithium Hydroxide (LiOH) concentration in the coolant and the standardized corrosion rate coefficients of the alloys which constitutes the mentioned loop. Differently, the DTT analyses were focused on the data set development from scratch using available data, assumptions and engineering judgements. The assessment have been done with two different configuration of the purification system (i.e., Chemical and Volume Control System (CVCS)) to compare the effects of the two configurations on the distribution of activities in the circuit with the goal of identify a suitable purification system configuration. The modeling section of the second part of this dissertation, deals with one of the theoretical tools limitation: the effect of magnetic field on ACP behaviour has never been explored yet. To address this, a preliminary physical model was developed to improve the OSCAR-Fusion code ability to simulate how ACP transport and deposition may be affected by the magnetic field. The magnetophoresis phenomenon was investigated and modification of already-built models have been proposed, with new mass transfer coefficients defined to account for magnetic field effects, and a new correlation for the deposition phase. Preliminary experimental investigations have recently confirmed the outcomes, such the enhancement of corrosion layer thickening in presence of magnetic field. This study contributes to the understanding of corrosion processes in FPP, offering insights that are essential for refining safety and design considerations. This has been done by systematically investigating the corrosion mechanisms that occur under the extreme conditions typical of fusion environments - such as high radiation flux, elevated temperatures, and the presence of reactive components. The findings are useful for informing the design and operational frameworks necessary to enhance the structural integrity and longevity of FPP. Furthermore, this work lays the foundation for future studies aimed at addressing corrosion-related challenges and more reliable safety analyses, ultimately supporting the progression toward the commercialization of fusion energy by improving the reliability and safety of FPP over their operational lifespan.