Impact of runaway electrons generated during disruptions on the First Wall of the tokamak reactors

Nuclear fusion is considered a promising alternative for power generation, potentially superior to fission, due to reduced operation radioactivity, the low nuclear waste production, and the intrinsic impossibility to develop a diverging reaction. Extensive research in the development of this power source is ongoing for several decades now. The next generation of Tokamak devices (the ITER reactor under construction and, later, DEMO), is the first expected to produce more power than that needed to sustain the nuclear reactions. One of the possible show-stoppers in the operation of the tokamak fusion reactors is the occurrence of disruptions, un-controlled events leading potentially to the deposition of a few GJ energy onto the wall in a few ms. On a DEMO-size machine, a disruption could potentially damage the wall unrecoverably. From a theoretical point of view, disruptions can be caused by a variety of events including MHD instabilities, malfunctioning of control systems, etc. A considerable fraction (from a few MeV to several tens of MeV) of the energy dissipated during a disruption can be carried by runaway electrons, a beam of electrons accelerated to relativistic energies by the intense electric field developing during such an event. Their generation and evolution are not yet fully understood by the fusion community, which further complicates the problem of the selection of structural materials and the design of the cooling system able to handle the high heat loads involved in the disruptions and the impact of Runaway Electrons (REs). The thesis analyzes the impact of runaway electrons generated during disruptions on different tokamak reactors including the First Wall of the EU-DEMO tokamak reactor, using the Monte Carlo-based code FLUKA. Firstly, the runaway current damage to the first wall of current tokamak reactors is presented along with its generation mechanisms and mitigation systems. The Dreicer mechanism is the primary generation mechanism for the current reactors while in future reactors the avalanche mechanism is expected to take this role. The mitigation systems are the Massive Gas Injection (MGI), Shattered Pellet Injection (SPI), Resonant Magnetic Perturbation (RMP), and Magnetic Energy Transfer (MET). However, the MGI and SPI can lead to an additional generation of runaways, so further investigations are required for all these mitigation systems. Consequently, a reproduction of a case study done by ENEA has been carried out. This consisted of bombarding a sample geometry of the EU-DEMO First Wall with REs having an energy of 20 MeV. It was seen that according to the angle of impact, a different amount of energy is deposited in the structure as well as different deposition curves are obtained according to the structural materials considered. In the future, this work would be useful to reproduce, to a certain extent, the measurements given by the diagnostics in the JET fusion device where the diagnostics measure the Soft and Hard X-rays. These X-rays are the secondary particles generated by the REs and could be traced by the virtual detectors embedded in FLUKA. Therefore, this would show the capabilities of FLUKA in reproducing the real measurements, clearing the picture of how REs interacts with the different structural materials present in the main chamber of JET. Afterward, the study could be scaled up to EU-DEMO where Tungsten will be the main plasma-facing component to handle the REs.