ARC reactor: neutronic and activation analysis of high-entropy alloys for the vacuum vessel

We live in a time when we need to increase energy production and reduce CO2 emissions. In this perspective, nuclear energy is characterized by one of the lowest emissions values per unit of energy produced and at the same time it may provide energy in a constant and controllable way. The objective of this thesis is the modelling of the vacuum vessel for Affordable Robust Compact (ARC) fusion reactor: an approximately 190 MWe tokamak reactor of small size in order to reduce the cost and the complexity of such a facility. ARC is under development by the Massachusetts Institute of Technology (MIT) and PSFC (Plasma Science and Fusion Centre). The schematisation of its geometry was made gradually, starting from a very simplified geometry to a D shape model, to best approximate reality. Tritium breeding ratio, neutron flux, neutron spectra and neutron currents were computed by OpenMC. The study firstly analysed the single blanket module geometry with a first wall of tungsten, Inconel 718 as the structural material, FLiBe as breeding blanket and coolant, and beryllium as neutron multiplier. An additional layer made of Tungsten carbide was inserted as a shield and reflector to increase the TBR. To optimise the TBR we replaced the structural material with a high-entropy alloy and perform a sensitivity study on it to obtain the best composition. An assessment of the 184W tailoring technique to study the influence on the TBR was made on the alloys. Through the use of high-entropy alloys, it was possible to remove the beryllium layer, as it is expensive, toxic and contains traces of uranium. The plasma source needed to be modified to best represent the real case. Starting with a homogeneous, isotropic source with a simple box geometry, the simulations were carried out with a tokamak plasma source from the ‘OpenMC-plasma-source’ package. Finally, the FISPACT-II program studied the neutron induced activation of materials throughout the vacuum vessel and the effect that impurities can have on these materials. Materials activation is expected to affect the reactor economy in terms of decommissioning and management of radioactive waste as well as affecting public acceptance of this energy source, as activated materials could be associated with the problem of long-life waste typical of fission reactors. In this regard, the activation analysis showed the superiority of high-entropy alloys, since they allow a faster decay after being irradiated than Inconel-718.