Application of Monte Carlo and deterministic codes for neutronics in PWRs of different scales and in distinct operational conditions: initial core loading and load-following operations

This thesis explores the wide topic of fuel evolution (or fuel irradiation) inside Pressurized Water Reactors (PWRs). This subject is studied in this thesis via the application of two codes for neutronics, i.e. the diffusion of neutrons in matter: a Monte Carlo code and a deterministic code; they are codes that have been developed as tools for reactor physics, and in this thesis they are used in two different application cases, each of them corresponds to an internship done during the academic year 2024-2025. The first one was carried out at the LPC-Caen laboratories, it was a part-time internship and lasted from September to December 2024. The second internship was a full-time job at the SPRC/LE2C laboratory in CEA-Cadarache, it lasted from March to August 2025. The first application case concerns the activity of the irradiated fuel, and in particular the neutron radiation emitted by it during the loading procedure of a PWR. This study is embedded in the bigger SALMON project, which aims at using a pulsed neutron source to keep the reactivity monitored during core reloading. The intrinsic neutron radiation coming from the irradiated fuel is treated as noise when applying the experimental Sjostrand method: that's why it is important to quantify it before performing the real experiment in a real power plant. The fuel evolution and the simulation of the detectors are modelled using the Monte Carlo code Serpent2. The second application case studies how fuel evolution in a core changes in the case of a reactor that undergoes load-following patterns, i.e. its power output is adapted to the electricity demand. The benchmark proposed in this context is different from the previous one, indeed even though the simulations concern a PWR, the modelled core here is a an already-made neutronic image of a Small Modular Reactor (SMR): PRATIC. Moreover, in this case a deterministic code (Apollo3) is used. The reasons behind these changes in the methodology are clear and are explained in the thesis when they are introduced. In this thesis an overview of the necessary concepts of reactor physics is presented, then the methodology is explained, where also the description of the algorithms used by the deterministic code are shown, together with the modelled geometries. The results of direct fuel evolution calculations are presented at assembly scale, on the other hand the results of the simulations performed at core scale are conclusive for the application cases.