Modellering av oavsiktlig och olycksrelaterad drift under termohydrauliska transienter i franska PWR

This thesis investigates a deterministic approach aimed at improving the understanding of post-incident and post-accidental thermohydraulic transients in second-generation French Pressurized Water Reactors. The methodology is based on coupling the thermohydraulic system code CATHARE3 with the Instrumentation and Control simulation code DIAGRAM and the Fault Operating Procedure (FOP) modeling tool APE900.f. The simulation of FOPs is particularly challenging due to their intrinsic complexity and their variability across reactor designs. To address this issue, APE900.f was developed as an automated tool capable of modeling these procedures. The development of innovative models for post-incident and post-accident operational rules represents a step forward in safety analysis, as it enhances both the efficiency and the reliability of thermohydraulic transient simulations. The validation and improvement of such a methodology require a comprehensive understanding of thermohydraulic and neutronic phenomena in nuclear power plants, as well as a strong grasp of nuclear safety principles. It also demands the ability to critically analyze simulation outputs and compare them with reference studies. The primary objective of this thesis was to adapt the deterministic analysis of a well-known thermohydraulic transient to this new coupled methodology and to compare the results with those obtained from a reference simulation in which FOPs were modeled manually. To perform this non-regression study, CATHARE3 input datasets compatible with APE900.f were progressively modified in order to reproduce the reference case. Upon achieving satisfactory agreement, a second objective consisted of extending the transient calculation using APE900.f. The results demonstrate that the coupling of the three tools was successfully validated, as differences with the reference study were minimal. Furthermore, the use of APE900.f enabled an extension of the transient duration and led to improved safety margins, highlighting the robustness and added value of the proposed deterministic methodology. Hopefully APE900.f will, in the future, be officially adopted for the comprehensive simulation and analysis of all categories of thermohydraulic transients.