In the framework of Gas Cooled Reactor design assessment, an important point to calculate is thetemperature fields on the main structures in nominal and accidental situations in order to determine theconsequences on the reactor lifetime. This document presents such thermal-hydraulic and thermal-mechanicalstudies for the HTGR (High Temperature Gas-cooled Reactor) vessel system in normal operation and pressurizedLOFC (Loss Of Forced Coolant) accidents. Thermal-hydraulic calculations address the key issues forpressurized LOFC transients and evaluate the contribution of main design and modeling parameters. Thesecalculations are performed using the CFD (Computational Fluid Dynamics) code STAR-CD.For these transients where the primary system remains pressurized, it is necessary to carry out mechanicalanalyses on the structures to assess the damage levels reached.Sensitivity studies are conducted taking into account different irradiation levels and types of graphite,different assumptions of mass exchanging rate between the stagnant helium beside the vessel and the coolant inthe annular channel between core barrel and vessel, and different assumptions regarding the natural convectionof helium.To determine the structure temperatures, the thermal-hydraulic studies show that the conductivity value ofgraphite reflectors is the main factor for both the normal operation and the accidental situation considered(pressurized LOFC). The thermal-mechanical analyses allow evaluating the consequences of these loadingsituations for the lifetime assessment of the main metallic structures, namely the core barrel and the pressurevessel. The results obtained show that, for both structures, the damage levels remain below design limitations.
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