, L'un des principaux enjeux pour l'étude de l'évolution d'un réacteur consisteà modéliser la propagation de ces neutrons, décrite par l'équation de Boltzmann, tout en prenant en compte les phénomènes multiphysiques, c'est-à-dire les interactions entre la neutronique, la thermohydraulique et la thermomécanique. En effet, il existe un fort couplage entre ces différentes physiques : l'effet modérateur et l'effet Doppler dans le combustible notamment ont un effet stabilisant sur la puissance neutronique. L'étude de ce système multiphysique est, L'énergie produite dans un réacteur nucléaire provient des interactions entre les neutrons et les noyaux lourds présents dans le combustible

, Le premier regroupe les méthodes déterministes, qui résolvent l'équation numériquement en discrétisant l'espace des phases. Le second regroupe les méthodes stochastiques, aussi appelées méthodes Monte-Carlo , qui reposent sur le tirage aléatoire d'un grand nombre de trajectoires neutroniques, sur lesquelles une moyenne et une incertitude sont déterminées. Les méthodes Monte-Carlo offrent une résolution exacte du système, au prix de cette incertitude statistique sur le résultat. Pour la réduire, la solution la plus naturelle consisteà augmenter la statistique, c'est-à-dire le nombre de trajectoires simulées

, Le Monte-Carlo est utilisé ponctuellement comme outil de référence pour les calculs stationnaires, c'est-à-dire sans dépendance temporelle. En revanche, les calculs non stationnaires, comme les transitoires accidentels par exemple,étaient traités jusqu'à récemment par les codes déterministes uniquement. Pour les configurations non stationnaires, le développement de méthodes Monte-Carlo qui prennent en compte la dépendance en temps du système neutronique, mais aussi le couplage avec les autres physiques (méthodes dynamiques ), a pour but de servir de référence aux calculs déterministes. Avec l'augmentation des moyens de calcul, de telles simulations deviennent de plus en plus faisables, C'est pourquoi en général les méthodes déterministes sont utilisées pour la conception et l'opération des réacteurs commerciaux

C. Dans-ce-contexte, Il a consistéà mettre en place une chaîne de calcul pour la simulation couplée neutronique Monte-Carlo, avec le code TRIPOLI-4, en conditions non stationnaires et avec prise en compte des contre-réactions thermohydrauliques dans le but de servir de référence aux calculs de transitoires liésà des insertions de réactivité sur des réacteurs de type REP

, Ce travail est organisé en trois parties

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