Le calcul de la dynamique xénon requiert un modèle physico-numérique alimenté par des données ou paramètres et des conditions initiales. Les sources d’erreur de ce calcul peuvent provenir à la fois du modèle ainsi que des données. Dans cette thèse, nous proposons d'améliorer la prévision de la dynamique xénon par le recalage de l'état initial de cette dynamique en utilisant toutes les informations disponibles (modèle numérique, estimation a priori de l'état initial et mesures) complétées par le niveau de confiance dans chacune de ces informations. La méthode utilisée est un schéma d’assimilation de données de type 4DVAR. Des expériences jumelles nous permettent d’estimer la qualité des concentrations initiales ainsi recalées. Les résultats rencontrés permettent d'envisager la mise en place d’un outil d’aide au pilotage continuellement réajusté grâce aux mesures in-situ. ABSTRACT : Xenon-135 is a nuclear fission product which is known to be source of undesired roughly one day period density axial oscillations in pressurized water reactors. Xenon dynamics are non linearly coupled to another fission product the iodine-135. Such a coupling represents a challenge for the oscillation prediction. In order to improve xenon estimation, we investigate the feasibility of using variational data assimilation methods. The aim is to obtain a better estimation of initial 1D concentrations of xenon and iodine. Data assimilation techniques are widely used in meteorology and oceanography to improve initial states and forecasts. Such methods combine all kind of information about the system (model, a prior estimate of the true state and data). These information are weighted according to their accuracy expressed in error covariance matrices. The state resulting from the assimilation process is called analysis. 3DVAR and 4DVAR schemes for xenon dynamics are developed within the framework of twin experiments. This means that observations are obtained through numerical computation. Such a procedure allows an evaluation of produced analysis quality. The model developed for this purpose (CIREP1D) includes a monodimensional xenon dynamics linked to a monodimensional thermic and thermohydraulic model. Linear tangent and adjoint of this model are obtained through automatic differentiation. Observations are of three kinds : integrated powers over several nodes, power axial offset and boron concentration. This work figures out improvements on the estimation of iodine and xenon initial concentrations. Such encouraging results allow to set up tuning tool for an operator guiding syste
