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Theses

Évaluation de l'effet de l'aspersion sur les pressions et températures dans l'enceinte de confinement des réacteurs à eau pressurisée français en conditions accidentelles

Abstract : Containment analysis for the French fleet of pressurized water reactors is performed with the PAREO computer code, owned and developed by EDF. This containment computer code is based on a lumped-parameter (LP) single-volume representation of the containment building, but also on penalized hypothesis and physical models in order to calculate the conservative evolution of the pressure and temperature of the atmosphere inside the containment building. During the code development, the partial lack of validation data led to penalizing choices that, in turn, favorized the apparition of a superheated atmosphere in the containment building. Moreover, the physical models used in PAREO show low impact of the activation of the containment spray system (CSS) that sprays cold water into the containment atmosphere on lowering its temperature. The work performed in this thesis is focused on the assessment of the effect of the CSS on the containment atmosphere’s pressure and temperature evolutions. CSS impact on the atmosphere is studied at three different scales: single-drop scale (Micro), spray scale (Meso) and containment scale (Macro). In order to perform this analysis, the GOTHIC computer code has been chosen mainly because of its capacity to perform both LP and 3D calculations using the same underlying physical models. For the Micro scale, a bibliographical study of mechanistic heat and mass transfers (HMT) models allowed the construction of a mechanistic model, similar to the one used in the GOTHIC code. This model was validated against the CARAIDAS tests in order to be next compared against the PAREO HMT model between CSS water and the atmosphere. This comparison illustrates that the PAREO model is highly conservative due to its neglecting the vaporization of the CSS drops. For the Meso scale, the effect of a spray (collection of individual drops) on the pressure and temperature evolutions has been investigated through the comparison with the M5-MASP experimental series performed in the MISTRA facility. The GOTHIC spray HMT model is able to reproduce the pressure and temperature drop driven by the spray activation. The analysis is then extended to several sprays and the effect of the CSS on the upper part of a reactor building is investigated through the use of a large cubical volume. The 3D calculations show that the effect of the CSS in this configuration can be assimilated to a single-volume LP effect. These calculations confirm once again the conservatism of the PAREO CSS model. The Macro scale focuses on the analysis of the CSS effect using a reactor-like geometry in which the upper part is directly covered by the CSS sprays and the lower part can be considered as a rather cluttered volume in which the CSS has only an indirect effect. This analysis confirms once again the conservatism of the PAREO’s CSS HMT model when compared to GOTHIC results. GOTHIC 3D calculations show that the CSS activation leads to an immediate pressure and temperature drop, as in a single-volume LP calculation. In the studied configuration, the large water vapor release at the break creates a strong convection loop which persists even after the CSS activation and entrains some of the CSS drops, impacting their residence time in the atmosphere. This work represents a first step in studying the effect of the CSS on the pressure and temperature drop in the containment building, that will have to be completed with a refined geometry and the study of other accidental scenarios.
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Submitted on : Monday, April 11, 2022 - 5:39:09 PM
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  • HAL Id : tel-03637652, version 1

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Cristina Ghitiu. Évaluation de l'effet de l'aspersion sur les pressions et températures dans l'enceinte de confinement des réacteurs à eau pressurisée français en conditions accidentelles. Autre. Université de Lyon, 2021. Français. ⟨NNT : 2021LYSEC025⟩. ⟨tel-03637652⟩

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