Improvement of the nuclear safety code CATHARE based on thermal-hydraulic experiments for the Jules Horowitz Reactor

Abstract: The Jules Horowitz Reactor (JHR) is a material testing research reactor under construction at CEA-Cadarache (France). One of the computer codes employed in the safety analysis of this reactor is the thermal-hydraulic system code CATHARE. The physical models implemented in CATHARE have been developed and optimized for commercial Light Water Reactors which significantly differ from JHR in terms of both core geometry and operational conditions. In view of this, it is crucial to carefully assess the capabilities of CATHARE with respect to the JHR characteristics. The current thesis aims at improving the physical correlations used in CATHARE for JHR modeling. The work is based on the SULTAN-JHR experiments in narrow rectangular channels, that were carried out at CEA-Grenoble in order to investigate the thermal-hydraulics of the JHR core channels. The first part of the study is related to the assessment of the correlations for single-phase friction coefficients and for single-phase forced convection heat transfer. A more comprehensive modeling of single-phase flow in CATHARE is proposed by including a laminar-turbulent transition region. In addition, it is found that the turbulent heat transfer coefficient may be significantly under-estimated by standard correlations (e.g. Dittus-Boelter correlation) at high Reynolds numbers. Thus, new ad-hoc correlations were developed from the SULTAN-JHR data by making use of a best-fitting procedure. In the second part, the CATHARE two-phase heat transfer modeling is revised. Several correlations have been tested against the SULTAN-JHR experiments. The results show that the simplified Forster-Greif correlation may accurately predict the heat transfer in JHR-type channels under fully developed boiling conditions. Such a relationship is then added in CATHARE.