Assessment and improvements of thermal-hydraulic correlations and methods for the analysis of the Jules Horowitz Reactor

Abstract: Nuclear research reactors are used to test materials for current and future nuclear technologies, and to produce radioisotopes for medical purposes. Most of the existing Material Testing Reactors in Europe have operated for more than 50 years and new ones are needed. Therefore the Jules Horowitz Reactor (JHR) is under construction at the French Alternative Energies and Atomic Energy Commission (CEA), on the Cadarache site.

The JHR will allow irradiation experiments with high neutron fluxes, at fast and thermal energies. In order to cope with the considerable heat fluxes generated during operations, the core configuration consists of fuel assemblies with parallel narrow channels, where coolant flows at high velocity. Such a design is unique and specific simulation capabilities have to be developed for the analysis.

This doctoral research investigates possible improvements of the thermal-hydraulic modeling of the JHR, and is arranged in three parts.
In the first part, correlations for the single-phase turbulent friction and heat transfer, for the fully developed boiling heat transfer, and for the critical heat flux, respectively, are assessed and their accuracy is quantified, against the SULTAN-JHR experiments. These experiments were carried out in heated narrow channels comparable to the JHR ones. It is shown that the single-phase turbulent correlations valid for standard nuclear systems, can perform poorly when applied to the typical conditions of the JHR. Thus, new best-fitting relationships are derived. For fully developed boiling in narrow channels, the Forster-Greif correlation can be considered a reliable option. As regards the modeling of the critical heat flux, the Sudo correlation can provide satisfactory predictions. These results are then used to modify the thermal-hydraulic system code CATHARE for the purpose of a more realistic analysis of the JHR.

The second part is focused on the onset of flow instability, which is a primary concern in systems with parallel channels as the JHR, since it can lead to undesirable boiling crisis. In view of this, several criteria are evaluated with experiments in narrow channels from both the SULTAN-JHR program and the literature. Conservative predictions can be obtained with Saha-Zuber KIT correlation. Furthermore, some criteria are optimized with respect to the available experimental data for narrow channels.

In the third part, the analysis of a postulated accident in the JHR, namely a station black-out, is performed with a best-estimate plus uncertainty approach, combined with the CATHARE code as modified in the first part of the thesis. As a result, the impact of different input and modeling uncertainties on the simulation is estimated, and the most influential uncertain parameters are identified.