Condensation efficiency in two-phase computer code FRACAS

In order to achieve a better understanding and quantification of the safety injection efficiency, the COSI test program has been launched, with test conditions representative of both actual cold leg SI geometries and typical small break conditions. The results of the first test series have shown relatively high SI efficiency, amounting to almost perfect condensation in a lot of cases. The phenomenology of the concensation process has been established and divided into four contributors : on the SI jet itself, in the mixing zone created by the impact of the jet, in the cold leg outside the mixing zone, and finally in the downcomer in case of low liquid level. After data reduction, the thermalhydraulic network code FRACAS has been modified with the condensation model derived from the COSI tests. A typical small break calculation has been performed which shows a significant improvement in terms of core mass inventory due to direct and indirect effects of greater SI efficiency, and demonstrates the conservatism of the current model. (orig.)