Inherent Dilution: Qualification of TRIO_U on a ROCOM experiment

In this report, a generic test case concerning the “heterogeneous inherent dilution during a small break LOCA” is analyzed. During the heat transfer phase of a small break in the primary circuit, vapour, which is formed in the core region, can condense in the steam generator tubes what yields pure, unborated water. Under certain conditions, in particular in the absence of natural circulation, this pure water can accumulate in the steam generator and/or the U form intermediate leg. When the natural circulation starts, the pure water can be transported into the reactor pressure vessel with the possible risk of a criticality accident.

For the validation of Trio_U to the application to such accidents, a generic ROCOM experiment with a constant flow rate in one loop in the magnitude of natural circulation and 10% density difference between ECC and loop water was analysed. The following results have been concluded:

The Trio_U calculation shows a good agreement with the experiment, provided that the exact ROCOM geometry is used.
The LES approach led not only to a good qualitative representation of the experiment, but also a good quantitative accordance. This is true for the two tested sub-grid models, the Smagorinsky model and the WALE model.
It seems that the WALE model treats more correctly the injection phase and the formation of the cold plume in the upper downcomer region. However, the detected azimuthal displacement of the cold plume with the WALE model can lead to errors in the prediction of the core region which is affected by the injection.
The Smagorinsky model seems to underestimate the mixing phenomena during the injection phase. However, the location of the cold plume at the lower end of the downcomer was better predicted than with the WALE model
The transient of the tracer concentrations in the downcomer has been calculated on one hand for the 1:5 KONVOI geometry and on the other hand for the ROCOM geometry. The comparison of these results has shown the importance of using very precise CAD data when applying CFD.