Fluent analysis of a ROSA cold leg stratification test

One of the OECD ROSA project tests, investigating temperature stratification in the cold legs and the downcomer during ECCS water injection under single-phase natural circulation conditions was analysed with the FLUENT code. The guidance given in the “Best Practice Guidelines for the Use of CFD in Nuclear Reactor Safety Applications” of the OECD GAMA group was followed. Steady-state calculations were performed with the Standard k–ɛ, the Realizable k–ɛ and the Reynolds Stress Model, the last one being closest to the measured results. The calculations indicate the predominance of buoyancy effects in the cold leg caused by the density difference between cold and hot water, while in the test it seems, as if mixing between the cold plume and hot water would be the prevailing mechanism. It is shown that the temperature distribution in the downcomer is strongly influenced by correct modelling of the cold leg-downcomer connection. A model with an abrupt transition leads to the colder fluid flowing to the core barrel, while in the test it was flowing down along the vessel wall. Modelling the rounded transition of the ROSA facility shifts the cold stream towards the vessel wall.