Assessing water-wall behaviour for a light-water Small Modular Reactor with the aid of CFD analyses

The paper summarises the results of a work performed in the frame of the EU ELSMOR project in the aim to characterise the behaviour of a reference Small Modular Reactor equipped with a passive containment system cooled by a surrounding water-wall. The work started with a review of the available literature, aimed at identifying experimental data possibly in a suitable scaling range, useful to draw conclusions about the decay heat rejection capabilities of such systems at full scale. This search for suitable data encountered difficulties, since the downscaling in height of most of the existing experimental facilities does not allow for reaching the target Rayleigh numbers envisaged for free convection in full scale reactor conditions (Ra similar to 10(16)). In the lack of sufficient experimental information, it was firstly tried to reproduce downscaled experimental data by CFD; then, CFD was used to predict the water-wall behaviour in full scale configurations, on the basis of parametric assumptions about size and boundary conditions. The results help in sketching the phenomena that can be expected in reactor conditions at full scale, providing inter alia suggestions about pool mixing and the time needed to reach bulk boiling conditions. 2D, 3D and lumped parameter models were adopted using a range of assumptions to assess relevant phenomena. A study of the effect of geometrically downscaling facilities was also performed by CFD analyses. The obtained predictions of the system behaviour with different scales and assumptions address the present knowledge gap and are useful to highlight relevant phenomena on which experimental activities should be focused in the future.