GAS ENTRAINMENT PHENOMENA AFFECTING SAFETY ISSUES OF GEN IV NUCLEAR REACTORS

In the perspective of a future development of nuclear reactors, many targets should be reached in order to satisfy the GEN IV requirements. Among them, beyond the safety criteria and targets, the economy of energy produced is an important issue. With this aim, mainly for Sodium cooled Fast Reactors, reactor designs with compact vessels and high thermal power are being studied and designed. Nevertheless, owing to the high coolant velocity, the cover gas entrainment into sodium results as one of the most important safety issues in SFRs design. The gas entrainment phenomenon, in a SFR, may cause reactivity increase or fluctuation in the core, heat transfer capability reduction, entrained gas activation and instrumentation noise increase. Several studies, aimed at avoiding the formation of vortices leading gas entering into the pumps, were carried out with reference to several hydraulic systems. More recently, several studies have also been carried out. The gas entrainment, due to free surface vortex, is a very complex and unsteady phenomenon and it has been deeply studied by means of three different approaches: analytical, experimental and numerical. In this paper, several studies and models already proposed will be critically analysed with the aim as identifying advantages, disadvantages and specific limits for each model, in the view at developing an innovative approach.