Void Fraction In A Co-Current Two-Phase Flow Through A Prototypical Pwr Spent Fuel Assembly

The Fukushima accident has led in the last decade to numerous studies on the Spent Fuel Pools (SFP) accident phenomenology. In this context, the IRSN has launched an experimental project named DENOPI with the aim to study the behavior of spent fuel pools under loss of cooling and loss of coolant conditions. Within this project, the MEDEA facility is intended to study the thermal-hydraulics of a fully covered or uncovered spent fuel bundle. It consists in a hydraulic loop connected to an experimental test section in which a simulated one meter high PWR spent fuel bundle is located. The MEDEA-overflow experiments, presented in this study, consist in an adiabatic air/water flows in a fully covered unheated bundle. The pressure is measured all along the fuel bundle using several pressure transducers and the void fraction evolution is estimated and analyzed. Two different flow re-gimes are put in evidence in the MEDEA-overflow test series. Indeed, at low gas superficial velocities, a bubbly flow set up within the bundle whereas a transition to slug flow was obtained at larger ones. Finally, the results of several drift flux models are analyzed and compared to the experimental measurements.