Water Hammer Induced by the Vapor Condensation during a postulated LOCA in the Pressure Suppression System of ITER Nuclear Fusion Reactor: Experimental and Numerical Analyses

This paper deals with the qualification of the main safety systems of the nuclear fusion reactor ITER: the Vacuum Vessel Pressure Suppression System (VVPSS). This safety system manages the Loss of Coolant Accident (LOCA) that could occur in the Vacuum Vessel (VV) in order to avoid that a significant increase of the internal pressure could breach the primary confinement barrier. Steam condensation occurs at sub-atmospheric pressure. This condition is original respect the usual applications of steam direct condensation in nuclear fission reactors. An extensive experimental research program, funded by ITER Organization, on reduced scale experimental rigs (scale 1/22 and 1/10) have been carried out at the University of Pisa. Unstable condensation regimens occurred during some tests originating high intensity vibrations of the whole experimental rig. These events can be classified as Condensation Induced Water Hammer. Pressure impulses and occurrence frequencies of CIWH have been estimated elaborating the acceleration signals and implementing numerical models of the sparger. The preliminary conclusion is that the condensation at sub atmospheric pressure and the present geometry of the sparger determine a larger band of CIWH occurrence frequency than similar phenomena at atmospheric pressure. A positive achievement is that steam condensation occurs also at very small values of sub-cooling.