Design methodology of multi-hole spargers to prevent steam coalescence at sub-atmospheric pressure

In case of an in-vessel Loss Of Coolant Accident (LOCA), flash steam can be released in the Vacuum Vessel (VV) of the International Thermonuclear Experimental Reactor (ITER) causing its pressurization. To avoid this, the safety system named Vacuum Vessel Pressure Suppression System (VVPSS) will intervene sending the steam to four Vapour Suppression Tanks (VSTs) through a multi-hole sparger and condenses via Direct Contact Condensation (DCC).

To support the design of the multi-hole sparger, which is a key safety component of VVPSS, at the University of Pisa two testing facilities were designed and built in order to study and qualify the VVPSS, named Small Scale Test Facility (SSTF) and Large Scale Test Facility (LSTF).

During the experimental tests performed using LSTF with a VVPSS multi-hole full scale sparger, under certain conditions, the coalescence of the steam jet plumes resulted in the formation and collapse of large, isolated steam bubbles which produced high pressure loads at low frequency on the structure and flow reversal of the pool water inside the sparger.

To limit these large pressure loads, a methodology is needed to prevent the coalescence of the steam jet plumes.

With this aim, an image analysis of 15 experimental tests performed using SSTF was performed to develop and validate a correlation of the ratio between the maximum radius of the steam jet plumes and the hole diameter. Subsequently, two limiting radii for multi-hole spargers (named r₁ and r₂) were determined which allow avoiding the partial and the transitional complete coalescence of the steam jet plumes when compared to the maximum radius. The proposed methodology is new and quite innovative, and it was applied and validated by using the several videos recorded during the transient test performed using sparger B, consisting of DN450 pipe with 1000 holes.

The correlation estimates that partial coalescence and transitional to complete coalescence regions are avoided when the water subcooling temperature ranges between 37–45 °C and 25–31 °C, respectively, as observed in the recordings of the cameras. Results allow to identify the sparger design dimensions preventing the steam jet plumes coalescence, and avoiding the onset of excessive dynamic loads.