Experimental Characterization of the Emergency Heat Removal System of IRIS reactor,

Today several projects regarding innovative nuclear power plants include Natural Circulation (NC) based systems for core cooling during accident scenarios. IRIS reactor is an innovative Small-Medium size Reactor (335MWe) developed by an international consortium led by Westinghouse. The main safety system of IRIS reactor is the Emergency Heat Removal System (EHRS), a closed, two-phase flow, natural circulation loop linking the in-vessel helically coiled steam generators to a condenser submerged into the Refuelling Water Storage Tank (RWST). The loop is designed with the aim of transferring a power larger than the core decay from the reactor coolant to the environment. The system starts its operation following a closure of main steam and feed lines valves and the opening of cold leg valve. The absence of a loop pressurizer simplifies the layout and leads to a °ßsliding pressure°® behavior of the system, with operation conditions (temperature, pressure, flow rate, rejected power) that depend mainly on the loop water inventory. The EHRS Filling Ratio (FR), i.e. the ratio between the stored water mass in the loop during its operation and the maximum storable water mass in cold conditions, is a key parameter for the loop performances.Experimental investigations are needed to duly characterize the system behavior and to validate computational codes for power plant simulation.The experimental campaign was carried out at SIET-LEAP labs in Piacenza (Italy) on a full height (∼20m) loop, simulating the IRIS Emergency Heat Removal System. The heat source is the 32m length, electrically heated, helically coiled tube, simulating one full scale tube of the IRIS Steam Generator. The heat sink is a 250 litres, water pool, open to the atmosphere, where a single, slightly inclined, condenser pipe (1 m length, 59 mm i.d.) is submerged.The high surface-to-volume ratio for the scaled loop, if compared with the full scale one, required to compensate the thermal losses via electrical heating of cold and hot legs.Several Filling Ratio conditions were explored during the campaign, to capture its impact on the loop behavior. An experimental matrix including electrical power, steam generator inlet throttling and non condensable presence parameters was tested.An operation map, linking the loop operative pressure with the pool condenser extracted power and the Filling Ratio was obtained, showing the monotonically increasing effect of power and Filling Ratio on the system pressure.Steam generator inlet throttling, despite the significant impact on loop flowrate, have shown to be a second order effect on operative pressure.Two main thermalhydraulic dynamic features were found during loop operation. The first one was detected in all the explored runs and consisted in small amplitude pulsations of the flow rate with periods of the order of ten seconds (high frequency oscillations-HFO). Fast Fourier Transform (FFT) was applied in order to capture the main frequencies involved in HFO. First harmonic was correlated with system pressure showing the strong link between loop operative pressure and first harmonic of HFO. The probable role of vapour phase compressibility in determining HFO first harmonic period has been suggested.The second dynamic feature characterized by very low frequency (time periods of hundreds of seconds), high amplitude oscillations of flow rate, pressure and temperatures has been detected in the highest explored Filling Ratio (FR=0.79). This type of oscillations is characterized by the presence of a very low mixture quality (few percents) flowing in the riser of the circuit. A simple mechanistic explanation based on the multiple feedbacks between riser gravitational pressure drops, steam generator inlet subcooling and loop flow rate is proposed.