This paper deals with experimental and theoretical analyses of the steam direct condensation at sub-atmospheric pressure. These operation conditions occur in the Vacuum Vessel Pressure Suppression System (VVPSS) of ITER Nuclear Fusion Reactor. This safety system permits to manage the Ingress of Coolant Event (ICE cat. IV) accident condition, postulated to occur in the ITER Vacuum Vessel (VV) as a consequence of the rupture of the shielding blanket cooling piping or the rupture of the divertor cassette piping. Experimental tests of steam direct condensation at sub-atmospheric pressures in a 1:22 scale facility were performed at the laboratory Guerrini of DICI-University of Pisa. Presently a full-scale facility is being building at DICI. This paper illustrates the similitude analysis elaborated to scale up the experimental results obtained in the reduced scale facility. The main variables which influence the steam direct condensation were analyzed and the relative scale laws determined. CFD numerical simulations of direct steam condensation in the actual geometry and in a vessel of equal volume permitted to verify some of the assumptions on which the scale laws are based. Scaling studies were first performed in order to calculate the transient of steam mass flow rate occurring during the ICE IV event in the scale apparatus, starting from the thermal hydraulic studies performed at ITER and successfully compared with the experimental results carried out in scaled apparatus. This confirmed the suitability of scale laws and in the same time the capability of the VVPSS to condense the injected steam at sub-atmospheric pressure, matching the safety goal to reduce the system pressurization.
In the International Thermonuclear Experimental Reactor (ITER), a postulated Loss Of Coolant Accident (LOCA) in the Vacuum Vessel (VV) has to be managed with a pressure suppression system working at sub-atmospheric pressure. The operating conditions considerably differ from those experienced in the fission nuclear power plants such as BWR, since the ITER Tokamak works at very low pressure conditions and can withstand a maximum pressure of 0.15 MPa. For this reason, the pressure value must not exceed 10 kPa for a water temperature of 30 °C inside the Vapour Suppression Tanks (VSTs) that are the fundamental components of the Vacuum Vessel Pressure Suppression System (VVPSS). During a LOCA some non-condensable gases (mainly hydrogen and oxygen gases due to the water radiolysis or thermolysis) may be mixed in the steam and this could impair the condensation efficiency. In order to investigate the effects of non-condensable gas on DCC, we conducted a research program funded by ITER Organization at the laboratory of the University of Pisa: we designed and built a small-scale experimental rig to study the steam Direct Contact Condensation (DCC) with the presence of non-condensable gas and simulate the behaviour of a VST. Since DCC can occur with different characteristics, we ran 12 closed mode tests exploring all condensation regimes injecting a certain mass of air with the steam discharged in the subcooled water. The tests started at the saturation pressures corresponding to water temperatures ranging from 40 °C to 80 °C and ended when the free space volume reached the atmospheric pressure. From the analysis of the data acquired during the tests we observed that the condensation efficiency remained higher than 95%. We observed that despite this, the presence of a certain quantity of non-condensable gas has negative aspects on condensation: the condensation regime never reaches stability (the regimes quickly shift towards instability). Furthermore, the presence of air triggers a turbulence of the flow which interferes with the transfer of heat from the steam to the water. Not only the introduction of air into the flow increases linearly the pressure above the water head but its high temperature further contributes to the pressure increase. The air mixed with the flow also forms eddies that can trap the steam and transport it out of the water, preventing it from condensing. Apart from condensation, the most noticeable problem is the rapid increase in pressure inside the condensation tank.
This paper deals with experimental tests of steam condensation in a water pool at atmospheric pressure and temperature in the range 15-100 °C. The activity is performed in the frame of a research program, funded by the ITER organization, for the study of dust deposition, produced in the ITER Vacuum Vessel and entrained by the steam and non condensable gas into the Pressure Suppression Tanks in the case of a Loss of Coolant Accident. The steam condensation into a subcooled water pool has been investigated to characterise the condensation regimes occurring during dust deposition tests. The dust distribution on the tank walls strongly depends on the steam jet length and on the effective heat transfer coefficient. Few grams of dust reduce the water transparency, therefore separated tests without dust overcome this drawback. Measurements of the lengths and surfaces of the steam jets (which permit to calculate effective average heat transfer) have been performed by means of image analyses and have been compared with theoretical correlations. The comparison showed a good agreement between the experimental data and theoretical correlations. Useful data have been obtained for implementing numerical models of dust deposition and for understanding the dust distribution on the tank wall obtained in the experimental tests.
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