The impact of the distribution of the condensation energy adsorbed in the water pool on the DCC efficiency at sub-atmospheric pressure

The safety of a fusion reactor like ITER relies on the reliability of the pressure suppression system (for keeping pressure below 0.15 MPa during accidental scenarios), and, in turn, on the efficiency of Direct Contact Condensation (DCC). In this study, the role played by the distribution of the condensation energy in the water is analysed considering the experimental results of temperature and pressure recorded during steam condensation tests in a closed volume at sub-atmospheric conditions. The investigation of the mechanisms involved in the condensation at the water-steam interface is fundamental to verify that the subcooled water mass fully participates into the steam condensation for all the foreseen condensation regimes. Particularly, the paper analyses the distribution of the condensation energy in a water pool for different test conditions and with reference to the large-scale facility under operation at the DICI-University of Pisa. The subcooled water volume of the condensation tank in this system was assumed subdivided in 104 annular discrete volumes corresponding to the location of temperature and pressure sensors (testing monitoring points). Moreover, this discretization was used to implement a model to visualize the experimental data and in particular the accumulation of the condensation energy in the different part of the water pool and of the vacuum space. The experimental results showed that most of the tests had a participating fraction of water mass greater than 50%. Nevertheless, the non-uniformity and stratification of the temperature in the water pool requires a greater water mass for condensing a given steam mass. The analyses of steady state pure steam condensation tests determined that it needs to increase 1.39 times the water mass respect to the case of full participation of the water at the steam condensation.