This work presents the results of tests and simulations of the behaviour of an experimental facility called Natural Circulation Loop (NCL) when different volumes of noncondensables are trapped in the top header of the heat exchanger and with the system operating in a two-phase flow. The main objective of this project is to study the behaviour of a passive emergency cooling system of advanced reactors. The results show that, for lower air mass fractions in the heat exchanger, the behaviour of the NCL is equivalent to the operation of a single-phase flow. For higher air mass fractions in the heat exchanger, the behaviour changes radically, presenting higher temperatures, flows and pressure variations. The air mass fraction and the geometry of the circuit at the inlet of the heat exchanger determine the two-phase flow transient. The NCL simulations are performed with the RELAP5/MOD3.3 code presenting good agreement with experimental data for smaller air mass fractions. For higher air mass fractions, the results are satisfactory only after the circuit reaches the saturation temperature.
Keywords: natural circulation, two-phase flow, advanced reactors, noncondensable gases, research and development, R&, amp, D, emergency cooling systems, nuclear reactors, nuclear energy, nuclear power, simulation, heat exchangers