G. Fruttuoso

Experimental Investigation of Mixing Phenomena Inside a Concrete Containment Cooled by an Innovative Passive System

Several advanced nuclear plant concepts are characterized by the use of innovative cooling systems that remove the heat released inside the containment following a hypothetical accident, such as a loss-of-coolant accident, through passive heat transfer mechanisms. The design and installation of a localized passive containment cooling system (PCCS) inside a double-wall concrete containment requires the reliable knowledge of temporal and spatial distribution of noncondensable gas concentration, especially hydrogen, in a multicompartment geometry. Testing was conducted in the Large-Scale Containment Test Facility located at the Westinghouse Science and Technology Center in Pittsburgh, and the testing was modified to simulate in approximately one-tenth scale the main features of a concrete containment, designed by the Italian National Electric Utility (ENEL), in which the heat is removed through internal heat exchangers (HX) located in the dome region, and connected by an intermediate fluid loop to external HXs placed outside the double barrier concrete containment. No active component like pumps or human intervention are required for the operation of the system. The facility instrumentation, the test program, and the experimental results are described along with the first results obtained in the application of the FUMO code to the analysis of these experimental tests. The experimental data measured during the tests include temperature distributions inside the containment, helium concentrations at four internal locations, and laser Doppler anemometer measures to determine the atmosphere mixing under different simulated accident conditions. The experimental results indicate that helium, which simulates the hydrogen that may be released during some accident sequences, is distributed rather homogeneously inside the facility. The very good mixing exhibited by the helium indicates that the localized PCCS induces efficient convective motions inside the containment atmosphere, and this is a positive indication for safety analysis.