Experimental investigation of gas transport induced by containment spray activation during a scaled severe accident scenario

Abstract

Abstract We present the results of an experimental study on the activation of sprays representative of those used in a generic light water reactor containment for severe accident mitigation. Depending on the initial conditions, the spray nozzle type and the spray water flow rates, the spray may cause higher hydrogen concentration during depressurization due to steam condensation, or it may erode the hydrogen stratification by enhanced mixing. To investigate these phenomena, the tests were performed in the PANDA facility at Paul Scherrer Institut, Switzerland in the frame of the ERCOSAM-SAMARA project. The experiments followed a test sequence based on a postulated severe accident scenario scaled from a nuclear power plant design. The scaled scenario consisted of the formation of a stratified helium-rich (substitute for hydrogen) atmosphere in one of the PANDA vessels followed by the activation of a spray. Temporal evolution and spatial distribution of the gas temperature and the gas concentrations at selected positions were measured using thermocouples and mass spectrometers. Four tests were performed with two different spray nozzle types, two spray flow rates and two initial vessel wall temperatures of 105 °C and 135 °C, which created condensing and non-condensing environments, respectively. The different initial conditions lead to different density stratification. The effect of these different initial and boundary conditions on the transport and mixing of gases in the vessels due to the activation of the spray is revealed by these tests. The spray causes a fast breakup of the stratification and a complete mixing in the vessel in which it was injected. The concentration measurements in the adjacent connected vessel showed the presence of helium at the bottom of the vessel suggesting the possibility of hydrogen transport to the lower parts of the containment in a three gas mixture environment.