Parametric study highlighting cooler and spray activation on the transient mixing of a three-gas composition in the PANDA facility

Abstract

In postulated accident scenarios with the release of steam and hydrogen in the containment, for some nuclear power plant designs it is foreseen that sprays and coolers are activated to condense steam and mitigate the containment pressure build-up. The operation of the sprays and the coolers induce flow with the effect of mixing a stratified atmosphere which reduces local hydrogen concentration. However, steam condensation could lead to the formation of combustible air-hydrogen mixtures. Therefore, for the management of a severe accident, it is crucial to know the overall effect of spray and cooler activation on the containment pressure history and gas-mixture concentration evolution. In this article, the results of a series of five experiments, identified as HP4 (HYMERES PANDA series 4) series, are presented. The HP4 series was part of the OECD/NEA HYMERES project, and the main objective was to investigate the combined effect of a cooler and a spray in the containment atmosphere. The HP4 series was performed in the large-scale thermal-hydraulic facility PANDA facility (PSI, Switzerland). In the HP4 test matrix, the cooler design (with cooling tube partially or fully enclosed in a cooler case) and the spray design (full cone or hollow cone nozzle) were varied. Moreover, the spray water temperature was an additional parameter. The HP4 series revealed that the containment pressure history and the gas mixture composition during the postulated scenarios are strongly affected by spray and cooler safety systems whereas the details of the spray and cooler design are of minor importance.The containment pressure during the experiments increased from atmospheric up to 2.5 bar and maximum power removed by the cooler was about 100 KW. The spray activation induced steam condensation and the highest de-pressurization induced by the combined spray and cooler operation was about 0.7 bar, i.e. from 2.5 bar to 1.8 bar.