Application of COCOSYS code for investigation of gas mixing in mistra test facility

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

In the case of a severe accident in a water-cooled nuclear power plant large amounts of hydrogen could be generated due to fuel claddings oxidation and released to the containment. At certain concentrations of steam air and hydrogen the hydrogen combustion could occur and challenge the structural integrity of the containment, which is a last barrier preventing from radioactive material release to the environment. Therefore, a detailed knowledge of containment thermal-hydraulics is necessary to predict the local distribution of hydrogen, steam and air inside the containment. This paper presents the experience of Lithuanian Energy Institute in simulation of the experiments performed in MISTRA test facility for the case of the International Standard Problem ISP47. The MISTRA facility is located in the Saclay center of France Atomic Energy Commissariat (CEA) and is related to the research of containment thermal-hydraulics and hydrogen safety. The MISTRA facility and its operating conditions are designed with reference to the containment conditions of a pressurized water reactor (PWR) in accident situation. The facility comprises containment inside which three condensers are set up and external circuits. Containment volume is ~100 m3, with an internal diameter of 4.25 m and a height of 7.3 m. Containment is not temperature regulated, but preheated by steam condensation and thermally insulated. The relevant physical phenomena for simulation are the following: 1)centered steam and helium (instead of hydrogen) injection in the containment; 2) pressure and temperature increase in the containment; 3) wall condensation at regulated wall temperature; and 4) flow pattern in the containment and resulting gas temperature and concentration distribution. Test sequence consisted of several transient and steady state stages, when the measurements of the gas temperature and gas concentration profiles where performed. The presented analyses were performed employing the code COCOSYS versions V2.0v2 and V2.3 developed at GRS mbH (Germany). COCOSYS is a lumped-parameter code for the comprehensive simulation of all relevant phenomena, processes and plant states during severe accidents in the containment of light water reactors. The free convection, forced convection, radiation heat transfer and condensation may be considered in the analysis. The condensation model is based on the heat and mass transfer analogy (Stefan’s law). The water and gas flows are calculated separately, i.e. different junctions have to be specified for these flows. Several zone models could be selected by the user. The EQUIL._MOD zone model assumes the perfect steam, gas and water mixture inside a zone. Each component of the mixture is in thermal equilibrium. NONEQUILIB model considers the water and gas mixture, which is not necessarily in thermal equilibrium, i.e. water and gas may have different temperatures and calculated separately in the energy balance. The experimental and analytical analyses showed that gas stratification was not observed and well-mixed atmosphere conditions were reached for the investigated case.