Modelling of debris bed reflooding in PEARL experimental facility with MC3D code

• Published in: Nuclear engineering and design 2018-04, Vol.330, p.450-462
• Main Authors: Kokalj, Janez, Uršič, Mitja, Leskovar, Matjaž, Piar, Libuse, Meignen, Renaud
• Format: Article
• Language: English
• Subjects: Atmospheric models; Boundary conditions; Bypasses; Computer simulation; Configurations; Damage prevention; Debris; Debris bed coolability; Debris bed reflooding; Detritus; Entrainment; Fluidity; Heat transfer; Mathematical analysis; Nuclear accidents & safety; Nuclear energy; Nuclear engineering; Nuclear power plants; Nuclear safety; Physics; Porosity; Porous media; Regression analysis; Severe accident; Steam; Three dimensional models; Two dimensional models; Water flow; Water temperature
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2018.02.016

•MC3D simulations of PEARL experiments are presented.•The general trends of the flows are recovered and explained.•The 2-D behaviour, with faster progression in the bypass, is highlighted and explained.•The impact of porosity in the 2-D reflooding configurations is presented. A hypothetical severe accident in a nuclear power plant has the potential for causing severe core damage, including a meltdown. To prevent or in the case of an already formed debris bed to limit the in-vessel core degradation, the basic severe accident management strategies consider the in-vessel reflooding to ensure the debris bed coolability. The purpose of our research was to understand the key processes and conditions related to the in-vessel debris bed coolability in the bottom reflooding conditions. Recently, experimental tests in the PEARL facility (IRSN, France) were performed to highlight the behaviour of the steam and water flow in a hot porous medium and to provide experimental data to validate 2-D and 3-D models for the debris bed reflooding. Our aim was to analyse chosen PEARL experiments performed at the atmospheric pressure. The objective was to analyse the importance of the uncertainties in the initial and boundary conditions on the simulation results and to assess the heat transfer modelling approaches. Simulations were performed using the MC3D code (IRSN, France). In general, the performed simulations are in good agreement with the experiments. The general features, in particular the water preferential entrainment in the bypass are recovered and the analysis of calculation gives further information on the mechanisms. In particular, the mechanism of water deviation in the bypass (2-D behaviour) is described. The hypothesis of water dragged by steam coming from the debris bed region cannot be supported. However, the simulation results are indicating a noticeable impact of the actual conditions as the water temperature and the initial support bed and bypass temperature. The simulations, varying the porosity of the test section, showed that this impact affects the flow configuration and is important for cases with the 2-D configuration. The reflooding capabilities in this configuration may depend strongly on the characteristics of the debris bed. Changes in the heat transfer modelling do not have greater effect on the simulation results.