Debris Bed Formation HOME > RESEARCH > Severe Accident> Debris Bed Formation

This numerical simulation is related with the accident progress in ex-vessel Severe Accident for Light Water Reactors (LWRs), or Sodium-cooled Fast Reactors (SFRs). When the molten core material penetrates the lower plenum of pressure vessel, it is settled into the cavity bottom with fragmented debris particle formation by the interaction with coolant (could be water or sodium). The formation of debris bed is a crucial factor in terms of long-term coolability because the internal and external structures of debris bed determine the inflow degree of coolant [1,2,3]. Therefore, the experimental and numerical approaches for debris particle settlement were performed in Kyushu University, and the two-dimensional DEM (Discrete Element Method) simulation was used in the numerical part [4]. However, there is some limitations for showing realization such as multi-dimensional effects.

Fig. 1. Contact theory of inter-particle collision in DEM.
In this research, the three-dimensional two-way coupling CFD-DEM simulation was calculated and the results are compared between experimental and numerical data [4]. The comparison shows that there are some good agreements with the experimental and numerical data. Also, the characteristics of two-way CFD-DEM algorithm and the analysis for simulation results are shown in this research.
Fig.2. Schematic diagram of two-way coupling.

Fig.3 shows the particle sedimentation with transient time from 0 s to 1 s. This figure shows the three-dimensional data results by POSTECH. Also, the quantitative data for comparison are maximum dispersion angle, falling time of first particle, maximum bed height, and particle bed distribution.

Fig. 3. Numerical data of of POSTECH with 0 s, 0.5 s and 1 s.

There are a number of application using the CFD-DEM algorithm. First, the Discrete Element Method (DEM) algorithm adopts the simplified model for particle-particle interaction, to simulate massive particle flows in various engineering applications including powder technology, food engineering, civil engineering, and more. In addition, in the special condition like the fluid flow affects largely on the trajectory of a solid particle such as mineral processing, fluidized bed, or bubble column reactor, the analysis for fluid regime is a key parameter which can be calculated by the Computational Fluid Dynamics (CFD) using the CFD-DEM algorithm.

[1] W. Ma and T. N. Dinh, The effects of debris bed’s prototypical characteristics on corium coolability in a LWR severe accident, Nuclear Engineering and Design, 2010.
[2] E. Kim, W. H. Jung, J. H. Park, H. S. Park, and K. Moriyama, Experiments on Sedimentation of Particles in a Water Pool with Gas Inflow. Nuclear Engineering and Technology, 48, No.2, 457-469, 2016.
[3] E. Kim, M. Lee, H. S. Park, K. Moriyama, and J.H. Park, Development of an ex-vessel corium debris bed with two-phase natural convection in a flooded cavity. Nuclear Engineering and Design, 298, 240-254, 2016.
[4] M. Shamsuzzaman, B. Zhang, T. Horie, F. Fuke, T. Matsumoto, K. Morita, and Y. Tobita, Numerical study on sedimentation behavior of solid particles used as simulant fuel debris. Journal of Nuclear Science and Technology, 51, No.5, 681-699, 2014. p>