Experimental study on 3D large-scale debris bed formation using sub-millimeter particles for core catcher design optimization

• Three representative debris-bed morphologies were identified in underwater sedimentation experiments. • Debris-bed formation showed a clear regime transition around a Stokes number of unity. • Fluid-driven lateral transport controlled bed shape at low Stokes numbers. • Particle inertia and interparticle interactions dominated bed formation at high Stokes numbers. • Empirical correlations for outer slope angle reproduced the data within ± 13%. In a core disruptive accident of a sodium-cooled fast reactor, molten fuel is rapidly quenched by liquid sodium and fragmented into fine debris particles, which subsequently settle and form a debris bed. The debris bed shape plays a crucial role in determining coolability; however, the physical mechanisms governing debris-bed formation under three-dimensional conditions remain insufficiently understood. In this study, large-scale three-dimensional water-pool experiments were conducted using sub-millimeter to millimeter-sized particles to investigate debris-bed formation during underwater sedimentation. The debris bed shape was characterized using several geometric indicators, including the inner cavity diameter, outer diameter, maximum bed height, and outer slope angle. The results show that debris bed formation can be interpreted as the cumulative outcome of multiple physical processes occurring during particle sedimentation, namely momentum conversion associated with a granular jump near the bottom surface, particle transport toward the outer region driven by fluid motion and dispersion, and particle rearrangement near the bed periphery. By comparing the geometric indicators with relevant dimensionless parameters, a clear transition in the dominant physical behavior was identified around a Stokes number of unity, separating particle–fluid interaction–dominated and particle–particle interaction–dominated regimes. Based on this regime distinction, empirical correlations for the outer slope angle were formulated using physically motivated dimensionless parameters. The proposed correlations reproduce the experimental data with prediction errors comparable to the experimental repeatability, demonstrating that the dominant features of debris-bed formation can be quantitatively captured through dimensionless scaling grounded in experimentally observed physical mechanisms.