A Fuel Rod Debris Packing Model

The nonrandom packing of fuel rod debris around and above the surviving fuel rod segments in a degraded core was analyzed with the spacer grids modeled as a porous floor. The irregular shape of the debris was simulated by assuming that all of the spherical particles terminate their migration within the debris bed at their first two-point contact. The analytical approach was verified by comparing the computational results with experimental data for nonrandom packing. Specific calculations for the Three-Mile Island Unit 2 geometry reveal an average (horizontally integrated) nonrandom packing density between the fuel rods of ∼0.30. If simulated vibrations are imposed, this value increases to 0.50. If the debris bed builds up above the fuel rod stubs, the average (horizontally integrated) packing density above these rods reaches a value of ∼0.38 without vibrations; loosely packed gravel has an average random packing density of 0.45.