The Settled Bed Fast Reactor (SBFR) concept features a packed bed of fuel (directly cooled with sodium) which must be highly resistant to consolidation during power operation in order to avoid reactivity excursions resulting from sudden increases in bed solid fraction. In the SBFR design, the stability of the packed fuel bed is considered accpetable if a 12-g lateral shock produces a change in bed solid fraction of less than 0.002 (e.g., from 0.630 to 0.632), equivalent to a 10¢ reactivity change. Experimental results show that beds settled from fluidization can be compacted to exhibit 1/6 of this change when shock tested. The particle interlocking effect of simulated coolant downflow gives a substantial extra measure of stability.