[en] The changes in reactivity resulting from the initial fuel and clad motions that occur during an unprotected loss-of-flow (ULOF) accident in LMFBRs strongly influence the accident's course. To study these accident-induced phenomena, a series of in-pile experiments using small bundles of pins (the Sandia Transient Axial Relocation [STAR] experiments) were performed. In these experiments, ULOF-accident conditions were simulated, and the accident-induced clad motion and fuel dispersal were recorded using high-speed photography. The major parameters varied in the series were power level, type of fuel (fresh versus preirradiated), and the number of pins. The results of the first four experiments are presented here. The observations made in these experiments show that early fuel disruption caused by fission products occurs in ULOF accidents and is followed almost immediately by both fuel sweepout and axial clad relocation. 36 refs., 22 figs., 2 tabs