Two fuel elements containing 0.05 wt% B4 C in 6% enriched UO2, were irradiated in the Saxton reactor to burnups of ∼1000 and 5000 MWd per metric ton of uranium at peak heat ratings of 17.2 and 19.8 kW/ft, respectively. These elements were fabricated by vibratory compaction to densities of 88 ± 2% of theoretical with local boron concentrations maintained within a variation of ∼ ± 20% of the nominal loading. The postirradiation examination revealed no significant dimensional changes in either element and no axial boron redistribution of any consequence. However, the boron migrated radially outward in both irradiated fuel elements. The boron redistribution does not appear to be a function of burnup but depends heavily on the thermal gradient during irradiation. Its effect on core physics was analyzed using the THERMOS program, and the changes in ηf(ratio of neutrons produced to thermal neutrons absorbed) and Δρ (difference in core reactivity) were found to be minimal.