A floating absorber for safety at transient (FAST) is a passive safety system to enhance the inherent safety of sodium-cooled fast reactors (SFRs). This study optimized and improved the original FAST design in terms of the initial position, barycenter, eccentricity, reactivity worth, and diameter ratio of FAST to pin (k) in advanced burner reactor (ABR) cores. In contrast to previous studies based on average assembly analysis, a comprehensive design optimization study was conducted through transient simulations based on multiassembly analysis. Applying a damped FAST, which is an improvement over the original FAST design, enhanced the negative reactivity feedback in the oxide cores depending on the diameter ratio. Oscillatory behaviors were observed when the k of the damped FAST system was within 0.1 ∼ 0.55 for $1 of the FAST reactivity under an unprotected loss of flow event (ULOF). A damped FAST with a k of 0.7 maintained the lowest coolant temperature while suppressing the oscillatory behavior for $1 of the FAST reactivity. The results showed that damped FAST systems support negative reactivity feedback for the ABR oxide core and should be properly designed to avoid power and temperature oscillations depending on the FAST reactivity worth.