Floating absorber for safety at transient (FAST) is a passive safety device for sodium-cooled fast reactors. It is designed to insert negative reactivity in case of coolant temperature rise or coolant voiding in an inherently passive way. This study investigates behavior of FAST device during the anticipated transients without scram (ATWS) in a sodium-cooled breed-and-burn fast reactor (B&BR). Unprotected loss of flow (ULOF), unprotected loss of heat sink (ULOHS) and unprotected transient overpower (UTOP) scenarios are simulated at both MOL (middle of life) and EOL (end of life) conditions of the B&BR to investigate the impacts of FAST on the reactor safety. It is confirmed that FAST can very successfully mitigate the consequences of ULOF, ULOHS and UTOP even in the B&BR core with strongly positive coolant temperature coefficient. In addition, this paper provides key aspects of FAST design optimization for practical applications. (C) 2019 Elsevier Ltd. All rights reserved.