Passive safety device to improve coolant temperature coefficient of reactivity for sodium-cooled fast reactors

Abstract

This paper presents FAST (Floating Absorber for Safety at Transient) which is a passive safety device for Sodium-cooled Fast Reactors (SFR). FAST is designed to insert negative reactivity in case of coolant temperature rise or coolant voiding in an inherently passive way. Sinking or emerging of the floating absorber module in a FAST device is equivalent to insertion of negative or positive reactivity, respectively. First, design principles and methodologies of the FAST device are described in detail in this study. Behavior and effects of the FAST during the anticipated transients without scram (ATWS) in a sodium-cooled breed and burn fast reactor (B&BR) are investigated to see its impacts on the B&BR safety. ULOF (Unprotected Loss of Flow), ULOHS (Unprotected Loss of Heat Sink) and UTOP (Unprotected Transient Overpower) scenarios are simulated at both MOL (Middle-of-Life) and EOL (End-of-Life) conditions of the B&BR core to investigate impacts of FAST on the reactor safety. The ATWS simulations are performed with an in-house computer code taking into account all possible reactivity feedback components including the FAST movement and core expansion coefficients at MOL and EOL. It is confirmed that FAST can very successfully mitigate the consequences of ULOF, ULOHS and UTOP even when the B&BR core has a strongly positive coolant temperature coefficient. In addition, this paper provides key aspects of FAST design optimization for practical applications.