FAST (FLOATING ABSORBER FOR SAFETY AT TRANSIENT) AS A SOLUTION FOR POSITIVE COOLANT TEMPERATURE COEFFICIENT IN SODIUM-COOLED FAST REACTORS

摘要

Floating absorber for safety at transient (FAST) is apassive safety device for sodium-cooled fast reactors. It isdesigned to insert negative reactivity in case of coolanttemperature rise or coolant voiding in an inherentlypassive way. Sinking or emerging of the floating absorbermodule in a FAST device is equivalent to insertion ofnegative or positive reactivity, respectively. First, designprinciples and methodologies of the FAST device aredescribed in detail in this study. Movement of the floatingabsorber module during the anticipated transients withoutscram (ATWS) is investigated in a sodium-cooled breedand burn fast reactor (B&BR). ULOF (Unprotected Lossof Flow), ULOHS (Unprotected Loss of Heat Sink) andUTOP (Unprotected Transient Overpower) scenarios aresimulated at both MOL (Middle-of-Life) and EOL (Endof-Life) conditions of the B&BR to investigate impacts ofFAST on the reactor safety. The ATWS simulations areperformed with an in-house computer code taking intoaccount all possible reactivity feedback componentsincluding the FAST movement and core expansioncoefficients. It is confirmed that FAST can verysuccessfully mitigate the consequences of ULOF, ULOHSand UTOP even when the B&BR core has stronglypositive coolant temperature coefficient. In addition, thispaper provides key aspects of FAST design optimizationfor practical applications.