Sensitivity Analysis on Safety Limits of Large FBR Cores for Seismic Vertical Vibration of Control Rods

Abstract

Liquid metal fast breeder reactor (LMFBR) plants are necessary to have system components with thin wall structures that are much different from those of LWR plants, and so some reactivity vibrations are induced by seismic vertical movements between the core and the control rod system. Dynamic response analyses during these vibratory transients were performed and the maximum allowable displacements of the core relative to the control rod system were evaluated with a view to avoid fuel failures. Two two-region homogeneous and axially heterogeneous oxide cores, and a two-region homogeneous metal core were taken as the reference large FBR cores. The amplitude and the frequency of control rod vibrations, three kinds of reactivity coefficients and scram conditions were selected as main parameters for sensitivity analyses, and integrity limits of fuel assemblies were evaluated and the results of reactivity amplitude were converted into vertical displacements to have the target requirements for the aseismic design. The allowable maximum reactivity amplitudes were 1.19, 1.15 and 0.89$, and the corresponding allowable relative displacements between the core and the control rod system were 44, 52 and 42 mm, respectively for the above-mentioned three cores in the severest condition. Effects of seismic vibration characteristic, control rod scram time and other affecting factors were discussed based on the final results.