Bowing reactivity analysis of FFTF core

Yamaoka, Mitsuaki; Hayashi, Hideyuki

A passive safety test phase IIB is planned at the FFTF (Fast Flux Test Facility) core to assess the reactivity feedback effect related to passive safety feature of FBRs, especially the effect due to core deformation. For pre-test analyses of the test, a bowing reactivity analysis has been carried out for FFTF core. The bowing reactivity is analyzed based on core displacement data evaluated postulating ULOF (Unprotected Loss of Flow) event at 30% rated flow. In the analysis, fuel reactivity worth distribution is expressed as function on the reference core without deformation and the bowing reactivity is calculated based on the first-order perturbation theory. This report summarizes the relationships between power to flow ratio and the bowing reactivity with clearance between subassembly load pads and that between the core and the core restraint system as parameters. Followings are main results. (1)As the power to flow ratio increases, a positive reactivity is added to the core by the core deformation until clearance between subassembly load pads doses. This is due to the inward displacement of active core caused by mechanical interactions of subassemblies. (2)After the closure of clearance between subassembly load pads, the active core begins to move outwards, and a negative reactivity is added to the core. (3)The deformation behavior of the outermost subassemblies of the core dominates the bowing reactivity since both the magnitude of deformation and the reactivity effect for unit displacement are large compared with those of others. For the analysis, a code for bowing reactivity calculation has been developed. The calculation method and the manual are also presented in this report.