Local feedback reactivities effect on power distributions during control rod withdrawal

Kawashima, Katsuyuki; Mizuno, Tomoyasu

As a part of the FaCT Project, the local feedback reactivities effect on power distributions during control rod withdrawal was studied for a large fast reactor. In the UTOP transient analysis based on the point reactor kinetics model, power distribution is usually given by the steady-state core neutronics calculation performed prior to the transient analysis. However, the power distribution during transient could be different from the one in the steady-state core condition because of local feedback reactivities effect induced by the temperature rise in the fuel assemblies adjacent to the withdrawn control rod. The core calculation model is configured to take into account the transient temperature rise. It is shown that the core radial power distribution skewing is significantly mitigated by considering the local feedback reactivities due to temperature rise. The calculated maximum power density ratio is 1.30, compared with 1.39 calculated under the steady-state core condition.