Neutronic characteristics of FLWR in the transition phase changing from high conversion core to breeder core

Akie, Hiroshi  ; Nakano, Yoshihiro ; Okubo, Tsutomu

Innovative Water Reactor for Flexible Fuel Cycle (FLWR) is a low moderation water reactor which can realize Pu breeding and multiple recycling. And for the introduction stage, a high conversion (HC) type FLWR is also proposed to keep technical continuity from current LWRs. When the HC type core is shifted to the breeder (BR) core, there exist both types of fuel assemblies in the same core configuration. The power distribution in the HC + BR assemblies mixed core configuration is studied, because there might appear a power peaking in the adjacent region between HC and BR assemblies due to the difference in neutron spectrum. As a result, though a power peaking can be very large in the adjacent regions between the assemblies, the power distribution can be effectively flattened by considering a rod-wise fuel enrichment distribution and by optimizing the fuel assembly loading pattern. It is expected that FLWR can be shifted from HC type to BR type without major neutronic difficulties.