Fast reactor

Purpose: To improve the operation performance, fuel economy, etc. by suppressing the increase in the maximum power density in the axial direction of the reactor core upon insertion of control rods. Constitution: Among the control rods disposed in a ring-like arrangement, power conditioning control rods comprise two kind of control rods to and from which neutron absorption materials are inserted or withdrawn from above and below the reactor core region in which nuclear fissionable materials are enriched. The strain in the axial power distribution is larger near the control rods and exponentially decreased as aparting from the control rods. In the case of a fast breeder, since the average free path of the neutrons till they undergo interaction with fuels and coolants is longer, the control rod effect reaches further as compared with the case of water-cooled reactor. Accordingly, two kind of control rods are alternately disposed as much as possible over the entire reactor core. In this way, the strain in the axial power distribution near the control rods can be decreased. The thermal margin is increased thereby enabling to improve the fuel integrity and also the fuel economy. (Kamimura, M.)