Nuclear reactor

Purpose: To improve the operationability and economical performances as well as increase the fuel margin by decreasing the number of exchange for control rod patterns. Constitution: The content of neutron absorbers in a control rod to be inserted into the reactor core during normal operation is decreased as compared with that in the control rods other than the above to thereby decrease the reactivity worth of the control rod while taking notice of the relationship between the control rod hystory effect and the neutron absorber content in the control rod. Furthermore, fuel assemblies of a relatively low reactivity are arranged around the control rods to be inserted within the reactor core during such a normal operation to thereby control the control rod hystory effect. This enables a reactor operation for a long period of time by an identical control rod pattern to thereby improve the operation and economical performance, as well as the safety of the reactor. (Yoshihara, H.)