Patent Number: 
Section: claims

1. A reactor core immersed in a liquid metal coolant in a core barrel of a liquid metal cooled reactor, the reactor core comprising:a plurality of fuel assemblies contained in the core barrel;a neutron absorber that absorbs a neutron in the reactor core; anda neutron moderator that moderates a neutron therein so as to control a reactivity of the reactor core,the neutron absorber and the neutron moderator constituting a mixture contained in reactivity control assemblies of the reactor core in the liquid metal coolant prior to immersion of the reactor core, and the neutron moderator being composed of zirconium hydride. 2. The reactor core according to claim 1, further comprising:a reactivity control assembly arranged at a predetermined portion in the fuel assemblies, said reactivity control assembly having a tube portion; anda neutron absorber rod assembly contained in the tube portion and having the mixture made of the neutron absorber and the neutron moderator,wherein said neutron absorber rod assembly has a plurality of neutron absorber rods each having a cladding tube and the mixture filled therein,said mixture is made of the neutron absorber and the neutron moderator,said absorber rod assembly is arranged at a center portion of the fuel assemblies,said neutron absorber and the neutron moderator is mixed according to a volume percent ratio,said volume percent ratio of the neutron moderator and the neutron absorber is X to Y, said X percent being bigger than the Y percent, andsaid volume percent ratios of the neutron moderator and the neutron absorber in the neutron absorber rod in the axial direction are different according to different positions in the axial direction of the core.