The present invention relates to a reactor core which is applied to a water cooling reactor, and in particular, to a reactor core which has improvements in core internal structural materials, fuel design and core arrangement structure. 
In general, a light water reactor, which is a water cooling reactor, is classified into a boiling water reactor and a pressurized water reactor.  A number of fuel assemblies are charged in a reactor core of a boiling water reactor in four groups.  In each fuel assembly, a fuel coating cladding) tube is filled with a fissile material as a nuclear fuel, and a heat generated by a nuclear reaction of the fissile material is removed by a coolant.  A water is used as a coolant for removing the heat in the light water reactor. 
Hydrogen contained in water has a high neutron moderation ability, and therefore, the conventional water cooling reactor has a high ratio of water, and a high energy neutron (fast neutron) generated by the nuclear fission is greatly moderated.  Thus, a low-energy thermal neutron (slow neutron) occupies most of neutrons.  In the case where the fissile material absorbs the low-energy neutron, a fissile reaction of newly generating about three neutrons is not caused, but a ratio of a neutron capture of absorbing the neutrons in an atomic nucleus without causing the nuclear fission, becomes great.  Therefore, the number of neutrons generated per neutron absorption is reduced in the nuclear fission reaction by a low-energy neutron. 
On the other hand, in a high-energy neutron (fast neutron), since a ratio of neutron capture reaction is low and the fissile reaction is great, two or more average neutrons per neutron absorption can be generated inclusive of the neutron capture effect.  One of two or more fast neutrons newly generated is used for maintaining chain reaction, and on the other hand, the reminder thereof is absorbed in a parent material (nuclear material) such as .sup.238 U (U-238), thus, a fissionable material being effectively produced. 
In a case where a ratio of production and annihilation of the fissionable material is 1 or more, it has been found that fuel breeding is performed and a resource energy can be secured.  This is the reason why various countries have made a research and development of a breeder reactor which newly produces fissionable materials by a speed more than the development of consuming a nuclear material. 
However, in a conventional water cooling reactor, the ratio of water, which is coolant, to fuel ranges from about 2.0 to 2.5, and accordingly, a fast neutron generated by a fissile reaction is moderated, and then, becomes a low energy.  Thus, the breeding is not performed, and a ratio of production and annihilation of the fissile material was 1 or less, for example, a value of about 0.5.  Therefore, in a breeder reactor, an uranium resource, which can be theoretically converted into a thermal energy at 100%, has mot been effectively utilized, and the uranium resource effectively utilized has been merely about 1%. 
In the case of making use of a high-energy spectral neutron, in the conventional large-scale fast breeder reactor, there is the possibility that a reactivity (void reactivity) becomes positive due to the boiling of a coolant.  However, in a water cooling reactor, it is important to make negative the void reactivity in view of stability and safety of a reactor core.