Nuclear reactor core

In a reactor core loaded with fuel assemblies using steams as main heat removing coolants, transuranium elements minor actinide (MA) such as neptunium (Np), americium (Am) and curium (Cm) are enriched in at least a portion of fuel rods in the fuel assembly. Then, the fuel assembly is cooled by steams flowing between the fuel rods axially. In the steam-cooled reactor, assuming pressure as 13 MPa, and the temperature of the saturation steams as 331degC at the inlet and 550degC at the outlet, the steam density during rated power operation causes a change from about 0.06g/cc to about 0.03g/cc as distribution in the reactor. MA addition factor is increased for effective utilization while considering the difference. This contributes to the increase of reactor shutdown margin upon flooding of a steam cooled fast neutron spectra reactor and annihilation of MA nuclides during reactor operation, thereby improving the reactor core performance. (T.M.)