Method of reactor operation

Purpose: To make the ratio of number of hydrogen atoms and fuel atoms closer to the optimum value thereby improve the excess reactivity control amount and the fuel economy. Method: For the central portion of the reactor core, a 4-batch dispersion system of replacing fuel assemblies once for 4 operation cycles is employed and, in the peripheral portion of the reactor core, fuels at an advanced burning stage having stayed for 4 cycles in the central portion of the reactor core are loaded and taken out of the reactor core after one cycle of stay. In this case, at the first to the third cycle, fuels having a greater diameter water rod are loaded and, at the forth and fifth cycles, they are exchanged with fuel assemblies in which graphite rods of small neutron mederation effect are inserted into the larger diameter water rods within a range of 1/2 from the lower end of the effective fuel length. (Y.H.)