Initially charged nuclear reactor core

Purpose: To flatten the reactor power by changing the composition of fuel assemblies in the initially charged nuclear reactor core. Constitution: The amount of fissionable materials in fuel assemblies are changed depending on the reactor-core-staying time from charging to discharging so as to decrease the amount of the fissionable materials in the initially charged reactor core. The average amount of the fissionable materials of the fuel assemblies taken out at the former half period is made small, where latter half period, that is, uranium fuels or low plutonium-enriched materials with high initial reactivity are used. The average amount of the fissionable materials in the fuel assemblies taken out at the latter half period is made larger than that taken out at the former half period, and the fuel assemblies mainly composed of fissionable materials of high plutonium-enriched fissionable materials are used so as to have low initial stage reactivity. As a result, the difference in the reactivity at the initial stage of the reactor core cycle between the fuel assemblies taken out at the initial stage and the latter stage can be decreased to reduce the power peaking coefficient. (Kamimura, M.)