CORE OF FAST REACTOR

PURPOSE:To obtain a core of a fast reactor which enables attainment of a larger output with an improvement in safety, by lessening a factor of an increase in the positive reactivity of the core at the time of abnormality such as the time when the flow rate of a coolant lowers, by reducing the void reactivity of the core due to the coolant to be small. CONSTITUTION:A core of a fast reactor comprises a core region 22 wherein a nuclear fuel material is held and void assemblies 33 which are provided through this core region in parallel to the center in the axial direction of the core region. The void assembly 33 is formed of a cladding tube 53 which has a gas cavity 41 formed in the inner upper part, an exothermic heat generating material provided in the lower part in the axial direction and a coolant inflow port 44 and a coolant outflow port bored in the lower part and in the middle part in the axial direction.