A nuclear reactor provided with a core including a new fuel part which contains uranium and a burning part in which fuel burns, wherein the burning part moves in a direction toward the new fuel part from the beginning to end of the operation cycle. The nuclear reactor is provided with a reactivity applying mechanism to apply the reactivity which can change the power of the core when the temperature of the coolant which flows through the inside of the core changes and performs control to change the temperature of the coolant which flows through the inside of the core in accordance with the change of power which is demanded for the core. The reactivity applying mechanism includes a gap adjusting plate which supports fuel members. This plate is configured to expand when the core coolant temperature rises. The expansion increases distance between the fuel members.