A fast neutron nuclear reactor contains a nuclear reactor core having an array of device locations. Some device locations in the nuclear reactor core contain fissile and fertile nuclear fuel assembly devices. One or more other device locations in the nuclear reactor core contain Doppler reactivity augmentation devices that amplify the negativity of the Doppler reactivity coefficient within the nuclear reactor core. In some implementations, a Doppler reactivity augmentation device can also reduce the coolant temperature coefficient within the nuclear reactor core. Accordingly, a Doppler reactivity augmentation device contributes to a more stable nuclear reactor core.

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.

A core of a fast nuclear reactor has at least one gas expansion module, at least one neutron absorber, and at least one neutron moderator. The gas expansion module has a hollow tubular structure with one end closed and the other end opened. The at least one neutron absorber adjoins an outer face, in a radial direction of the core, of each gas expansion module. The at least one neutron moderator adjoins an outer face, in a radial direction of the core, of each gas expansion module. The at least one neutron absorber also adjoins the at least one neutron moderator.