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.

Some embodiments include a method comprising: flowing a molten salt out of a molten salt reactor at a first temperature, heating the molten salt reactor to a second temperature above the melding point of the second salt mixture causing the second salt mixture to melt; flowing the second salt mixture out of the molten salt reactor; flowing a third salt mixture into the molten salt reactor; and cooling the molten salt reactor from the second temperature to a third temperature causing the third salt mixture to solidify on the interior surface of the housing. In some embodiments, the molten salt may include a first salt mixture comprising at least uranium. In some embodiments, the first temperature is a temperature above the melting point of the first salt mixture.

Disclosed is an entropy-controlled solid solution matrix BCC alloy having strong resistance to high-temperature neutron radiation damage. The entropy-controlled solid solution matrix BCC alloy includes three or more multicomponent main elements selected from the element group consisting of Zr, Al, Nb, Mo, Cr, V, and Ti selected based on a neutron absorption cross-sectional area and a mixing enthalpy. Each of the elements is included in an amount of 5 to 35 at %, and the entropy-controlled solid solution matrix BCC alloy is a BCC-structure solid solution matrix alloy in a medium-entropy to high-entropy state. In this invention, damage caused by neutron radiation is reduced, and entropy is controlled to thus ensure a solid solution matrix BCC structure having a slow diffusion speed, and accordingly, resistance to void swelling due to radioactive rays is high.

Disclosed is an entropy-controlled solid solution matrix BCC alloy having strong resistance to high-temperature neutron radiation damage. The entropy-controlled solid solution matrix BCC alloy includes three or more multicomponent main elements selected from the element group consisting of Zr, Al, Nb, Mo, Cr, V, and Ti selected based on a neutron absorption cross-sectional area and a mixing enthalpy. Each of the elements is included in an amount of 5 to 35 at %, and the entropy-controlled solid solution matrix BCC alloy is a BCC-structure solid solution matrix alloy in a medium-entropy to high-entropy state. In this invention, damage caused by neutron radiation is reduced, and entropy is controlled to thus ensure a solid solution matrix BCC structure having a slow diffusion speed, and accordingly, resistance to void swelling due to radioactive rays is high.

A control assembly for a nuclear reactor includes a first reactivity control assembly having a first neutron modifying material, a second reactivity control assembly having a second neutron modifying material, and at least one drive mechanism coupled to the first neutron modifying material and the second neutron modifying material. The first neutron modifying material and the second neutron modifying material are selectively repositionable relative to a fuel region of the nuclear reactor. The at least one drive mechanism is configured to provide the first neutron modifying material and the second neutron modifying material in different directions through the fuel region thereby shifting a flux distribution within the fuel region away from the second neutron modifying material.

The modular lower moving system for nuclear fuel handling includes: a lower reactor vessel assembly including nuclear fuel loaded therein; a carrier having a space allowing the lower reactor vessel assembly to be accommodated therein; a rail extending from a reactor area to a fuel handling area; a transfer cart horizontally movable along the rail; a lifting device installed at the transfer cart, movable upward or downward with respect to the transfer cart; and a drive device configured to supply power to the transfer cart and the lifting device. The method of refueling nuclear fuel using the modular lower moving system includes a carrier lifting process, a lower reactor vessel assembly detachment process, a process of accommodating the lower reactor vessel assembly in the carrier, a carrier lowering process, a transfer cart movement process, a nuclear fuel offloading process, and a nuclear fuel loading process.

A mitigation assembly for a nuclear reactor including a box with an upper portion forming the head of the assembly housing an upper neutron shielding device, including a head including removable lock and a slug installed free to move in translation relative over a given travel distance, the lock being configured such that locking/unlocking between the head and the box can be made by displacement of the slug with an extraction grab with its pawls attached in the slug. The lower part of the upper neutron shielding device includes a cone-shaped sealing block with the tip of the cone oriented downwards, cooperating with a cone-shaped internal surface of the box, a sealing device being formed between the two, the assembly created forming a removable sealing plug.

A mitigation assembly for a nuclear reactor including a box with an upper portion forming the head of the assembly housing an upper neutron shielding device, including a head including removable lock and a slug installed free to move in translation relative over a given travel distance, the lock being configured such that locking/unlocking between the head and the box can be made by displacement of the slug with an extraction grab with its pawls attached in the slug. The lower part of the upper neutron shielding device includes a cone-shaped sealing block with the tip of the cone oriented downwards, cooperating with a cone-shaped internal surface of the box, a sealing device being formed between the two, the assembly created forming a removable sealing plug.

A reflector assembly for a molten chloride fast reactor (MCFR) includes a support structure with a substantially cylindrical base plate, a substantially cylindrical top plate, and a plurality of circumferentially spaced ribs extending between the base plate and the top plate. The support structure is configured to encapsulate a reactor core for containing nuclear fuel. The MCFR also includes a plurality of tube members disposed within the support structure and extending axially between the top plate and the bottom plate. The plurality of tube members are configured to hold at least one reflector material to reflect fission born neutrons back to a center of the reactor core.

A heat transfer (exchange) composition comprising a halide salt matrix having dispersed therein nanoparticles comprising elemental carbon in the absence of water and surfactants, wherein said halide is fluoride or chloride, wherein the halide salt may be an alkali halide salt (e.g., lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, sodium chloride, potassium chloride, rubidium chloride, and eutectic mixtures thereof) or an alkaline earth halide salt (e.g., fluoride or chloride salt of beryllium, magnesium, calcium, strontium, or barium), and wherein the nanoparticles comprising elemental carbon may be solid or hollow, and wherein the composition may further include nanoparticles comprising a fissile material (e.g., U, Th, or Pu) dispersed within the composition. Molten salt reactors (MSRs) containing these heat transfer compositions in coolant loops in thermal exchange with a reactor core, as well operation of such MSRs, are also described.