A variable diameter fuel rod of a nuclear reactor assembly is disclosed. The variable diameter fuel rod includes an elongated cladding tube configured to house a plurality of fuel pellets including a fissile material arranged in a fuel stack orientation. The elongated cladding tube includes first and second axial reflector regions and a middle axial region therebetween. The middle axial region comprises an outer diameter defined as d.sub.1. The first and second axial reflector regions include an outer cladding diameter defined as d.sub.2 and d.sub.3, respectively. The variable diameter fuel rod further includes a transitional region between the diameter d.sub.1 of the middle axial region and the diameter d.sub.2 of the axial reflector region. The diameter d.sub.2 of the axial reflector region is greater than the diameter d.sub.1 of the middle axial region.

A variable diameter fuel rod of a nuclear reactor assembly is disclosed. The variable diameter fuel rod includes an elongated cladding tube configured to house a plurality of fuel pellets including a fissile material arranged in a fuel stack orientation. The elongated cladding tube includes first and second axial reflector regions and a middle axial region therebetween. The middle axial region comprises an outer diameter defined as d.sub.1. The first and second axial reflector regions include an outer cladding diameter defined as d.sub.2 and d.sub.3, respectively. The variable diameter fuel rod further includes a transitional region between the diameter d.sub.1 of the middle axial region and the diameter d.sub.2 of the axial reflector region. The diameter d.sub.2 of the axial reflector region is greater than the diameter d.sub.1 of the middle axial region.

Provided is a secondary shutdown structure of a nuclear reactor, which uses sliding doors, and more particularly, to a secondary shutdown structure of a nuclear reactor, which uses sliding doors and is capable of shutting down a nuclear reactor reliably with a simple structure without using a boric acid solution.

Provided is a secondary shutdown structure of a nuclear reactor, which uses sliding doors, and more particularly, to a secondary shutdown structure of a nuclear reactor, which uses sliding doors and is capable of shutting down a nuclear reactor reliably with a simple structure without using a boric acid solution.

An aft plenum assembly for use with a nuclear thermal reactor including a pressure vessel and a nozzle assembly having a top plenum plate disposed within the pressure vessel, the top plenum plate defining a first plurality of fuel flow apertures, a bottom plenum plate disposed within the pressure vessel, the bottom plenum plate being parallel to the top plenum plate thereby defining a plenum space therebetween, the bottom plenum plate defining a second plurality of fuel flow apertures, and a plurality of tubular connections extending between the first plurality of fuel flow apertures of the top plenum plate and the second plurality of fuel flow apertures of the bottom plenum plate, wherein the aft plenum assembly is disposed between the pressure vessel and the nozzle assembly.

A modular nuclear reactor system includes a lift-out, replaceable nuclear reactor core configured for replacement as a singular unit during a single lift-out event, such as rather than lifting and replacing individual fuel assemblies and/or fuel elements. The system includes a reactor vessel and a power generation system configured to convert thermal energy in a high temperature working fluid received from the reactor vessel into electrical energy. The reactor vessel includes: a vessel inlet and an adjacent vessel outlet arranged near a bottom on the vessel; a vessel receptacle configured to receive a unified core assembly; locating datums in the base of the vessel receptacle and configured to constrain a core assembly in multiple degrees of freedom; and an interstitial zone surrounding the vessel receptacle and housing a set of control or moderating drums.

A composite structure is disclosed comprising a neutron-absorbing metal hydride phase contained within a matrix having a density of greater than 95%. In various embodiments the metal hydride is a hydride of one or more of the following: Gadolinium, Hafnium, Europium, Samarium. The composite structure is utile as a shield for fusion or fission reactors.

A composite structure is disclosed comprising a neutron-absorbing metal hydride phase contained within a matrix having a density of greater than 95%. In various embodiments the metal hydride is a hydride of one or more of the following: Gadolinium, Hafnium, Europium, Samarium. The composite structure is utile as a shield for fusion or fission reactors.

The present relates to the integration of the primary functional elements of graphite moderator and reactor vessel and/or primary heat exchangers and/or control rods into an integral molten salt nuclear reactor (IMSR). Once the design life of the IMSR is reached, for example, in the range of 3 to 10 years, it is disconnected, removed and replaced as a unit. The spent IMSR functions as the medium or long term storage of the radioactive graphite and/or heat exchangers and/or control rods and/or fuel salt contained in the vessel of the IMSR. The present also relates to a nuclear reactor that has a buffer salt surrounding the nuclear vessel. During normal operation of the nuclear reactor, the nuclear reactor operates at a temperature that is lower than the melting point of the buffer salt and the buffer salt acts as a thermal insulator. Upon loss of external cooling, the temperature of the nuclear reactor increases and melts the buffer salt, which can then transfer heat from the nuclear core to a cooled containment vessel.

The reactor core includes at least one module, a solid neutron moderator, and a liquid neutron moderators. Each module contains a casing, at least one heat pipe, one fuel element and thermal insulation. The heat pipe comprises a casing, a wick, and a coolant. The fuel element is made of nuclear fuel, arranged along an evaporation area of the heat pipe, around the heat pipe casing, in thermal contact with the heat pipe casing, and enclosed in a can. Liquid metals are used as the coolant of the heat pipe. Thermal insulation is arranged between the can and the module casing. At least one hole is made in the solid neutron moderator. Each module is arranged within a respective hole of the solid neutron moderator. The space between the module casing and the solid neutron moderator is filled with a liquid neutron moderator.