Methods, systems, and devices for producing hot fluid from a reactor are disclosed. A mix of moderator particles, fuel particles, and reflector particles are configured to produce heat by nuclear reactions.

Methods, systems, and devices for producing hot fluid from a reactor are disclosed. A mix of moderator particles, fuel particles, and reflector particles are configured to produce heat by nuclear reactions.

A control rod for a nuclear fuel assembly is described herein that includes a neutron absorbing material having a melting point greater than 1500 C. that does not form a eutectic with a melting point less than 1500 C., and may further include a cladding material having a melting point greater than 1500 C. The cladding material is selected from the group consisting of silicon carbide, zirconium, a zirconium alloy, tungsten, and molybdenum. The absorbing material is selected from the group consisting of Gd.sub.2O.sub.3, Ir, B.sub.4C, Re, and Hf. The metal cladding or the absorbing material may be coated with an anti-oxidation coating of Cr with or without a Nb intermediate layer.

A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.

A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.

The present invention relates to a small modular reactor including small fuel assemblies. The small modular reactor comprises: a reactor body; and a nuclear fuel material which is accommodated in the reactor body, includes a plurality of fuel assemblies, and has a section arrangement region. The section arrangement region includes a first arrangement region and a plurality of second arrangement regions that are located outside the first arrangement region and spaced apart from each other. The fuel assemblies include: first nuclear fuel assemblies that each have a first size and are closely arranged in the first arrangement region; and second nuclear fuel assemblies that each have a second size smaller than the first size and are arranged in the second arrangement region.

The present invention relates to a small modular reactor including small fuel assemblies. The small modular reactor comprises: a reactor body; and a nuclear fuel material which is accommodated in the reactor body, includes a plurality of fuel assemblies, and has a section arrangement region. The section arrangement region includes a first arrangement region and a plurality of second arrangement regions that are located outside the first arrangement region and spaced apart from each other. The fuel assemblies include: first nuclear fuel assemblies that each have a first size and are closely arranged in the first arrangement region; and second nuclear fuel assemblies that each have a second size smaller than the first size and are arranged in the second arrangement region.

A method is provided for operating a nuclear reactor. The method includes operating the nuclear reactor for at least one plutonium equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies; subsequently, operating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced with transition nuclear fuel assemblies and then with uranium-equilibrium nuclear fuel assemblies; and then operating the nuclear reactor for at least one uranium equilibrium cycle.

A method is provided for operating a nuclear reactor. The method includes operating the nuclear reactor for at least one plutonium equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies; subsequently, operating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced with transition nuclear fuel assemblies and then with uranium-equilibrium nuclear fuel assemblies; and then operating the nuclear reactor for at least one uranium equilibrium cycle.

A method of stabilizing density wave oscillations in boiling water reactor cores is disclosed. The invention introduced a thin metallic fuel element made of fissile isotope baring metal encased and bonded with metallic cladding. The thin construction and the metallic material guarantee very short thermal time constant compared with the oscillation period. Although the feedback of the processes involved in density waves are negative, i.e. oppose any initial perturbation, sufficiently strong feedback may result in unstable behavior because of the time delay inherent in the propagation of the density wave and the heat conduction delay in the traditional fuel rods made of ceramic pellets encased in zircaloy tubes. The new fuel element of this invention introduces fast, not delayed, thermal energy to the coolant in response of any neutron flux perturbation, and thus introduces a stabilizing feedback. Boiling water reactor fuel bundles may benefit from this invention by including the new fuel element as part of its array of fuel rods, preferably filling in the space vacated by part-length fuel rods.