Abstract: Methods and apparatuses are provided for removing thermal energy from a nuclear reactor, which are fault tolerant. The apparatus includes at least one heat pipe configured to absorb thermal energy produced by the nuclear reactor. In addition, the apparatus includes a first compartment thermally coupled to the at least one heat pipe. The first compartment is configured to contain a first gas. Furthermore, the apparatus includes a second compartment thermally coupled to the at least one heat pipe. The second compartment is configured to contain a second gas and configured to isolate the second gas from the first gas.

Abstract: In accordance with the present invention, a nuclear reactor with a recirculating heat transfer fluid has a bi-level core which provides enhanced flexibility in fuel arrangement. The bi-level core includes two sets of fuel units, one set arranged on a first level, the other set arranged on a second level. Preferably, fuel units of the second level are arranged in vertical alignment with fuel units of the first level. This permits a fuel unit of the first level to be accessed by removing only the adjacent fuel unit of the second level. During refueling operations, fuel units can be shifted from one level to the other, providing additional flexibility in arranging units at various stages of burnup. Preferably, fuel units of the first level are inverted relative to the fuel units of the second level.

Abstract: Fresh fuel bundles are inserted into core locations with small coolant flow orifices, while mid-life fuel bundles are inserted into core locatiosn with large coolant flow locations. Thus, fissile fuel production is promoted at the expense of fissioning early in a bundle lifetime, increasing the effective quantity of fuel available for energy production. Moreover, fissioning is promoted at the expense of conversion later in the bundle lifetime to enhance reactivity of the remaining fissile material and to minimize the further production of fissile fuel. The net result is longer fuel lifetimes and reduced radioactive waste.

Abstract: A reactor core includes upper and lower matrices of fuel bundles. Fuel rods in the upper matrix have their plenums oriented upward, while fuel rods in the lower matrix have their plenums oriented downward. Refueling involves removal of a first bundle in the upper matrix, removal and retirement of the bundle in the lower matrix directly below the original position of the first bundle, inversion and installation of the first bundle in the lower matrix, and installlation of a new bundle in the upper matrix. The new bundle is installed plenum-side up. The bi-level core provides greater flexibility in repositioning fuel bundles for longer burnups and lower high-level waste. In particular, problems with axial spectral variations in neutron flux can be compensated using the disclosed core arrangement and refueling procedure.

Abstract: A fuel assembly comprises a channel box, upper and lower tie plates fixed to the upper and lower portion of the channel box and a bundle of fuel rods enclosed in the channel box and retained by the upper and lower tie plates. The bundle of fuel rods includes ordinary fuel rods containing fissile material not containing gadolinia, and gadolinia-containing rods each containing gadolinia therein and having a larger outer diameter than that of the ordinary fuel rod. The gadolinia-containing rod has pellets enclosed in a closed cladding, and the outer diameters of the pellets also are larger than these of fuel pellets of the ordinary fuel rods.

Abstract: A nuclear reactor is provided with novel composite assemblies at selected locations at the periphery of the reactor core in order to reduce the rate of radiation exposure to a critical pressure vessel weld. The composite assemblies include a shield zone at a selected location close to the weld and a fuel zone incorporating fissile material. This approach reduces the rate of radiation exposure at at the weld while also contributing to production of power by the core.

Abstract: The axial power distribution of a nuclear reactor in a region of high burn-up degrees should desirably be flat. In consideration of the fact that void coefficients have a distribution in the axial direction, a mean infinite multiplication factor in an axially upper region of a reactor core is made lower than a mean infinite multiplication factor in a lower region. Especially, this becomes possible in such a way that a mean enrichment factor of fuel located in the uppr region is made lower than a mean enrichment factor of fuel located in the lower region.

Abstract: An open lattice elongated nuclear fuel assembly including small diameter fuel rods disposed in an array spaced a selected distance above an array of larger diameter fuel rods for use in a nuclear reactor having liquid coolant flowing in an upward direction. Plenums are preferably provided in the upper portion of the upper smaller diameter fuel rods and in the lower portion of the lower larger diameter fuel rods. Lattice grid structures provide lateral support for the fuel rods and preferably the lowest grid about the upper rods is directly and rigidly affixed to the highest grid about the lower rods.