A method of manufacturing nuclear fuel elements may include: forming a graphite base portion of the fuel element; depositing a first layer of graphite spheres on the base portion; depositing a first layer of fuel, burnable poison and/or breeder particles on the first layer of graphite spheres; forming a second layer of graphite spheres on the first layer of particles; depositing a second layer of fuel, burnable poison and/or breeder particles on the second layer of graphite spheres; and forming a graphite cap portion of the fuel element. Fuel, burnable poison and/or breeder particles of the first layer may be are spaced apart by substantially the same distance, and fuel, burnable poison and/or breeder particles of the second layer may be spaced apart by substantially the same distance. The fuel element may be a spherical fuel pebble. The fuel particles may be tri-structural-isotropic (TRISO) particles without an overcoat.

A nuclear reactor may comprise: fuel comprising or breeding plutonium-239; a neutron moderator, such as ZrH.sub.x, where x is about 1.6, YH.sub.2, TiH.sub.2 and/or ThH.sub.2, which behaves as an Einstein oscillator and as the temperature of the reactor increases the moderator increases the energy of thermal neutrons into the Pu-239 neutron absorption resonance; and a neutron absorbing element with strong neutron absorption around 0.3 eV added to one or more components of a reactor core of the nuclear reactor, wherein the neutron absorbing element is provided in an amount calculated to suppress, at any time during the life of the fuel, a reactivity gain with temperature due to the neutron moderator increasing the energy of thermal neutrons into the Pu-239 neutron absorption resonance.

A nuclear reactor may comprise: fuel comprising or breeding plutonium-239; a neutron moderator, such as ZrH.sub.x, where x is about 1.6, YH.sub.2, TiH.sub.2 and/or ThH.sub.2, which behaves as an Einstein oscillator and as the temperature of the reactor increases the moderator increases the energy of thermal neutrons into the Pu-239 neutron absorption resonance; and a neutron absorbing element with strong neutron absorption around 0.3 eV added to one or more components of a reactor core of the nuclear reactor, wherein the neutron absorbing element is provided in an amount calculated to suppress, at any time during the life of the fuel, a reactivity gain with temperature due to the neutron moderator increasing the energy of thermal neutrons into the Pu-239 neutron absorption resonance.

A nuclear fission reactor fuel assembly adapted to permit expansion of the nuclear fuel contained therein. The fuel assembly comprises an enclosure having enclosure walls to sealingly enclose a nuclear fuel foam defining a plurality of interconnected open-cell voids or a plurality of closed-cell voids. The voids permit expansion of the foam toward the voids, which expansion may be due to heat generation and/or fission gas release. The voids shrink or reduce in volume as the foam expands. Pressure on the enclosure walls is substantially reduced because the foam expands toward and even into the voids rather than against the enclosure walls. Thus, the voids provide space into which the foam can expand.

A nuclear fission reactor fuel assembly adapted to permit expansion of the nuclear fuel contained therein. The fuel assembly comprises an enclosure having enclosure walls to sealingly enclose a nuclear fuel foam defining a plurality of interconnected open-cell voids or a plurality of closed-cell voids. The voids permit expansion of the foam toward the voids, which expansion may be due to heat generation and/or fission gas release. The voids shrink or reduce in volume as the foam expands. Pressure on the enclosure walls is substantially reduced because the foam expands toward and even into the voids rather than against the enclosure walls. Thus, the voids provide space into which the foam can expand.

The new features of an accelerator driven subcritical reactor disclosed by this invention include the multiple intake ports connected to the reactor vessel for delivering protons from one or more accelerators to accommodate the full length LWR spent fuels for furnishing the desirable neutron distribution in a subcritical core to incinerate nuclear wastes. This is based on the notion of adopting the spent fuels in intact form to feed directly to the newly designed subcritical core. External modulators in the proton intake ports have the ability of splitting the fluxes and adjusting their energy from one or more accelerators to form multiple proton streams arriving at different axial locations in the spallation target for creating multiple neutron sources. The new design could combine the cycles of reprocessing spent fuels, manufacturing fuels for reuse, and incinerating minor actinides into one single cycle.

The new features of an accelerator driven subcritical reactor disclosed by this invention include the multiple intake ports connected to the reactor vessel for delivering protons from one or more accelerators to accommodate the full length LWR spent fuels for furnishing the desirable neutron distribution in a subcritical core to incinerate nuclear wastes. This is based on the notion of adopting the spent fuels in intact form to feed directly to the newly designed subcritical core. External modulators in the proton intake ports have the ability of splitting the fluxes and adjusting their energy from one or more accelerators to form multiple proton streams arriving at different axial locations in the spallation target for creating multiple neutron sources. The new design could combine the cycles of reprocessing spent fuels, manufacturing fuels for reuse, and incinerating minor actinides into one single cycle.

An aqueous assembly has a negative coefficient of reactivity with a magnitude. The aqueous assembly includes a vessel and an aqueous solution, with a fissile solute, supported in the vessel. A reactivity stabilizer is disposed within the aqueous solution to reduce the magnitude of the negative coefficient of reactivity of the aqueous assembly during operation of the aqueous assembly.

An aqueous assembly has a negative coefficient of reactivity with a magnitude. The aqueous assembly includes a vessel and an aqueous solution, with a fissile solute, supported in the vessel. A reactivity stabilizer is disposed within the aqueous solution to reduce the magnitude of the negative coefficient of reactivity of the aqueous assembly during operation of the aqueous assembly.

A fuel assembly for a nuclear reactor, a fuel rod of the fuel assembly, and a ceramic nuclear fuel pellet of the fuel rod are disclosed. The fuel pellet includes a first fissile material of UB.sub.2, The boron of the UB.sub.2 is enriched to have a concentration of the isotope .sup.11B that is higher than for natural B.