Energy spectrums of slowed neutrons in heterogeneous uranium

Monte Carlo simulations using Geant4 and OpenMC were performed to study the energy spectra of moderated neutrons in various fission media. The study examined homogeneous uranium dicarbide and uranium dioxide, as well as heterogeneous structures involving uranium/carbon and thorium/carbon channels with different compositions and lattice parameters. In the homogeneous uranium dicarbide medium, the spectral maximum was found between 20 to 50 keV, which is relevant for designing fast single-channel reactors operating in a traveling wave fission mode with a soft fast neutron spectrum. A heterogeneous thorium and carbon medium was identified that produces a thermal neutron spectrum, enabling the realization of a traveling wave mode of neutron nuclear fission on thermal neutrons. By adjusting channel lattice parameters and introducing burnable absorbers such as Cadmium or Indium, a super thermal neutron spectrum ranging from 1 to 20 eV was demonstrated in a heterogeneous uranium and carbon medium. These findings are presented as useful for developing nuclear transmutation reactors and next-generation reactors operating in the traveling wave mode.

The research establishes a clear physical link between the material composition and achievable neutron energy spectra within fission media, directly addressing design considerations for advanced reactor concepts. The progression from moderately fast neutron spectra in homogeneous materials to highly thermalized or super-thermal spectra in heterogeneous systems highlights a pathway toward tailoring neutronics environments. The concept of utilizing thermal neutrons for fission, as demonstrated in the thorium-carbon system, suggests an avenue for novel reactor modes beyond conventional fast spectrum operation. Furthermore, demonstrating the ability to engineer a narrow energy range (1 to 20 eV) through lattice engineering and burnable absorbers moves the discussion from simple spectral observation to active control over neutron population dynamics. The implication is that material science parameters are not merely passive structural constraints but active variables in controlling the functional mode of nuclear processes. Future investigation should focus on scaling these findings to larger-scale, coupled system simulations to assess the practical engineering challenges and feedback mechanisms inherent in these postulated traveling wave modes within transmutation reactor designs.