Highly enriched uranium subcritical assemblies with and without low atomic number (low-Z) reflectors are modeled. The neutron and photon transport process in the assemblies initiated by a pulsed neutron source is simulated using the Monte Carlo code. When the size or mass density of the fissile region and the reflector changes, the effective multiplication factor k_eff of the fissile region or the whole system changes accordingly (for assemblies without reflectors, the fissile region refers to the whole system). For each case, the time dependence of leakage γ count rate, neutron population in the fissile region and their ratio is obtained and analyzed to investigate the applicability and limitation of the pulsed neutron source (PNS) method for subcritical systems with and without reflectors. It is indicated that the PNS method is suitable for systems of which the fissile region is near-critical, i.e., the k_eff value of the fissile region is close to 1. The closer the fissile region is to the critical state, the more accurately the leakage γ count rate can reflect the fissile region's fission decay properties during the specific time window. For the highly enriched uranium assemblies described in this paper, the k_eff threshold of the fissile region ensuring the applicability of the PNS method is given. In addition, the possibility of applying the PNS method under dynamic conditions are also discussed according to the calculation results.

Using density functional theory at B3LYP/6-311+G(d, p) level, the spectral characteristics and dissociation characteristics of (-0.05~0.05 a.u.) monofluorodichloroethane molecules under different applied electric fields are studied. It includes the ground state structure, total energy, dipole moment, highest occupied orbital energy level, lowest vacant orbital energy level, energy gap, infrared spectrum, Raman spectrum, UV-visible absorption spectrum and potential energy curve of between C and Cl bond under electric field. In the y-axis direction, with the increase of negative electric field, the distance between C atom and Cl nucleus increases, the highest occupied orbital energy level decreases, the total energy of the system, the lowest unoccupied orbital energy level and the energy gap increase first and then decrease, and the dipole moment decreases first and then increases. The applied electric field can affect the absorption intensity and absorption peak frequency of the infrared spectrum, Raman spectrum and UV-visible light absorption spectrum of monofluorodichloroethane. The infrared spectrum, Raman spectrum and UV-visible light absorption spectrum appear red shift or blue shift with the change of electric field. The barrier between C and Cl atoms gradually decreases with the increase of negative electric field and reaches -0.05 a.u. When C atom breaks with one of the Cl atoms, when a C—Cl bond in the molecule breaks, the applied strength is -0.04 a.u. When the electric field of, another C—Cl bond is broken, and the molecule is gradually dissociated under the electric field. The research results improve the theoretical data of the influence of external electric field on mono-fluorodichloroethane molecules.