Neutron transport and thermalization studies with beryllium oxide.
Creators: name: Rainbow, M. T
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis presents the results of some experimental investigations into the behaviour of neutron populations in the polycrystalline moderator beryllium oxide. The first part describes an examination of the decay in time of 'thermalized' neutron populations in eighteen small moderator assemblies of various shapes and sizes. The experimental results confirm the existence of continuous energy modes and reveal the existence of a rather sharp transition between exponential and non-exponential decay of the neutron population at a buckling of 3.0 x 10-2 cm-2. The observed upper bound to decay constants which could be associated with a discrete asymptotic energy mode is not consistent with the limit predicted by theory. This incon- sistency is considered to be evidence for the existence of neutron population decay which is associated with a 'pseudo discrete' energy mode. The transition region exhibits a slight dependence on the shape of the moderator assembly and there is evidence of shape-dependence in the decay of neutron populations in assemblies with large buckling. These measurements constitute the first detailed examination of the decay of neutron populations in small moderator assemblies and of the theoretical predictions concerning continuous energy modes. In addition, they provide an explanation of some confusing experimental observations of the past. The second part of the thesis is concerned with a pulsed thermalization experiment which involved the use of plutonium-239 as a spectral indicator. An experimental technique has been developed to include a consideration of spatial effects and enable the measurement of space-indepen- dent time-dependent reaction rates of spectral indicators. This technique has been applied successfully in measurements with a boron (1/v) detector and the spectral indicator plutonium-239. The experimental results indicate that the theoretical basis of the technique, viz the assumption that the neutron flux in a finite moderator assembly may be expanded in terms of Fourier spatial modes, is very good indeed; even at times immediately after the injection of a fast neutron pulse into the large (60. 96 x 60. 96 x 58.~-2 cm3) moderator assembly. The experimental technique has involved the measurement of the time-dependent reaction rate of the detectors associated with the various Fourier spatial modes. These appear to exist as quite separate identities associated with different energy spectra and are, therefore, directly comparable with the results of time-dependent zero-dimensional multigroup calculations. The measured time-dependent reaction rates of plutonium-239 and boron corresponding to the fundamental Fourier spatial mode have been compared with the results of zero-dimensional multigroup diffusion calculations which involved the use of a crystal scattering kernel for beryllium oxide. The theoretical and experimental results are in disagreement. This discrepancy is discussed and possible explanations are presented. The boron measurements have enabled the time-dependent response due to the non-1/v (resonance) part of the plutonium-239 fission cross section to be extracted from the total time-dependent response of the plutonium-239 detector. This response has been examined for evidence of the existence of higher discrete energy modes for beryllium oxide. That part of the time-dependent response due to the non-1/v part of the plutonium-239 fission cross section which reflects the decay of the Oa3 eV neutron population associated with the fundamental Fourier mode exhibits quite marked non-exponential behaviour. This is considered to be an indication that no higher discrete energy modes exist for the polycrystalline moderator beryllium oxide. The lack of any evidence in the experimental results of the existence of a higher discrete energy mode, combined with the theoretical prediction that no such mode exists for polycrystalline moderators and the results of the invest- igations of neutron population decay in small moderator assemblies reported in this thesis, suggests that time dependent neutron spectra in beryllium oxide must be represented by the sum of a single discrete energy mode (which may, in some circumstances, not exist) and a continuous mode.