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Neutron transport and thermalization studies with beryllium oxide.

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Rainbow, M. T 1969 , 'Neutron transport and thermalization studies with beryllium oxide.', PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis - PhD
Authors/Creators:Rainbow, M. T
Keywords: Neutrons
Copyright Holders: The Author
Copyright Information:

Copyright 1969 the author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.) - University of Tasmania, 1969. Bibliography

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