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Pulsed neutron experiments in thorium

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Pheng, Moo Siew 1973 , 'Pulsed neutron experiments in thorium', PhD thesis, University of Tasmania.

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Abstract

This thesis presents the results of some experimental and theoretical
investigations into the behaviour of fast neutrons in a pulsed thorium assembly.

The experiments constitute the first detailed examination of the space-time
characteristics of fast neutrons in a heavy metal system. The space and time
dependent fission rates of 237Np, 235U and 239 Pu are measured following
the injection of a short burst of fast neutrons of mean energy - 2.7 MeV into
the centre of a thorium assembly of size 0.4 x 0.4 x 0.4 m3 .

Analysis of the measured spatial distributions
have been directed towards the extraction of the space independent .
time dependent reaction rates .associated with the three-dimensional
Fourier spatial modes. The time dependent reaction rates corresponding
to the fundamental Fourier spatial mode and the associated
values of the instantaneous decay constant have been compared with
calculations from a multigroup diffusion theory code which uses
the asymptotic reactor theory to describe leakage. The. thorium
cross sections employed are the ABBN 26 group set and two 50 group
sets from the WINFRITH data file and the ENDF/B-II data file.

The calculated 237Np fission rates using the
three data sets differ substantially from each other and decay
faster than measurements for the first 30 ns or so after the
initial neutron pulse. At later times measurements lie within
the calculated curves.

The calculated 235U fisslon'rates using the
three data sets are in close agreement with each other. All
three sets, however, predict too fast a decay rate for the first
70 ns or so after the initial neutron pulse compared with
measurements, the disagreement being well outside the experimental
errors. At later times the predicted values of the instantaneous
decay constant are close to the measured values. Similar trends are
observed for the 239Pu fission rates.

The discrepancies between measurements and
calculations are discussed and possible explanations are presented.
The time dependent 237Np and 235U fission rates measured at
several positions in the thorium assembly are also
compared with calculations from a simple Monte Carlo code.
Again the calculated reaction rates decay faster than the measured
reaction rates.

Some studies of the sensitivity of the measured
parameters to physically reasonable changes in the thorium cross
sections at various energies based on the multigroup diffusion theory
calculations are presented. The measured parameters are shown to be
insensitive to changes in the fission and radiative capture cross
sections but are very sensitive to the elastic and inelastic cross
sections.

Item Type: Thesis - PhD
Authors/Creators:Pheng, Moo Siew
Keywords: Thorium, Fast neutrons
Copyright Holders: The Author
Copyright Information:

Copyright 1973 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, 1975. Bibliography: l. 109-112

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