Seven surrogate precursors for modeling delayed neutron decay and predicting reactivity Page: 4 of 8
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Seven Surrogate Precursors for Modeling Delayed Neutron Decay and
David J. Loaiza and F. Eric Haskin
Delayed neutron activity has traditionally been represented by six delayed
neutron groups, whose yields and decay constants are obtained from
nonlinear least-squares fits to out-of-pile measurements.! The six-group
delayed neutron decay constants obtained in this manner are empirical fits to
the data. They cannot be matched with decay constants of specific delayed
neutron precursors. The traditional six-group delayed neutron decay constants
are, therefore, different for each fissionable nuclide. In addition, the
traditional group decay constants for any particular fissionable nuclide
changes with the neutron energy spectrum.
The use of a different set of group decay constants for each fissionable
nuclide complicates analysis of the dynamic behavior of fast reactors. A fast
reactor containing six principal fissioning nuclides of uranium and plutonium
must, in effect, be described by 36 delayed neutron groups.2 Additionally, the
use of group decay constants that depend on the neutron energy spectrum
makes it difficult to select values that describe the dynamic response of
epithermal systems because virtually all delayed neutron activity
measurements have been performed for fast or thermal-neutron-induced
Clearly, it would be desirable to have a single set of group decay constants
that could be applied to all fissionable nuclides. A set of seven fixed decay
constants is proposed herein. Each of the proposed decay constants is
associated with a specific, dominant delayed neutron precursor. In effect,
each group is represented by a single surrogate precursor. Using recently
measured delayed neutron activities for U-235 and Np-237,3 the proposed set
of decay constants actually improved the goodness of fit to the data. For
other fissionable nuclides lacking experimental data, a method has been
devised to obtain yields consistent with the proposed set of decay constants
from the traditional six-group parameters. This transformation is
accomplished without altering the traditional inferred reactivity scale.
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Loaiza, D.J. & Haskin, F.E. Seven surrogate precursors for modeling delayed neutron decay and predicting reactivity, article, December 31, 1997; New Mexico. (digital.library.unt.edu/ark:/67531/metadc708090/m1/4/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.