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Formerly the PEACO worked only in the second resonance energy range. Now, the upper energy limit
can be extended up to 961.12eV optionally.
1.5.5 Thermal Neutron Energy Range (cut-off-energy > E > 1.0E-5eV)
The cut-off-energy should be determined by considering the effect of up-scattering, resonance
levels around the cut-off-energy and the continuity of scattering cross-sections described at the
previous section. Judging from the comparisons with continuous energy Monte Carlo calculation
results, about 2.0eV is recommended, even if sharp resonance levels lower than this energy can not be
treated by the PEACO option. The Maxwellian distribution depending on temperature and 1/E is
assumed for the asymptotic spectrum.
1.6 Effective Resonance Cross-sections
The SRAC code installs three kinds of methods for calculation of effective resonance
cross-sections, they are, NR approximation, IR approximation and the PEACO calculation. One of
them is selected by the characteristics of the problem, the required accuracy or computation cost. The
NR approximation is defaulted unconditionally. If one of other options is specified, the cross-sections
obtained by the NR approximation are replaced by the values calculated by the IR or the PEACO
calculation, in the energy range where each option is active.
1.6.1 NR Approximation
The Fast and Thermal Libraries contain the infinite dilution cross-sections ( σ ) of every
nuclide and the tabulation of self-shielding factors for resonant nuclides (f-table). The effective
microscopic cross-section of reaction x of nuclide n in resonant mixture i is given by Eq.(1.6-1) using
the above quantities.
∞
σ
i
x,
n
= σ
∞,
x,
n
f
i
x ( σ 0,
n
, T
i
)
(1.6-1)
i i
where f x ( σ 0,
n , T ) is the resonance self-shielding factor obtained by the interpolation of the f-table
i
with a parameter: background cross-section σ 0,n defined by Eq.(1.6-2) and another parameter: the
i
temperature T of the mixture of which the nuclide is a constituent. The interpolation is carried out
by using the third order spline function. The background cross-section is given by the equivalence
theory between homogeneous system and heterogeneous system as
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