Epithermal Self-Shielding
The biggest weakness of the TRANSX code as described so far
occurs for thermal reactor analysis in the ``epithermal''
energy region from about 4 eV to around 200 or 300 eV. In
this region, the resonances are no longer ``narrow,'' and the
flux shape given by Eq.~(\ref{eq225}) is no longer sufficiently
accurate. This failure is normally only important for
resonance materials that are present in large concentration,
such as U-238.
TRANSX results for this energy range can be improved
by preparing special cross sections for the
U-238 using the NJOY flux calculator. As an example,
you could prepare a new
isotope called U238B with cross sections appropriate
for UO$_2$ in pins of a certain size surrounded by
a water region. You could then add this isotope to your
MATXS library using BBC. By asking for U238B
instead of U238 in your TRANSX run, you would get
improved results for resonance self-shielding in the
epithermal region.
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