Description of the "u235ib02o4cWg6anu" evaluation
=================================================

A.Trkov, R. Capote
IAEA, Vienna, Austria

August 2015


To demonstrate the compensating effects, we focused on
the new 235U(nth,f) PFNS evaluation based on the GMA
fit with an average neutron energy of 2.00 MeV. The
spectra between 0.5 MeV and 2 MeV were from the GANDR
fit. Above this energy all fission spectra were taken
from the Talou evaluation submitted to the CRP on PFNS.

The thermal nu-bar was reduced by 0.38% relative to the
ENDF/B-VII.1 evaluation to cancel the tuning done in
the ENDF/B-VII.1 (and the ENDF-B/VI) libraries for
better agreement with criticality benchmarks. Some
structure due to the resonances was tested as much as
could be supported by experimental evidence, but the
data are poor and there is some flexibility for additional
adjustments.

The rest of the fission data were taken from the 
ENDF/B-VII.1 evaluation because the fision cross sections 
are consistent with the IAEA Standards. 

Resonance parameters were taken from the ORNL evaluation 
submitted to the CIELO project by Leal, labelled "ornl4". 
Apparently, there was a new resonance file produced, but 
this was not available at the time of the present evaluation.
There is a marked decrease in the capture cross section near
the upper end of the resolved resonance range, but
apparently this is supported by the sodium void
coefficient data, reported by WPEC SG-29.

The capture cross sections above the URR were 
taken from JENDL-4, but scaled to be 2% lower than 
ENDF/B-VII.1 around 25 keV and 4% lower around 426 keV. 
The decrease in capture is indicated by the Wallner 
measurement based on the accelerator mass spectrometry 
technique.

All other cross sections were taken from EMPIRE calculations
using a dispersive optical model potential, RIPL 2408, 
that couples 8 levels of the ground state rotational
band. The fision model parameters were adjusted
to reproduce the Standards evaluation of the 235U(n,f)
cross section.

Preliminary trial calculations were done (described in the
big PFNS paper sent for publication recently, where it was
demonstrated that agreement with several fast and thermal
benchmarks can be restored with this evaluation, but the
adjustments are not unique. Particularly, the reduction in
the capture may give rise to problems in some benchmarks
sensitive to the epithermal capture of U-235. Adjustment of
nu-bar might be an alternative.

A more systematic study was initiated recently, based on
the sensitivity data available in the DICE package. A
preliminary report is available, but it represents
"work in progress". The report includes more benchmarks
sensitive to U-235 data.

The data file for U-235 has internal label "u235ib02o4cWg6anu"
and is available from the U235 tab.

The corresponding ACE file is also available.