Program - Brookhaven National Laboratory

are provided in An Expanded Criticality Validation Suite for MCNP (LA-UR-10-06230), which can be
obtained from the NNDC. The suite includes three fast, one intermediate, and two thermal subcategories
for 233 U, HEU, IEU, and plutonium systems and two thermal subcategories for LEU systems. The three
subcategories for fast benchmarks are characterized by the type of reflection: unreflected, reflected by a
heavy material, and reflected by a light material. The two subcategories for thermal benchmarks are lattices
and solutions. The intermediate subcategory has a variety of configurations because only a relatively small
number of critical experiments with intermediate spectra have been performed. The suite contains at least
one benchmark in each of those subcategories for each fuel type. ENDF/B-VII.1 was found to produce
improvements relative to ENDF/B-VII.0 for benchmarks that contain significant amounts of tungsten,
zirconium, cadmium, or beryllium. The results for the benchmarks with beryllium suggest that further
improvements still may be needed. A number of deficiencies previously identified for the ENDF/B-VII.0
library still remain in ENDF/B-VII.1. Those deficiencies include 235 U cross sections in the unresolved
resonance region, thermal cross sections for 239 Pu, and fast cross sections for copper, 237 Np, and 232 Th.
Fast cross sections for graphite and nickel also may need to be reviewed. The ENDF/B-VII.1 results for the
remaining benchmarks in the expanded criticality validation suite show only minor or negligible changes
in keff relative to ENDF/B-VII.0.
FE 3 11:40 AM
Validation of Cross Sections with Criticality Experiment and Reaction Rates: The
Neptunium Case
L.S. Leong, L. Tassan-Got, J.N. Wilson, C. Le Naour, C. Stéphan
CNRS, Institut de Physique Nuclëaire, UMR 8608, Orsay, 91406, France
L. Audouin
Univ Paris-Sud, Orsay, 91405, France
C. Paradela, D. Tarrio, I. Duran
Facultad de fisica, Universidade de Santiago de Compostela, 15782, Spain
n TOF Collaboration
n TOF Facility, European Organization for Nuclear Research CERN CH-1211 Genève 23 Switzerland
Critical experiments are valuable checks of nuclear data, in particular cross sections. We used such methods
to check recently measured neutron-induced fission cross sections of 237Np. This isotope is abundantly
produced in reactors and due to its long half-life, its high radiotoxicity and its large mobility in aquatic
systems it could be a candidate for incineration, provided its neutron cross sections are known with a
good accuracy. The neutron-induced fission cross section has recently been measured in a broad energy
range (from eV to GeV) at the n TOF facility at CERN and appears to be higher by 5-7% beyond the
fission threshold, when compared to previous measurements. To check the reliability of the n TOF data,
we simulated a criticality experiment performed at Los Alamos with a 6 kg sphere of 237Np. This sphere
was surrounded by enriched uranium 235U so as to approach criticality with fast neutrons. The simulation
performed with MCNP5 predicts a multiplication factor keff in better agreement with the experiment
(the deviation of -750 pcm is reduced to +250 pcm) when the ENDF-B7 evaluation is replaced by the
n TOF data. We also explored possible other effects which could be responsible for this underpediction
like deficiencies of the inelastic cross section in 235U and the nubar of 237Np, but we show that the variation
needed to cancel the keff deviation is hardly compatible with the measurements. These outcomes support
the hypothesis of a higher fission cross section of 237Np. However ratios of fission rates 237Np/235U
have been measured in different neutron energy spectra (GODIVA, COSMO, CGFR-Proteus). Those data
are quasi-direct measurements of 237Np integrated fission cross section relative to 235U or 239Pu. The
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