**Design of Nuclear Criticality Safety Experiments Containing Highly
Enriched Uranium and Gadolinium in Diverse Scattering Matrices**

J.S. Bennion, J. Nielsen (Idaho State U.), J.B. Briggs (Idaho National
Engineering and Environmental Laboratory) and R.G. Sanchez (Los Alamos
National Laboratory)

Abstract Number: bennion_95

**Introduction**: A comprehensive review of the scientific literature
has been performed to identify existing critical experiments containing
hydrogenously-moderated highly enriched uranium (HEU) with gadolinium as
a neutron absorber (HEU/H/Gd) ^1^. These experiments are vital to the
U.S. National Spent Nuclear Fuel Program (NSNFP) to verify and validate
computer models bounding postulated criticality scenarios for the
disposal of SNF in a geologic repository. Based on worst-case scenarios,
the NSNFP concluded that a neutron absorber must be incorporated into
the disposal package to preclude internal criticality; Gd is attractive
because several Gd compounds are effectively insoluble ^2^. Many
HEU/H/Gd critical experiments have been performed worldwide. However,
only a few series have been documented in sufficient detail to be
accepted as benchmarks and catalogued in the International Handbook of
Evaluated Criticality Safety Benchmark Experiments (IHECSBE) ^3^. All
HEU/H/Gd experiments included in the IHECSBE were performed at two
prestigious laboratories in Russia. Most experiments consisted of
aqueous UO~2~(NO~3~)~2~ solutions with different concentrations of
Gd(NO~3~)~3~. Two experiments involved water-moderated hexagonal
lattices of UO~2~ fuel rods with Gd absorber rods. Relatively large
biases in the calculated values of k~eff~ were observed in both the
homogeneous and heterogeneous experiments at low to moderate Gd
concentrations ^3^.

**Methods and Materials**: A series of new experiments has been designed
for the Planet critical assembly machine located at the Los Alamos
Critical Experiments Facility (LACEF) to provide additional, independent
data for heterogeneous systems to validate computer transport codes and
cross-sectional data ^4,5^. Experimental cores are constructed from
high-density polyethylene (PE) plates, which are recessed on each major
surface to accommodate square HEU foils and/or neutron-scattering
(matrix) and -absorbing materials. A PE plate with embedded fuel, matrix
and absorber comprise a unit cell, which are stacked to form a
rectangular parallelepiped with lateral and axial polyethylene
reflectors. The core is divided in two and assembled by remote control
by moving the bottom half of the core towards the stationary upper half
in increments using the traditional 1/*M* approach-to-critical method.
Experiments were designed using Al, SiO~2~, CaO, MgO, PE, Bi, and Zr as
matrix materials to modify the energy characteristics of the neutron
flux density. Gadolinium foils were arranged within the matrix in
various spatial configurations and mass combinations. The number of unit
cells required to achieve criticality was determined with the aid of the
MCNP computer code ^6^. A comparison of MCNP-calculated spectral indices
and group response functions (neutron flux density and the fission and
capture responses) demonstrates the new experiments provide excellent
bounding of the NSNFP criticality models. The new experiments have
similar spectral characteristics and will provide supplementary
benchmark data to help validate cross-sectional data for important
matrix materials expected in SNF disposal packages. Actual critical
benchmark experiments using the matrix materials and Gd foils are now
under way at LACEF.

**Acknowledgments**: This work was supported through a grant provided by
the U.S. Department of Energy National Spent Nuclear Fuel Program at the
Idaho National Engineering and Environmental Laboratory.

**References**: ^1^ J.S. Bennion and J.W. Nielsen, "Review of Nuclear
Criticality Safety Experiments for HEU/H/Gd Systems," TR-99/00-004,
Idaho State University College of Engineering, Pocatello, Idaho (April
2001); ^2^ *Evaluation of Codisposal Viability for Aluminum-Clad
DOE-Owned Spent Fuel: Phase II Degraded Waste Package Internal
Criticality*, BBA000000-01717-5705 Rev 0, Civilian Radioactive Waste
Management System-M&O, Las Vegas, Nevada (December 1997); ^3^
*International Handbook of Evaluated Criticality Safety Benchmark
Experiments*, NEA/NSC/DOC (95) 03, Nuclear Energy Agency, Organization
for Economic Co-operation and Development (Sept. 1999 ed.); ^4^ J.S.
Bennion and J.W. Nielsen, "Design of Nuclear Criticality Safety
Experiments for HEU/H/Gd Systems," Technical Report No. TR-99/00-003,
Idaho State University College of Engineering, Pocatello, Idaho (March
2001); ^5^ Nielsen, J.W., J.S. Bennion, J.B. Briggs, and R.G. Sanchez,
"Design of Critical Experiments Using Highly Enriched Uranium with
Gadolinium in Various Matrices," *Trans. Am. Nucl. Soc.*, **83**,
209-212 (2000); ^6^ "MCNP - A General Monte Carlo N-Particle Transport
Code," LA-12625-M, Version 4B, J.F. Briesmeister, Ed., Los Alamos
National Laboratory (1997).