An Evaluation of Monte Carlo Simulations of Neutron Multiplicity Measurements of Plutonium Metal

Conference · Thu Jun 21 04:00:00 EDT 2012

OSTI ID:1044820

Mattingly, John ^[1]; Miller, Eric ^[2]; Solomon, Jr, Clell J ^[3]; Dennis, Ben ^[2]; Meldrum, Amy ^[2]; Clarke, Shaun ^[2]; Pozzi, Sara ^[2]

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1. North Carolina State University
2. University of Michigan
3. Los Alamos National Laboratory

In January 2009, Sandia National Laboratories conducted neutron multiplicity measurements of a polyethylene-reflected plutonium metal sphere. Over the past 3 years, those experiments have been collaboratively analyzed using Monte Carlo simulations conducted by University of Michigan (UM), Los Alamos National Laboratory (LANL), Sandia National Laboratories (SNL), and North Carolina State University (NCSU). Monte Carlo simulations of the experiments consistently overpredict the mean and variance of the measured neutron multiplicity distribution. This paper presents a sensitivity study conducted to evaluate the potential sources of the observed errors. MCNPX-PoliMi simulations of plutonium neutron multiplicity measurements exhibited systematic over-prediction of the neutron multiplicity distribution. The over-prediction tended to increase with increasing multiplication. MCNPX-PoliMi had previously been validated against only very low multiplication benchmarks. We conducted sensitivity studies to try to identify the cause(s) of the simulation errors; we eliminated the potential causes we identified, except for Pu-239 {bar {nu}}. A very small change (-1.1%) in the Pu-239 {bar {nu}} dramatically improved the accuracy of the MCNPX-PoliMi simulation for all 6 measurements. This observation is consistent with the trend observed in the bias exhibited by the MCNPX-PoliMi simulations: a very small error in {bar {nu}} is 'magnified' by increasing multiplication. We applied a scalar adjustment to Pu-239 {bar {nu}} (independent of neutron energy); an adjustment that depends on energy is probably more appropriate.

• Research Organization: Los Alamos National Laboratory (LANL)
• Sponsoring Organization: DOE/LANL
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1044820
• Report Number(s): LA-UR-12-22399
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
ACCURACY
BENCHMARKS
DISTRIBUTION
EVALUATION
LANL
MULTIPLICITY
NEUTRONS
PLUTONIUM
SANDIA NATIONAL LABORATORIES
SCALARS
SENSITIVITY
SIMULATION