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

Abstract

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 scalarmore » adjustment to Pu-239 {bar {nu}} (independent of neutron energy); an adjustment that depends on energy is probably more appropriate.« less

Authors:
 [1];  [2];  [3];  [2];  [2];  [2];  [2]
  1. North Carolina State University
  2. University of Michigan
  3. Los Alamos National Laboratory
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
DOE/LANL
OSTI Identifier:
1044820
Report Number(s):
LA-UR-12-22399
TRN: US1203398
DOE Contract Number:  
AC52-06NA25396
Resource Type:
Conference
Resource Relation:
Conference: 2012 American Nuclear Society Meeting ; 2012-06-24 - 2012-06-28 ; Chicago, Illinois, United States
Country of Publication:
United States
Language:
English
Subject:
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

Citation Formats

Mattingly, John, Miller, Eric, Solomon, Clell J. Jr., Dennis, Ben, Meldrum, Amy, Clarke, Shaun, and Pozzi, Sara. An Evaluation of Monte Carlo Simulations of Neutron Multiplicity Measurements of Plutonium Metal. United States: N. p., 2012. Web.
Mattingly, John, Miller, Eric, Solomon, Clell J. Jr., Dennis, Ben, Meldrum, Amy, Clarke, Shaun, & Pozzi, Sara. An Evaluation of Monte Carlo Simulations of Neutron Multiplicity Measurements of Plutonium Metal. United States.
Mattingly, John, Miller, Eric, Solomon, Clell J. Jr., Dennis, Ben, Meldrum, Amy, Clarke, Shaun, and Pozzi, Sara. Thu . "An Evaluation of Monte Carlo Simulations of Neutron Multiplicity Measurements of Plutonium Metal". United States. https://www.osti.gov/servlets/purl/1044820.
@article{osti_1044820,
title = {An Evaluation of Monte Carlo Simulations of Neutron Multiplicity Measurements of Plutonium Metal},
author = {Mattingly, John and Miller, Eric and Solomon, Clell J. Jr. and Dennis, Ben and Meldrum, Amy and Clarke, Shaun and Pozzi, Sara},
abstractNote = {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.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2012},
month = {6}
}

Conference:
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