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Computational Evaluation of Neutron Multiplicity Measurements of Polyethylene-Reflected Plutonium Metal

E. C. Miller, J. K. Mattingly, S. D. Clarke, C. J. Solomon, and B. Dennis, A. Meldrum, and S. A. Pozzi

Nuclear Science and Engineering / Volume 176 / Number 2 / February 2014 / Pages 167-185

Technical Paper / dx.doi.org/10.13182/NSE12-53

Simulations of neutron multiplicity measurements of a highly multiplicative plutonium sphere measured with a moderated array of 3He proportional counters have consistently overpredicted the mean and variance of the measured multiplicity distribution. In contrast, identical experiments using a 252Cf source have been accurately simulated. This paper outlines a sensitivity analysis of several key parameters that could account for the overprediction in the simulation of the plutonium sphere. Parameters that were analyzed include source-detector distance, detector dead time, variations in density and volume of the plutonium, and the value of  for v̅ 239Pu-induced fission. Of these parameters, the only factor that accounted for the overprediction within reasonable bounds was a change in the value of the 239Pu  v̅. The sensitivity analysis showed that a small change (1.14% reduction) in the value of v̅ dramatically improved the simulated results.

 
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