Monte Carlo Simulations of Multiplicity Measurements of the Plutonium BeRP Ball

Year
2009
Author(s)
E.C. Miller - Department of Nuclear Engineering & Radiological Sciences
S. A. Pozzi - Department of Nuclear Engineering & Radiological Sciences
B.D. Dennis - Department of Nuclear Engineering & Radiological Sciences
S.D. Clarke - Department of Nuclear Engineering & Radiological Sciences
Abstract
The objective of this work was to demonstrate the ability of the MCNP-PoliMi code to predict the detected neutron multiplicity distribution of a plutonium metal sphere. Various measure- ments were made at the Nevada Test Site using a 4.483-kg weapons-grade plutonium metal sphere in both bare and moderated configurations. The moderating material was high-density polyethylene of varying thicknesses. Neutrons from the source were detected using a bank of 15 3 He detectors. The measurements were simulated with MCNP-PoliMi. The MCNP-PoliMi postprocessor was modified to simulate the dead time effects in the 3He tubes and the associated detection system electronics. The Feynman-Y metric was chosen as the basis of comparison between the measured data and the simulated results. MCNP-PoliMi is able to simulate the Feynman-Y of the bare plutonium sphere within 8% of the measured value.