Plutonium sphere multiplicity simulations with MCNP-PoliMi

In order to improve the characterization methods for fissile materials, effort must be made to validate the computer codes that are used to simulate the behavior of these systems. For this work, measurements of a 4.4-kg sphere of weapons grade plutonium metal were taken. A detector array of ³He tubes embedded in polyethylene was used to measure the multiplicity of the system. The experiment was then modeled using the MCNP-PoliMi code and the simulated results were compared to the measured results using the Feynman-Y metric. MCNP-PoliMi is able to correctly predict the measured value of the Feynman-Y vale within 5% for all of the moderated cases and within 22% for the bare sphere.