Anomalous Behavior of Monte Carlo Uncertainties near Reflecting Boundaries

Knolls Atomic Power Laboratory – Bechtel Marine Propulsion Corporation, Schenectady, New York, USA

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Abstract

It has been observed that statistical uncertainties on tallies in Monte Carlo iterated-fission-source calculations tend to be larger for tally regions near reflective boundaries than for similar tally regions far from such boundaries. Using a theoretical methodology developed by Brissenden and Garlick, the cause of this behavior was found to be a greater correlation between fission generations in the regions near the reflective boundaries. The theory was found to be in qualitative agreement with Monte Carlo results for a simple test problem. It is also shown that applying the coarse mesh finite difference method eliminates the anomalous behavior.