Anomalous Behavior of Monte Carlo Uncertainties near Reflecting Boundaries

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