A MONTE-CARLO VARIANCE REDUCTION APPROACH FOR NON-BOLTZMANN TALLIES

Quantities that depend on the collective effects of groups of particle s cannot be obtained from the standard Boltzmann transport equation. M onte Carlo estimates of these quantities are called non-Boltzmann tall ies and have become increasingly important recently. Standard Monte Ca rlo variance reduction techniques were designed for tallies based on i ndividual particles rather than groups of particles. Experience with n on-Boltzmann tallies and analog Monte Carlo has demonstrated the sever e limitations of analog Monte Carlo for many non-Boltzmann tallies. In fact, many calculations absolutely require variance reduction methods to achieve practical computation times. A description is given of how variance reduction techniques can be applied when non-Boltzmann estim ates are desired.