[en] In small-diameter pipes, the Pulsed-Neutron Activation (PNA) technique is known to measure mass flow of single-phase and two-phase flows independent of changes in flow profiles or flow regimes. In large-diameter pipes, both neutrons and gamma rays are attenuated significantly in the fluid, and PNA mass flow measurements must be corrected for density and flow-regime variations. The flow velocity reading is corrected by introducing an effective time exponent into the basic velocity equation. The density reading is corrected by a theoretical factor applied to the basic density measurement. The computer program PNASIM (Pulsed Neutron Activation SIMulator), described in this report, can be used to calculate the optimized (for minimum error) time exponent to be used in the velocity equation, and for the correction factor to be applied to the density measurement. Also, the program can be used to optimize the experimental setup of a PNA system (detector size, source detector, spacing, etc.) as well as the experimental procedure (width of time channel, number of pulses to be accumulated, etc.)