Editing Input syntax manual (section)

=== set pop ===

 '''set pop''' ''NPG NGEN NSKIP'' [ ''K0 BTCH NEIG'' ]
Sets parameters for simulated neutron population in criticality source mode. Input values:

{|
| <tt>''NPG''</tt>
| : number of neutrons per generation
|-
| <tt>''NGEN''</tt>
| : number of active generations
|-
| <tt>''NSKIP''</tt>
| : number of inactive generations
|-
| <tt>''K0''</tt>
| : initial guess for ''k''<sub>eff</sub> (default value: 1.0)
|-
| <tt>''BTCH''</tt>
| : batching interval (default value: 1)
|-
| <tt>''NEIG''</tt>
| : number of independent parallel eigenvalue calculations (default value: 1)
|}

<u>Notes:</u>
*Using the pop card sets the mode to criticality source simulation. 
**External source simulation is invoked using the [[#set nps|set nps]] option. (The two cards are mutually exclusive).
*Simulation sequence:
**First, run for a number of inactive generations to allow the fission source to converge
**Followed by a number of active generations, during which the results are collected. 
***The statistics are divided in batches, and by default each generation forms its own batch.
*Convergence of fission source can be monitored using Shannon entropy (see [[#set his|set his]] and [[#set entr|set entr]] options).
**Setting an initial guess value manually may get the simulation going if it terminates on the first generation because of poor initial guess. 
***The value does not affect fission source convergence.
*See detailed descriptions on [[fission source convergence]] and [[statistical effects of batching]].
*The number of neutrons per generation also affects the memory usage together with [[#set nbuf|set nbuf]] option.
**With a large value, lowering the buffer size might be necessary for the simulation to be runnable.
*Since [[#set_ifp|IFP]] based [[Output_parameters#Adjoint_weighted_time_constants_using_IFP|time constants]] and [[Sensitivity calculations|sensitivity calculations]] start to collect their event history during the inactive generations, you should take this in account in the number of inactive generations by increasing the number of inactive generations by the number of generations in the event history (IFP chain length or number of latent generations in sensitivity calculations) above the number needed for source convergence.