Input syntax manual

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Revision as of 18:13, 17 November 2015 by Jaakko Leppänen (Talk | contribs) (Input options)

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This page will contain the whole input syntax of Serpent 2

This is a reference to one article [1]

This is another reference to the same article [1]

Input cards

mat (material definition)

surf (surface definition)

cell (cell definition)

det (detector definition)


Input options

set delnu

set delnu <dnu>

Sets delayed neutron emission on or off. Input values:

<dnu> -- option to switch delayed neutron emission off (0/no) or on (1/yes)


  • Delayed neutron emission is on by default in neutron criticality source and off by default in external source simulations.

set gcut

set gcut <gmax> 

Sets generation cut-off for neutrons. Input values:

<gmax> number of simulated generations before cut-off

The generation cut-off can be used in neutron external source simulations, to limit the length of fission chains.


  • Applicable only to neutron external source simulation (invoked using (set nps)
  • Generation or time cut-off (set tcut) is always needed for neutron external source simulations in super-critical systems.

set nps

set nps <ppb> [ <btch> ] 

Sets parameters for simulated particle population in external source mode. Input values:

<ppb> number of particles per batch
<btch> number of batches

The simulation is for a number batches using the given batch size.


  • Using the nps card sets the mode to external source simulation. Criticality source simulation for neutrons is invoked using set pop.
  • Running an external source simulation requires a source, defined by the src card. Source definition also sets the transported particle type.
  • Neutron external source simulations are limited to sub-critical systems, unless time cut-off (set tcut) or generation cut-off (set gcut) is invoked.
  • Neutron external source simulations in multiplying systems may require adjusting the neutron buffer (set nbuf).
  • Delayed neutron emission is switched off by default in neutron external source simulation (for compatibility with MCNP). Delayed neutrons can be included with set delnu.

set pop

set pop <npg> <ngen> <nskip> [ <keff0> <btch> ] 

Sets parameters for simulated neutron population in criticality source mode. Input values:

<npg> number of neutrons per generation
<ncyc> number of active generations
<nskip> number of inactive generations
<keff0> initial guess for k-eff
<btch> batching interval

The simulation is first run for a number of inactive generations to allow the fission source to converge. This is 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.


  • Using the pop card sets the mode to criticality source simulation. External source simulation is invoked using set nps.
  • Convergence of fission source can be monitored using Shannon entropy (input parameters set his and set entr).
  • See detailed description on the statistical effects of batching.

set tcut

set tcut <tmax> 

Sets time cut-off for neutrons and photons. Input values:

<tmax> time limit for simulated particle histories (in seconds)

The time cut-off can be used in both neutron and photon external source simulations, to limit the length of particle histories.


  • Time or generation cut-off (set gcut) is always needed for neutron external source simulations in super-critical systems.
  • Note to developers: this should take independent values for photons and neutrons


  1. ^ Leppänen, J. "Development of a new Monte Carlo reactor physics code." D.Sc. Thesis, Helsinki University of Technology, 2007. (VTT Publications 640)