Installing and running Serpent

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Compiling the source code

Serpent is written in the C programming language and compiled using the standard Make utility. The compilation (of version 2.1.29) should look something like:

~/src/serpent2$ make
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c addbranching.c
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c addbuf.c
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c addbuf1d.c
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c zaitoiso.c
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c zdis.c
gcc -Wall -ansi -ffast-math -O3 -DOPEN_MP -fopenmp -Wno-unused-but-set-variable -pedantic -c zonecount.c
gcc addbranching.o addbuf.o addbuf1d.o ... zaitoiso.o zdis.o zonecount.o -lm -fopenmp -lgd -o sss2
Serpent 2 Compiled OK.

The compilation should not produce any error or warning messages and it should produce an executable named "sss2"

Compiler options

There are number of compiler options that can be invoked by editing the Makefile. When recompiled, it is recommended to do a "make clean" before "make.

GD Graphics library

If the GD open source graphics library is not available in the system, line:

LDFLAGS += -lgd

must be commented out and line:


added. For more information, see the the GD Graphics library website. Also note that there are several threads addressing the installation of these libraries at the Serpent discussion forum.

Debug mode

The code can be compiled in the debugging mode by adding line


in the Makefile. This activates various pointer and value checks during the calculation. The code runs slower, but errors are more likely to be caught before inducing unexpected results. In case of crash or any unexpected behavior, the best way to proceed is to recompile the code in debug mode and repeat the calculation using the same random number seed (the "-replay" command line option). For more information see the Pitfalls and troubleshooting section.

Parallel calculation using MPI

Parallel calculation using MPI also requires changes in the Makefile. The calculation mode is activated by adding line:


The compiler needs to know where to look for the associated libraries. In some installations this can be accomplished simply by changing the compiler from “gcc” to “mpicc”.

Making backups

A time-stamped backup can be made using the "bk" -option:

~/src/serpent2$ make bk
zip "`date +'backup/'`" *.c *.h Makefile versions.txt
  adding: addbranching.c (deflated 90%)
  adding: addbuf.c (deflated 71%)
  adding: addbuf1d.c (deflated 70%)
  adding: surface_types.h (deflated 75%)
  adding: Makefile (deflated 83%)
  adding: versions.txt (deflated 83%)
cp sss2 "`date +'backup/serpent_2.1.30_%y%m%d%H%M'`"
cp "`date +'backup/'`" ./
chmod a-w "`date +'backup/'`"
chmod a-w "`date +'backup/serpent_2.1.30_%y%m%d%H%M'`"

The source code is zip-compressed in the "backup" subdirectory, which must be found in the source directory. The executable is copied in the same directory and renamed using the same time stamp, so that earlier versions can be called without re-compiling the source code.

Installing updates

Updates are sent to registered users in tar.gz-compressed format by e-mail. The installation of updates is carried out by overwriting the existing source files:

~/src/serpent2$ make bk
~/src/serpent2$ tar -xzvf
~/src/serpent2$ make clean
~/src/serpent2$ make

It is always good practice to make a backup of the source code before installing the update. It is also important to realize that any modifications made in the source code may be lost when the updated files are installed. The updates are cumulative in the sense that update "sssup2.1.X+1.tar.gz" contains all the modifications in update "sssup2.1.X.tar.gz" and earlier.

The code version can be checked after the compilation using the "-version" command line option:

~/src/serpent2$ sss2 -version

  _                   .-=-.           .-=-.          .-==-.       
 { }      __        .' O o '.       .' O o '.       /  -<' )--<   
 { }    .' O'.     / o .-. O \     / o .-. O \     /  .---`       
 { }   / .-. o\   /O  /   \  o\   /O  /   \  o\   /O /            
  \ `-` /   \ O`-'o  /     \  O`-'o  /     \  O`-`o /             
   `-.-`     '.____.'       `._____.'       `.____.'              

Serpent 2 beta

A Continuous-energy Monte Carlo Reactor Physics Burnup Calculation Code

 - Version 2.1.29 (June 12, 2017) -- Contact:

 - Reference: J. Leppanen, et al. "The Serpent Monte Carlo code: Status,
              development and applications in 2013." Ann. Nucl. Energy,
              82 (2015) 142-150.

 - Compiled Jun 12 2017 08:21:11

 - MPI Parallel calculation mode not available

 - OpenMP Parallel calculation mode available

 - Geometry and mesh plotting available

 - Default data path set to: "/xs"


This also prints information about the availability of parallel calculation, geometry and mesh plotting and the default data path.

Setting up the data libraries

Serpent reads continuous-energy cross sections from ACE format data libraries. The directory file is different from the "xsdir" file used by MCNP and the conversion between the two formats is made using the "" perl script:

~/xsdata/jeff31$ sss_jeff31u.xsdir > sss_jeff31u.xsdata

The output is written in a new file, which is read by setting the set acelib input parameter.

The conversion script checks that each library file in the original xsdir exists and it is important that the first line in the file defines the correct data path. If the data is found in another location, the xsdir file must be edited before running the script. If all file paths are set correctly, the xsdir file should be directly usable by MCNP and the conversion to Serpent format should work without problems.

Two entries are written for each nuclide, one using the standard MCNP convention ( and another one using the element symbol and the isotope mass (e.g. 92235.03c and U-235.03c for U-235). Either name can be used to identify the nuclide in the material compositions.

The script assumes that nuclides in isomeric states are identified by setting the third digit in the ZA to 3 (e.g. 61348 for Pm-148m or 95342 for Am-242m). If other convention is used, the isomeric state number (5. entry) must be set manually.

Radioactive decay and fission yield data is read in standard ENDF format that requires no modifications. Photon transport calculations require additional physics data ( that can also be downloaded separately. The file path for this data is set using the set pdatadir input option.

The default search path for cross section, decay, fission yield and isomeric branching data can be defined by setting environmental variable "SERPENT_DATA". This allows accessing the files in this directory using the file name only. The default cross section library can be defined using variable "SERPENT_ACELIB". When set, the set acelib is not required for running the simulation. These two environmental variables can be set automatically in the ".bashrc" initialization file (or similar). For example:



Running Serpent

Serpent is run from the Linux command line interface. The general syntax is:

sss2 INPUT [ options ]

Where INPUT is the input file name and the available options are:

-version  : print version information and exit
-replay  : run simulation using random number seed from a previous run
-his  : run only burnup history in coefficient calculation
-coe  : run only restarts in coefficient calculation
-plot  : stop after geometry plot
-checkvolumes N  : calculate Monte Carlo estimates for material volumes by sampling N random points in the geometry (see detailed description)
-checkstl N M  : check for holes and errors in STL geometries by sampling M directions in N points (see detailed description)
-mpi N  : run simulation in MPI mode using N parallel tasks
-omp N  : run simulation in OpenMP mode using N parallel threads
-disperse  : generate random particle or pebble distribution files for HTGR calculations (see detailed description)
-rdep  : read binary depletion file from previous calculation and print new output according to inventory list
-tracks N  : draw N particle tracks in the geometry plots or invoke track plot animation
-comp MAT [ ID ]  : print pre-defined composition of material MAT that can be copy-pasted into the inputfile (see detailed description)
-elem SYM DENS [ ID ]  : decomposes natural element identified by symbol SYM at density DENS into its constituent isotopes (see detailed description)
-qp  : quick plot mode (ignore overlaps)
-nofatal  : ignore fatal errors
-mix  : decompose mixtures in file (see detailed description)
-norun  : stop before running the transport simulation
-noplot  : ignore geometry plots

Most of the input options are self-explanatory, the rest are described below.

Running parallel calculations

Miscellaneous input options

Monte Carlo volume calculation routine

Incorrect material volumes can lead to a number of problems in burnup calculation and the normalization of reaction rates. To deal with these issues Serpent provides a Monte Carlo based routine for checking and calculating the volumes of complicated material zones. The volumes are evaluated by sampling a large number of random points in the geometry, and the estimate represents the exact volumes seen by the particles during the transport simulation. Checking the material volumes is also a good means to confirm that the geometry was properly set up.

The Monte Carlo based volume calculation routine is invoked by command line option "-checkvolumes", followed by the number of random points and the name of the input file. The calculation works also in OpenMP parallel mode. The usage is best illustrated by an example:

sss2 -omp 24 -checkvolumes 10000000 bwr


Calculating material volumes by Monte Carlo...

Estimated calculation time: 0:00:09
Realized calculation time:  0:00:08

Volumes (2D problem, the values are in cm2) :

Material fuel1         : 5.9038E-01 5.9015E-01 (0.00562) : -0.00038   (100.0 % den.)
Material fuel2         : 2.3615E+00 2.3635E+00 (0.00366) :  0.00084   (100.0 % den.)
Material fuel3         : 3.5423E+00 3.5525E+00 (0.00281) :  0.00288   (100.0 % den.)
Material fuel4         : 1.1808E+00 1.1795E+00 (0.00436) : -0.00108   (100.0 % den.)
Material fuel5         : 1.1808E+01 1.1798E+01 (0.00154) : -0.00081   (100.0 % den.)
Material fuel6         : 2.8338E+01 2.8316E+01 (0.00090) : -0.00078   (100.0 % den.)
Material fuel7         : 5.9038E+00 5.9094E+00 (0.00202) :  0.00096   (100.0 % den.)
Material clad          : 1.6336E+01 1.6340E+01 (0.00111) :  0.00026   (100.0 % den.)
Material box           : 0.0000E+00 1.3505E+01 (0.00128) :      N/A * (100.0 % den.)
Material cool          :        N/A 9.5257E+01 (0.00036) :      N/A * (100.0 % den.)
Material moder         : 0.0000E+00 5.5451E+01 (0.00055) :      N/A * (100.0 % den.)

Volumes written in file "bwr.mvol"

The volumes are printed separately for each material. If automated depletion zone division is used, the volumes are also printed for each sub-zone. The first column of results shows the volume actually used in the calculation and the next column the Monte Carlo estimate together with the associated relative statistical error. The next column gives the difference between the used and estimated volume, accompanied by '*' if the difference is suspiciously large compared to statistical accuracy (note that the results are random variables so it is possible that the estimate is off by chance). The last column shows the relative density compared to the input value in the material card. The value can be below 100% if the multi-physics interface is used. It should also be noted that in 2D geometries the calculated volumes are actually cross-sectional areas in cm2.

The volume calculation routine also prints an output file [input].mvol, which for the previous example is:

% --- Material volumes:

% Produced Fri Sep 16 09:18:39 2016 by MC volume calculation routine by
% sampling 10000000 random points in the geometry.

set mvol

fuel1           0 5.90149E-01 % (0.006)
fuel2           0 2.36348E+00 % (0.004)
fuel3           0 3.55245E+00 % (0.003)
fuel4           0 1.17947E+00 % (0.004)
fuel5           0 1.17979E+01 % (0.002)
fuel6           0 2.83158E+01 % (0.001)
fuel7           0 5.90941E+00 % (0.002)
clad            0 1.63403E+01 % (0.001)
box             0 1.35046E+01 % (0.001)
cool            0 9.52565E+01 % (0.000)
moder           0 5.54508E+01 % (0.001)

The set mvol card is one of the options to define material volumes and the output from the volume checker can be copy-pasted into the input or the entire file linked using the include command. The volume calculation routine can be run automatically using the set mcvol input option.

Checking for holes in STL geometries

Particle disperser routine

Standard compositions

Serpent has a built-in list of more than 350 pre-defined material compositions. These materials cannot be used directly in the input, but the compositions can be printed and copy-pasted into the input file. The materials are numbered and the full list is printed with option "-comp list":

sss2 -comp list

List of available material compositions:

  1  "A-150 Tissue-Equivalent Plastic (A150TEP)"
  2  "Acetone"
  3  "Acetylene"
  4  "Air (Dry, Near Sea Level)"
  5  "Alanine"
  6  "Aluminum"
  7  "Aluminum Oxide"
  8  "Aluminum, Alloy 2024-O"
353  "800H"

The output is printed in Serpent material card format with the mass density included. The usage is illustrated by an example:

sss2 -comp 4

% --- "Air (Dry, Near Sea Level)" [PNNL-15870, Rev. 1]

mat m4 -1.20500E-03

 6000  -1.24000E-04
 7000  -7.55268E-01
 8000  -2.31781E-01
18000  -1.28270E-02

 6012  -1.22564E-04
 6013  -1.43645E-06
 7014  -7.52324E-01
 7015  -2.94416E-03
 8016  -2.31153E-01
 8017  -9.35803E-05
 8018  -5.34540E-04
18036  -3.88624E-05
18038  -7.70386E-06
18040  -1.27804E-02

The output includes both elemental compositions for photon transport and isotopic compositions for neutron transport calculations. Optional second parameter is the library ID, which is printed after each nuclide ZA:

sss2 -comp 269 06c

% --- "Steel, Boron Stainless" [PNNL-15870, Rev. 1]

mat m269 -7.87000E+00

 5000.06c  -1.00000E-02
 6000.06c  -3.96000E-04
14000.06c  -4.95000E-03
15000.06c  -2.28000E-04
16000.06c  -1.49000E-04
24000.06c  -1.88100E-01
25000.06c  -9.90000E-03
26000.06c  -6.94713E-01
28000.06c  -9.15750E-02

 5010.06c  -1.84309E-03
 5011.06c  -8.15691E-03
 6012.06c  -3.91413E-04
 6013.06c  -4.58738E-06
14028.06c  -4.54739E-03
14029.06c  -2.39265E-04
14030.06c  -1.63344E-04
15031.06c  -2.28000E-04
16032.06c  -1.41126E-04
16033.06c  -1.14910E-06
16034.06c  -6.70834E-06
16036.06c  -1.67133E-08
24050.06c  -7.85070E-03
24052.06c  -1.57439E-01
24053.06c  -1.81960E-02
24054.06c  -4.61478E-03
25055.06c  -9.90000E-03
26054.06c  -3.92204E-02
26056.06c  -6.38452E-01
26057.06c  -1.50084E-02
26058.06c  -2.03234E-03
28058.06c  -6.15363E-02
28060.06c  -2.45201E-02
28061.06c  -1.08366E-03
28062.06c  -3.51174E-03
28064.06c  -9.23214E-04

Note that data for all nuclides may not be found in the Serpent cross section libraries. The complete list of pre-defined material compositions is available here.

Elemental decomposition

This command line option can be used to decompose natural elements in material cards into individual isotopes. The parameters include the element symbol and density or fraction (positive values for atomic and negative values for mass densities / fractions). Optional third parameter is the library ID, which is printed after each nuclide ZA. The usage is illustrated below with examples.

Decomposing natural zirconium with 97.5% mass fraction:

sss2 -elem Zr -0.975 06c

Isotopic composition for natural zirconium:

 40090.06c  -4.94385E-01
 40091.06c  -1.09014E-01
 40092.06c  -1.68461E-01
 40094.06c  -1.74438E-01
 40096.06c  -2.87019E-02

The isotopic fractions sum up to -0.975. Note that data for all nuclides may not be found in the Serpent cross section libraries. Similarly, decomposition of natural boron into atomic fractions:

sss2 -elem B 1.0 

Isotopic composition for natural boron:

  5010  1.99000E-01
  5011  8.01000E-01

The library ID is omitted.

Material decomposition

The -mix command line option processes all mixtures defined in the input and decomposes them into standard material compositions. The compositions are written in material input card format in file [input].mix.

For example, coolant can be defined as a mixture of two materials:

% --- Water: 

mat water  -0.76973  moder lwtr 1001
 1001.03c   0.66667
 8016.03c   0.33333

therm lwtr lwe7.10t

% --- Natural boron:

mat boron   1.00000  tmp 550
 5010.03c   0.19900
 5011.03c   0.80100

% --- Coolant:

mix cool
water      -0.99950
boron      -500E-6

Serpent decomposes mixture cool into a conventional material before running the transport simulation. The -mix command line option prints out the decomposed material composition:

% Material cool is a mixture of 2 components:

% -----------------------------------------
% Material             v. frac      m. frac
% -----------------------------------------
% water            9.99979E-01  9.99500E-01
% boron            2.14484E-05  5.00000E-04
% -----------------------------------------

mat cool  7.72309E-02 moder lwtr 1001

  1001.03c  5.14732E-02
  8016.03c  2.57362E-02
  5010.03c  4.26822E-06
  5011.03c  1.71801E-05