Overview
Serpent is a multipurpose threedimensional continuousenergy neutron and photon transport code, developed at VTT Technical Research Centre of Finland since 2004. The code is distributed with different license options for noncommercial research and educational use, and for commercial work. The distribution is handled by VTT and two data centers: the OECD/NEA Data Bank and the Radiation Safety Information Computational Center (RSICC) in the US. See the users subpage for more information on Serpent distribution.
The recommended publication for referencing Serpent is:
• 
Leppänen, J., et al. (2015)
"The Serpent Monte Carlo code: Status, development and applications in 2013."
Ann. Nucl. Energy, 82 (2015) 142150.

It should be noted, however, that this review article is largely outdated, and does not cover many of the features and capabilities available in the current version.
More publications are listed on a separate subpage.
Support for users is provided at the
Serpent Discussion Forum, and the
Serpent Wiki
acts as an online user manual.
Typical applications
The continuousenergy Monte Carlo method can be used for a wide range of particle transport
applications. The physics model in Serpent covers neutron, photon and coupled
neutronphoton simulations.
Serpent was originally developed as a reactor physics code, but the
scope has been considerably broadened over the years.
The most common applications for Serpent are introduced in the following, together with the
associated key features and capabilities.
One of the drawbacks of the Monte Carlo method is its high computational cost, which
emphasizes the significance of parallel computing. Serpent features a hybrid
MPI / OpenMP approach, which operates at CPU core and clusted node level. Modern
multicore workstations are sufficient for most applications, but large
(fullcore) burnup calculation problems, dynamic simulations and coupled multiphysics
calculations may require a large computer cluster to reach an acceptable overall running time.
Reactor modeling
Serpent has been used for the modeling of different types of
nuclear fission reactors since the beginning of the project.
The standard geometry model relies on a universebased constructive solid geometry (CSG) type,
which is sufficient for most reactor types based on a regular geometry. Additional geometry options
include an explicit particle / pebble fuel model for hightemperature gascooled reactors, and a CADbased geometry type for complicated irregular structures. The Monte Carlo method
is inherently threedimensional, and scalable to an arbitrary level of spatial detail.
Neutron interaction physics in Serpent is based on classical collision kinematics and ENDF reaction laws.
Cross sections are read from ACE format data libraries. The format was originally developed for the MCNP code
from Los Alamos National Laboratory, and is also used by other Monte Carlo codes, such as OpenMC and Geant4.
The continuousenergy interaction data is produced from
evaluated nuclear data files without major approximations. This also means that the best available knowledge
on neutron interactions can be used in the simulations asis. Monte Carlo codes can be used with
any reactor technology without applicationspecific limitations. ACE format cross section libraries
are preprocessed to a specific temperature. The temperatures can be further adjusted using
a builtin Doppler broadening routine.
Serpent features builtin burnup calculation capability for tracking the nuclide concentrations subject
to neutron interactions and radioactive decay. The methodology is applicable to nuclear fuel and
activated materials. Depletion zone division and the formation of transmutation and decay paths
is accomplished automatically, with minimal input from the user. Serpent uses the Chebyshev
Rational Approximation method for the solution of the Bateman depletion equations and provides
various time integration methods to perform the iterations between the neutronics and depletion solution.
To reduce the memory demand in burnup calculations, Serpent provides different optimization modes for small
and largescale systems. Features used to speedup the transport simulation can be switched off to improve memory
efficiency. The standard methodology is
applicable up to SMRscale fullcore burnup calculations. A collisionbased domain decomposition scheme enables
running burnup calculations in traditional large LWRs, albeit at high computational cost.
Standard detectors (tallies) in Serpent enable calculating flux, power and reaction rate distributions in
geometry cells and materials, as well as regular structures, such as lattices and superimposed meshes.
In addition to the standard (spacediscretized) volume and surfaceintegrated detectors, Serpent provides a methodology to
reconstruct spatial distributions into functional form. Such an approach relies on the
socalled functional expansion tallies (FET), based on orthonormal polynomial bases. FET's methodology
endows Monte Carlo estimates with a deterministic/hybrid capability.
Reactor modeling covers a wide range of applications with different methodologies. Other relevant
capabilities within this scope include a dynamic simulation mode with delayed neutron physics,
methods for sensitivity and uncertainty analyses and advanced heat deposition modes with
gamma heating. Coupling to other physics solvers is addressed separately below.
Group constant generation
One of the original intended uses for Serpent was group constant generation for deterministic fuel cycle simulator and
transient analyses codes. Serpent can produce all input parameters needed for nodal diffusion calculations,
including homogenized macroscopic reaction cross sections, isotopic microscopic cross sections,
diffusion coefficients, assembly discontinuity factors, poison cross
sections and pointkinetics and delayed neutron parameters. The calculation of most parameters relies on standard Monte Carlo
tallies. For diffusion coefficients Serpent uses the cumulative migration method (CMM), and effective delayed neutron
fractions are calculated using the iterated fission probability method (IFP). Group constant generation can be performed
as infinite or critical spectrum calculation.
Group constant generation requires repeating the calculation for a large number of burnup points and variations in the
thermal hydraulic and reactivity conditions. Managing the calculation chain involving thousands of runs
becomes a formidable task, which can be considerably simplified using builtin features. Serpent provides a branch
capability to invoke small variations in the operating conditions. The casematrix feature enables organizing the
history and restart calculations in an optimal way to be run in computer clusters. With sufficient computational
resources the Monte Carlo method can be considered a viable option for group constant generation.
The methodology used for group constant generation has been designed to be compatible with VTT's Ants nodal
neutronics code, used as part of the Kraken computational framework (see below). Serpent has also been widely used
for producing input data for other nodal codes, such as DYN3D and PARCS.
Coupled multiphysics applications and the Kraken framework
Modeling of an operating nuclear reactor requires solving a coupled problem between neutronics, thermal hydraulics and fuel
behavior. One of the challenges of coupling a Monte Carlo neutronics solver to such multiphysics calculation chain
is handling the twoway data transfer between the solvers. The approach in Serpent is to completely separate the
thermal hydraulic statepoint distributions from the userdefined geometry model.
Information on material temperatures and densities
is brought into the transport simulation via a universal multiphysics interface, which is essentially a threedimensional
structure superimposed over the underlying geometry.
For the code user this means that no modifications are needed in the geometry model when Serpent is coupled to
another solver. Internally the distributions are handled using an efficient rejection sampling based algorithm,
which allows both discretized and continuous changes in temperature and density. Temperature modifications are performed using
an onthefly Dopplerbroadening routine, so there is no need to store cross sections at multiple
temperatures. The multiphysics interface supports several distribution types, including an unstructured OpenFOAM
mesh, and a special interface type for fuel performance code coupling.
Since 2017 Serpent has been developed as part of VTT's new Kraken computational core physics framework, where
the code can be used either for producing group constants for the Ants nodal neutronics code
(reducedorder approach), or directly coupled to the other solvers (highfidelity approach).
Radiation transport and fusion applications
Even though Serpent was originally developed as a reactor physics code,
the implementation of new methods and capabilities has enabled applications
beyond the scope of fission reactors. The CADbased geometry type is a practical option for
the modeling of complicated and irregular structures. Such geometries are commonly encountered
in fusion and radiation transport applications. The same neutron
and photon physics routines
developed for fission applications are similarly
valid, regardless of the source from which the particles were emitted.
One of the major advantages of Serpent in radiation transport applications is that the
builtin burnup calculation capability provides the means to generate source terms
comprised of spent nuclear fuel or neutronactivated materials.
Practical examples include radiation shielding calculations carried out for
a spent nuclear fuel storage cask, or shutdown dose rate calculations for
a fusion reactor. The source term is formed automatically using the
material compositions obtained from a burnup/activation calculation, combined with
photon emission spectra read from ENDF format decay data files. No additional processing
is required from the user.
Radiation shielding calculations often require extensive use of variance reduction,
in order to get sufficient statistics in the heavily shielded parts of the geometry.
The variance reduction scheme in Serpent is based on the conventional
weightwindows method, with weightwindow boundaries defined by a super imposed
importance mesh. The importances can be read from MCNP WWINP format files, or
generated using a builtin response matrix method based solver. The builtin solver
supports calculating importances with respect to one or multiple responses, and
and global variance reduction option to populate the entire geometry. Mesh options include
rectangular and cylindrical types and selfadapted mesh that can be automatically
refined around heavily shielded structures.
Verification and validation
Throughout its development Serpent has been verified
by comparison to MCNP, in which the neutron physics model is based on the same ENDF reaction laws. The differences are generally
within the range of statistical accuracy
when the same ACE libraries are used in the calculations.
Differences to other Monte Carlo codes (Keno, OpenMC, Tripoli, etc.) are small,
but statistically significant discrepancies can be observed
in some cases. The photon physics model in Serpent differs to some extent from that in MCNP. Similar level of agreement
cannot be expected as in neutron transport simulations, but the differences should remain small.
Verification of burnup calculations becomes more complicated, due to the lack of a perfect reference code. The same applies to multiphysics simulations, in which
Serpent is coupled to thermal hydraulics and/or fuel performance codes.
In addition to discrepancies in the transport physics, there are additional factors related to data used in the calculations,
coupling algorithms, methods used in external solvers, and so on. The largest uncertainties in burnup calculations often
originate from the fundamental nuclear data, and approximations applied in the formulation of the transmutation chains.
Considerable effort from both Serpent developers and the user community has been devoted to the verification and
validation of Serpent by computational benchmarks. The calculation cases cover a wide range of applications, and some
benchmarks provide experimental data for comparison. Related publications and reports are collected in
Serpent Wiki.


Updates
on website 
October 4, 2022 
• 
Website for the
11th International Serpent UGM set up. 
August 2, 2022 
• 
The 11th Serpent UGM announced (from August 29 to September 1).

May 23, 2022 
• 
New base version 2.2.0 released to the OECD/NEA Data Bank, contents of the website completely revised. 
February 24, 2021 
• 
New code version released. 
Serpent Discussion Forum
Serpent Wiki
Base version:
2.2.0 (see distribution)
Current update:
2.2.0 (May 5, 2022)
Some recent and upcoming
events 
August 29  September 1, 2022
11th Serpent User Group meeting. in Garching, Germany. 
July 1, 2022
VTT and Studsvik Scandpower announce collaboration on the development and distribution of an NQA12015 certified version of Serpent as part of SSP's software family (see
press release for more information). 
May 23, 2022
Serpent 2.2 available at the OECD / NEA Data
Bank (PackageID
NEA1923/01)

May 1520, 2022
Kraken workshop at the
PHYSOR 2022 conference in Pittsburgh, PA, USA, May 1520, 2022.
(download material)

October 2730, 2020
10th International Serpent User Group Meeting organized as an online event by the Technical University of Munich.

October 1419, 2019
9th International Serpent User Group Meeting in Atlanta, GA, USA.

May 29  June 1, 2018
8th International Serpent User Group Meeting in Espoo, Finland.

April 2226, 2018
Serpent workshop at the
PHYSOR 2018 conference in Cancún, México, April, 2226, 2016. 
November 69, 2017
7th International Serpent User Group Meetingin Gainesville, FL, USA.

May 19, 2017
Ville Valtavirta defended his Doctoral Thesis:
Development and applications of multiphysics capabilities in a continuous energy Monte Carlo neutron transport code at Aalto University.

September 2629, 2016
6th International Serpent User Group Meeting in Milan, Italy.

October 1316, 2015
5th International Serpent User Group meeting in Knoxville, TN, USA 
May 8, 2015
Tuomas Viitanen defended his Doctoral Thesis:
Development of a stochastic temperature treatment technique for Monte Carlo neutron tracking at Aalto University.

September 1719, 2014
4th International Serpent User Group meeting in Cambridge, UK 
November 68, 2013
The Third International Serpent User Group Meeting in Berkeley, California, USA,
organized by the University of California, Berkeley.
(see meeting
website)

May 24, 2013
Maria Pusa defended her Doctoral Thesis on
Numerical methods for nuclear fuel burnup calculations at Aalto University.

September 1921, 2012
The Second International Serpent User Group Meeting in Madrid,
Spain, organized by the Universidad Politécnica de Madrid
(see meeting
website) 
January 31, 2012
Betatesting phase of Serpent 2 started 
September 1516, 2011
2011 Serpent International User Group Meeting, Dresden,
Germany (also see the
topic at the discussion forum and the meeting
website) 
March, 2010
Serpent 1.1.7 available at RSICC
(Code Number C00757)

January 15, 2010
Serpent cross section libraries released as a separate NEA package
(PackageID
NEA1854)

January 6, 2010
NEA Base version upgraded to 1.1.7
(PackageID
NEA1840)

May 26, 2009
Serpent 1.1.0 available at the OECD / NEA Data
Bank (PackageID
NEA1840)

April 8, 2009
Serpent 1.1.0 submitted
to the OECD / NEA Data Bank for public distribution 
