VTT’s Serpent Monte Carlo code is an expert tool for neutron and photon transport simulation, coupled multi-physics reactor analysis, fusion neutronics and radiation shielding calculations. Developed at VTT since 2004, Serpent is already used by hundreds of organisations worldwide. The Serpent-based Kraken is a multiphysics computational framework that provides reactor physics calculations and other tools for a wide range of nuclear applications.
Key facts
Serpent is a continuous-energy Monte Carlo neutron and photon transport code, and Kraken is a modular multi-physics framework for coupled reactor core analysis.
Applicable to all reactor types, including conventional PWRs and BWRs, SMRs and advanced non-LWR concepts.
Available under time-limited licences for commercial use and free of charge for non-commercial research and education.
Understanding how neutrons travel through a reactor core and how the chain reaction behaves is fundamental to every stage of nuclear reactor development – from early-stage concept design through safety analysis and regulatory licensing.
As the nuclear sector expands into small modular reactors, advanced reactor concepts and fusion energy, the demand for computational tools that can handle diverse and unconventional designs is growing.
VTT’s Serpent and Kraken software provide reactor physicists, design engineers and safety analysts with a flexible platform for addressing these challenges across both established and emerging reactor technologies.
Model the activity of any reactor type with a versatile Monte Carlo approach
The Monte Carlo method at the core of Serpent enables continuous-energy neutron and photon transport simulations without the application-specific limitations inherent in deterministic codes. This makes the software applicable to a wide range of reactor types and problems. It can be used to model anything from conventional light-water reactors to small modular reactors, molten salt designs and other advanced concepts that challenge traditional calculation tools.
Serpent can be used as a standalone code for neutronics and radiation shielding. As part of the Kraken framework, it serves as the high-fidelity neutronics solver in coupled multi-physics simulations that combine neutronics with thermal hydraulics and fuel behaviour modelling.
Serpent is also used in the generation of homogenised multi-group constants for reduced order neutronics calculations, for example, when using the Ants nodal neutronics code in the Kraken framework. To simplify your workflow, Serpent supports CAD-based geometry inputs.
Two computational routes – high fidelity and reduced order
Kraken offers users two complementary approaches to reactor core analysis. The high-fidelity route uses Serpent’s continuous-energy Monte Carlo method for best-estimate results.
Serpent also produces data for the reduced-order route, which shares the same framework, so the high-fidelity solution can serve as computational verification for the faster reduced-order calculations.
This flexibility is important in practice: early-stage scoping calculations can run on a standard workstation, while more detailed analyses may benefit from high-performance computing clusters.
What can you do with Serpent and Kraken?
- Fission reactor modelling: Coupled core physics analysis combining neutronics, thermal hydraulics and fuel behaviour. The Kraken framework handles the full coupled problem for stationary, fuel cycle and transient simulations. You can build a single computational model and apply it to multiple analyses to reduce the engineering effort associated with model preparation.
- Fusion neutronics: Serpent can do tritium breeding calculations, neutron dose estimates for reactor components, heat load assessments and activation analyses. The Kraken framework extends these capabilities with thermal hydraulics coupling for blanket coolant analysis.
- Radiation shielding: Serpent supports radiation protection and design assessment with Monte Carlo transport simulation of neutron and gamma radiation propagation through materials and structures. The software provides a streamlined workflow from source term generation to radiation transport with weight-window based variance reduction.
Developed for real-world reactor analysis
VTT originally developed Serpent and Kraken for its in-house reactor analysis needs – including safety analyses for Finnish nuclear power plants. The commercial distribution grew out of external interest in the tools. This origin means that we’ve developed the software to solve the real-world engineering problems that our nuclear experts have faced.
Serpent has been in public distribution for over fifteen years, with an international user base spanning research, academic and commercial organisations. Kraken, while newer, builds directly on Serpent’s established foundation.
We have an ongoing quality assurance programme to prepare Serpent for Nuclear Quality Assurance certification. The certification will facilitate the software’s use in regulatory licensing.
Working on the nuclear sector, we pay careful attention to quality assurance. Currently the software is widely accepted, and the feedback from users is overly positive. To further improve our QA, in 2025 we launched a separate QA programme to eventually give designers, operators and regulatory bodies the further confidence on the correctness of Serpent’s calculations.
How to get started with Serpent and Kraken?
Serpent and Kraken are installed locally on Linux-based systems. Computational requirements vary by application – simpler calculations can run on a modern laptop, while large-scale Monte Carlo simulations may require dedicated computing clusters.
Licences are time-limited, with pricing determined by the licence period and the number of users. Non-commercial research and educational licences are available free of charge through the OECD/NEA Data Bank and RSICC.
Technical support is available as a separate service from VTT’s development team – the same experts who build and use the software daily.
How to work with us
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Get in touch!
Tell us what you would like to do with Serpent or Kraken and how many licenses you would need.
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Proposal
We’ll create a proposal based on how many licenses and how much support you need.
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Execution and delivery
We deliver the software, establish licenses and support your experts in getting started.
