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1.         NAME AND TITLE

KRAKEN 1.2:            Computational Reactor Analysis Framework..

RSICC is authorized to distribute KRAKEN 1.2 for research and education purposes only. Commercial use is prohibited. RSICC can only distribute KRAKEN 1.2 to customers that are in the following “pre-approved” countries: member states of European Union, Australia, South Korea, Canada, Iceland, Japan, New Zealand, Switzerland, the United Kingdom, and the United States.

RSICC can only license and authorize the use of KRAKEN for peaceful and legal purposes. KRAKEN cannot be used in connection with the development, production, handling, operation, maintenance, storage, detection, identification, or dissemination of chemical, biological, or nuclear weapons or other nuclear explosive devices or the development, production, maintenance, or storage of missiles capable of delivering such weapons. The use of KRAKEN for any military purpose is strictly prohibited.


Ants : Nodal neutronics code.

Cerberus : Multi-physics driver package.

Cetus :Reactor simulator package.

FINIX :Fuel behavior module.

Kharon :Porous medium thermal hydraulics code.

KrakenTools :Python 3 package for pre- and postprocessing purposes.

SuperFINIX :Core level fuel behavior module.


VTT Technical Research Centre of Finland.


PYTHON, GCC GFORTRAN, CMAKE; Linux OS is supported, but auxiliary packages may function on other platforms. (RSICC ID: C00877MNYWS00).


The Kraken computational framework is a new modular calculation system designed for coupled core physics calculations. The development started at VTT Technical Research Centre of Finland in 2017, with the aim to replace VTT’s outdated legacy codes used for the deterministic safety analyses of Finnish power reactors. In addition to conventional large PWRs and BWRs, Kraken is intended to be used for the modeling of SMRs and emerging non-LWR technologies.

The main computational modules include the Ants nodal neutronics solver, the FINIX fuel behavior module and the Kharon thermal hydraulics code, all developed at VTT. The core physics solution can be further coupled to system-scale simulations. In addition to development, significant effort has been devoted to verification and validation of the implemented methodologies. The reduced-order Ants code has been successfully used for steady-state, transient and burnup simulations of PWRs with rectangular and hexagonal core geometry. The Ants–Kharon–FINIX code sequence is actively used for the core design tasks in VTT’s district heating reactor project.

A complete description of the project is found at the Kraken website:-


Ants is a modern multi-group nodal neutronics solver capable of performing steady state, burnup and transient calculations in rectangular and hexagonal fuel geometry. The solution is based on a combination of the analytic function expansion nodal method (AFEN) and the flux expansion nodal method (FENM), used for solving the diffusion equation. Ants has been specifically developed for the Kraken framework since 2017 and to utilize Serpent-generated group constant data. The main advantage of using the continuous-energy Monte Carlo method for group constant generation is that the same Serpent–Ants calculation sequence can be applied to a wide range of fuel and reactor types without application-specific limitations.

FINIX is a fuel behavior module developed at VTT since 2012 for the purpose of multi-physics simulations. The code was originally designed for internal coupling, for example, to replace outdated fuel models in VTT’s legacy transient analysis codes or to provide temperature feedback for Serpent simulations. The code has a small user base, with most of the users applying it as an internally coupled fuel behavior solver with Serpent.

FINIX solves the thermo-mechanical behavior of a single fuel rod under neutron irradiation conditions, taking into account both thermal effects and changes in rod geometry. The thermal and mechanical models are coupled by internal pressure and fuel-cladding gap heat conductance, which are functions of rod temperature and dimensions. Both the heat equation and the mechanical behavior are solved radially in one-dimensional cylindrical geometry independently for several axial nodes. The node-wise solutions are coupled together via internal pressure, which is solved simultaneously for the entire rod. This approach is generally referred to as the 1.5-dimensional model. The dependence of physical quantities on local parameters, such as temperature and burnup, are based on publicly available material correlations.

Kharon is a closed-channel two-phase steady-state thermal hydraulics module based on the porous-medium approximation. Being a closed-channel steady-state solver implies its applications are limited to time-independent simulations with a closed assembly geometry or no appreciable cross-flows. The original motivation for developing Kharon was to have a light-weight TH solver to complete the multi-physics coupling during the early development stages of the Kraken framework. Even though Kharon was essentially designed as a placeholder for a more advanced methodology, the code has shown good results in routine fuel cycle simulations coupled to Ants and SuperFINIX. The module has been used, for example, in the core design analyses of VTT’s LDR-50 district heating reactor concept.

Communication between the solver modules is handled using the Cerberus Python package. Cerberus is essentially a high-level code-agnostic interface that provides the methods for socket-based communication, exchange of field data, iteration algorithms, etc. The Python-based functionality is complemented by a reactor simulator module, which provides a user interface for reactivity control, core reloading operations and other features needed to perform routine fuel cycle simulations. The simulator also collects the relevant output data and evaluates several safety-related parameters, such as reactivity feedback coefficients, control rod worths, shutdown margins and power peaking factors.


Complex input files may require more compute resources.


The running time depends on the case and the calculation parameters.


At least 5 GB of RAM is recommended if the code is intended to be used for assembly burnup calculations.


Linux OS is supported, but auxiliary packages may function on other platforms. CMAKE, BOOST, GCC and GFORTRAN.

10.        REFERENCES

Leppänen, J., Valtavirta, V., Rintala, A., Hovi, V., Tuominen, R., Peltonen, J., Hirvensalo, M., Dorval, E., Lauranto, U. and Komu, R. "Current Status and On-Going Development of VTT's Kraken Core Physics Computational ramework." Energies, 15 (2022).

Hirvensalo, M., Rintala, A. and Sahlberg, V. "Triangular Geometry Model for Ants Nodal Neutronics Solver." In proc. M&C 2021, Virtual conference, Oct. 3-7, 2021.

Ikonen, T., Loukusa, H., Syrjälahti, E., Valtavirta, V., Leppänen, J. and Tulkki, V. "Module for thermomechanical modeling of LWR fuel in multiphysics simulations." Ann. Nucl. Energy, 84 (2015) 111-121.

Ikonen, T., Syrjälahti, E., Valtavirta, V., Loukusa, H., Leppänen, J. and Tulkki, V. "Multiphysics simulation of fast transients with the FINIX fuel behaviour module." EPJ Nuclear Sci. Technol., 2 (2016) Article 37.

Lauranto, U., Komu, R., Rintala, A. and Valtavirta, V. "Validation of the Ants-TRACE code system with VVER-1000 coolant transient benchmarks." Ann. Nucl. Energy, 190 (2023) 109879


The package is transmitted digitally as a zip file that includes source code and documentation.


April 2024.