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Open source detailed Thermal-hydraulics analysis code

Detailed Thermal-hydraulics analysis code JUPITER

JUPITER is a detailed thermo-hydraulic analysis code developed for the purpose of advanced understanding of gas-liquid two-phase flow in nuclear reactors and the transition phenomena of melted materials that occur during severe accidents. It can handle multiphase flows of arbitrary compositions consisting of gas, liquid, and solid phases, and has CUDA implementation in addition to MPI/OpenMP hybrid parallelization, and supports high-fidelity analysis using large-scale grids. It has been released as open source since April 2026.

Download in zip format Download from GitHub View manual
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What is JUPITER specs Usage guide

What is JUPITER?

Various functions available for nuclear system calculations

Diagram of bubble flow function

bubbly flow

Gas-liquid two-phase flow containing a large number of bubbles is possible with numerical coalescence of bubbles suppressed.

Diagram of turbulence function

turbulence

If a sufficient number of meshes can be secured, it is also possible to simulate turbulent flow.

Diagram of radiation function

radiation

Detailed evaluation of radiant heat transfer, which is important in thermal analysis of high-temperature melts

Diagram of eutectic reaction function

eutectic

Evaluating the effects of material interactions and melting point reduction during accidents by considering eutectic reactions

phase change diagram

phase change

Simulating melting/solidification, boiling/condensation using temperature recovery method

Diagram of porous flow

porous flow

Simulating the flow inside a porous body using the Darcy-Brinkman approximation

specs

Language used C, Fortran, CUDA Difference method/lattice Finite difference method, orthogonal equidistant grid
Operating environment/compiler LINUX / GNU C, Intel(R) C, PGI(R) C Shape input Direct import of 3D-CAD data (STL data)
Advection term difference differentiation 5th WENO method interface capture PLIC, THINC, THINC/WLIC, THINC/AWLIC
time integral Third-order accuracy TVD Runge-Kutta method turbulence model Model not used (DNS), ILES
Parallelization Massively parallel computing using MPI, OpenMP, and CUDA Curvature/surface tension calculation method CLSVOF/CSF model
phase change Temperature recovery method (model under consideration to improve accuracy) pressure solution MPI-OpenMP hybrid CG method with preprocessing
Additional physical model Porous flow (Darcy-Brinkman), radiation heat transfer (DO method) chemical reaction model Eutectic reaction, steam oxidation, hydrogen absorption reaction

Usage guide

Public version code download
Download in ZIP format / GitHub public page

Steps to start using (sample calculation)
From building to sample input and visualization, see HERE

manual
Japanese version manual / English version manual

References
S. Yamashita, T. Ina, Y. Idomura and H. Yoshida, “A numerical simulation method for molten material behavior in nuclear reactors,” Nucl. Eng. Des., 332, pp. 301-312 (2017). View paper

inquiry
For inquiries regarding JUPITER, please contact us below.
　fukuda.takanari[at]jaea.go.jp　(Takanari Fukuda: change [at] to @)
(Since JUPITER is released as open source, you may be able to obtain clues to problem solving by referring to the source code and manual to the generation AI. Please use these methods as well before contacting us.)

terms of service

BSD-2-Clause
Copyright 2026, Japan Atomic Energy Agency (JAEA) All rights reserved.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice,this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.