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International RELAP5 User Group Meeting (IRUG)
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Versatile modeling and simulation tool to predict a nuclear reactor's complex phenomena

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Connect with other RELAP5 users to share strategies and best practices for RELAP5 modeling

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Idaho National Laboratory’s RELAP5-3D is a robust modeling and simulation software designed to predict the intricate behavior of nuclear reactor systems. Originally developed in 1979, it has evolved beyond studying Light Water Reactors to successfully model the thermal and fluid performance of high-temperature gas-cooled reactors, super-critical CO2 reactors, sodium fast reactors, and molten salt reactors.

Accredited by international benchmarks and design bases analyses accepted by the Nuclear Regulatory Commission for reactor licensing, RELAP5-3D is recognized as the premier system analysis code. It is often the standard by which other system codes are measured.  RELAP5-3D has an extensive experiment database that validates the software’s modeling, offering confidence in predicting thermal and fluid performance.

With the ability to integrate into an analyst’s toolset, these codes provide a quick analysis of a system. When coupled with computational fluid dynamics (CFD) codes, such as openFOAM and STARCCM+, RELAP5-3D allows for comprehensive modeling.

RELAP5-3D is widely used for reactor safety analysis, design, operator training, and non-nuclear modeling and simulation.  The software is a Department of Energy 10 CFR 810 code, which means that RELAP5-3D is solely distributed through the INL Technology Deployment Office, which requires a license agreement, export control review, and approved use by the INL Code Oversight Group.

Meet our team

RELAP5-3D in the News

Lab’s FY-23 licenses outnumber previous year

Idaho National Laboratory researchers love seeing their technologies at work in the marketplace, bolstering the nation’s economy, energy industry and communities.

Key nuclear modeling and simulation software releases new version

Idaho National Laboratory has released the latest version of RELAP5-3D, a versatile modeling and simulation tool that predicts complex phenomena happening inside a nuclear reactor. This update is the first RELAP5-3D code release in five years.

How INL’s reactor safety code extends beyond nuclear

A lot can change in 50 years. Just ask a carmaker, a telephone operator or any middle-aged adult. Like other creations from 1966, Idaho National Laboratory’s most successfully licensed invention — a reactor safety code — has evolved a lot since its inception.

Frequently Asked Questions

RELAP5-3D operates optimally on Linux-based computers but also operates effectively on Windows-based computers. The code does not operate on macOS computers.

RELAP5-3D can model the following reactors: 

  • Light water reactors (pressurized water reactors and boiling water reactors)
  • Heavy water reactors
  • Supercritical water reactors
  • Gas-cooled reactors
  • Molten salt reactors
  • Liquid metal cooled reactors

RELAP5-3D can model 31 different fluids. 

  • light water
  • heavy water
  • hydrogen
  • lithium
  • potassium
  • helium
  • nitrogen
  • sodium
  • NaK
  • lithium-lead
  • ammonia
  • new light water (based on internal energy and pressure)
  • glycerol
  • blood
  • bismuth-lead
  • light water (Athena interpolation)
  • heavy water (Athena interpolation)
  • light water (IAPWS-95)
  • carbon dioxide
  • helium new style
  • xenon new style
  • helium-xenon new style
  • molten salt 1 new style
  • molten salt 2 new style
  • molten salt 3 new style
  • molten salt 4 new style
  • DowThermA new style
  • r134a new style
  • molten lead
  • vertrel
  • 95 light water (based on internal energy and pressure)

There are no trial versions for this export-controlled code. Each potential user must be approved through the INL export control process and the INL Code Oversight Group. We must also establish a license agreement for each organization.

Remote executable access (Level 1) means that the code is executed on the INL High Performance Computing (HPC) platform. The input decks, property files, etc., are stored on the INL HPC and are only visible to you. INL HPC benefits include multi-CPU access and large system memory. Local executable access (Level 2) means that the code executable is stored on your local computer system. Level 2 may also operate the code via the INL HPC.

A single user license grants an International RELAP5 User Group (IRUG) member organization, under Department of Energy rules, the legal right to have only one person work with RELAP5-3D. The contract providing the license rights must specify the technical point of contact (TPOC) and the name of the single user, and that one user must apply to and receive approval for access to the RELAP5-3D code by the INL Code Oversight Group. No one else at the IRUG member organization is legally permitted to work with the code. Unless the TPOC and the single user are the same person, the TPOC may not work with the code except to receive it, install it for the single user, and maintain contact with INL regarding the contract and other communications. In the event the organization, INL or the federal government terminates the single user’s work with RELAP5-3D, the member organization may, with INL notification and consent, and until the end of the contract, replace that single user with a new single user who in turn must be approved for RELAP5-3D work by the INL Code Oversight Group.

A multi-user license allows multiple users at the IRUG-member organization to work with the code; however, only those who have applied to the INL Nuclear Computational Resource Center and received notification of approval to use RELAP5-3D, may work with the code. All requests for RELAP5-3D access will be processed and approved on a person-by-person basis, not as a blanket organizational approval. The IRUG member organization may add and/or delete approved users who have been approved by the INL Code Oversight Group, with prior INL notification and consent, until the contractual multi-user license agreement expires.

RELAP5-3D is continuously developed by the INL RELAP5-3D development team, whereas RELAP5/Mod 3 stopped development in 2010. Some of the added capabilities include:

  • RELAP5-3D has the 3D hydraulics component.
  • Coupling to other Codes: Parallel Virtual Machine, in-memory, socket, Python.
  • Capable of modeling most reactor types including properties for 30 types of fluids.
  • Modern ANSI FORTRAN 2008 coding: Intel and GNU Fortran/C compilers.
  • Fast: Vectorized, SMD parallelized, structured coding.
  • Sequentially verified to 32 decimal places.
  • NQA Level 3 and has achieved commercial grade dedication for several innovative reactor design licenses.

Licensing fees vary with the six types of licenses offered. See the Licensing Fee page for detailed information on licensing fees and benefits associated with the selected license.

RELAP5-3D licenses are renewed on a one-, three- or five-year basis, as negotiated by INL’s Technology Deployment office and agreements administrator, per the terms of the organizational license agreement. Fees are assessed and collected each renewal period.

RELAP5-3D is continuously developed. The goal is to release a new code version annually. The most recent version of RELAP5-3D, Version 4.5.2, was released in October 2023.

RELAP5-3D is the premiere thermal hydraulics code in the nuclear industry. RELAP5-3D models the entire reactor system (i.e., reactor core, primary cooling system and secondary coolant loops) for every type of nuclear reactor, including light water, heavy water, molten salt, liquid metal and high-temperature gas reactors. RELAP5-3D is used across the world by universities, research institutions, nuclear power plant vendors, simulator companies, utilities and national laboratories.

New users of RELAP5-3D can request access through INL’s Nuclear Computational Resource Center at

Yes! International license agreements are available. The code is export controlled and access may be limited for some countries.

IRUG is the International RELAP5 User Group. An organization becomes a member of IRUG when a RELAP5-3D license is established. Meetings are held annually and all IRUG members are invited to participate in a RELAP5-3D training, technical seminar session and business meeting. The technical seminar allows organizations to share how RELAP5-3D is being used and what has been learned, creating a collective knowledge base and international connection between organizations. New code features can also be requested during IRUG.

Idaho National Laboratory

Dr. Paolo Balestra

Methods Lead for the Advanced Reactor Technology

Dr. Paolo Balestra obtained his master’s degree in energy engineering in 2012 and his Ph.D. in energy and environment with focus on nuclear engineering in 2017 at the “La Sapienza” University of Rome. He is currently the Methods Lead for the Advanced Reactor Technology - Gas Cooled Reactor (ART-GCR) DoE Program at the Idaho National Laboratory (INL). He is also work package manager of INL’s High Temperature Gas-cooled Reactor (HTGR) area of the multiphysics section of the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program. His research efforts are focused on two areas: the analysis and optimization of the new generation of advanced reactor design and the development and validation of dedicated advanced reactor designs simulation tools.

Dr. Victor Coppo Leite


Dr. Victor Coppo Leite is a researcher at Idaho National Laboratory who recently earned a Ph.D. in Nuclear Engineering from Pennsylvania State University. He also holds a Bachelor of Science and a Master of Science in the same field from the Federal University of Rio de Janeiro. His interests include incorporating machine learning methods into thermal hydraulic problems and multi-physics modeling. Currently, he applies his expertise to contribute to the development of RELAP5-3D, while also engaging in activities to maintain the code and conduct analyses.

Brandon Cox

Engineer & Software Developer

Brandon Cox is an engineer and software developer in the Thermal Fluid System Methods & Analysis Department at Idaho National Laboratory. In 2019 he graduated from Brigham Young University with a B.S. in Chemical Engineering. He has experience developing engineering software and computational fluid dynamics (CFD). He is currently a RELAP5-3D Developer and Analyst. In this capacity, he participates in various RELAP5-3D updates and development activities and corrects user errors. He also performs engineering calculations and analyses using RELAP5-3D and CFD. He has experience using Python, Fortran, Linux, and OpenFOAM. Outside of work he enjoys spending time with family and many outdoor activities including hiking, hunting, fishing, and photography.

Dr. Robert Kile

Advanced Reactor Research and Development Engineer

Dr. Robert Kile is an Advanced Reactor Research and Development Engineer at Idaho National Laboratory. His work focuses on code validation for gas-cooled reactors. Dr. Kile is also the coordinator for the Depressurized Conduction Cooldown problem of the OECD-NEA HTTF Benchmark.

Dr. George Mesina

Acting manager for Thermal Fluids Systems, Methods & Analysis

Dr. George Mesina earned his PhD in Applied Mathematics in 1988 from the University of Pittsburgh. He has served as the RELAP5-3D Code Architect for 25 years. He is the acting manager for INL’s Thermal Fluids Systems, Methods, and Analysis department, responsible for the RELAP5-3D code analysts and developers and leading the RELAP program.

RELAP5 team portraits: Carlo Parisi

Dr. Carlo Parisi

Nuclear Engineer

Dr. Carlo Parisi holds a MSc and PhD in nuclear engineering from Pisa University, Italy. He joined INL in 2015, and since then he has worked on the Light Water Reactor Sustainability (LWRS) program, Versatile Test Reactor (VTR) and improved Advanced Test Reactor (ATR) concepts, and the sodium loop for the TREAT reactor. Since 2020 he is responsible for the thermal hydraulic design of the microreactor "MARVEL”.

connie stevens

Connie Stevens

Project Coordinator

Ms. Connie Stevens works as the RELAP5-3D project coordinator. She received a Master of Legal Studies from the University of Utah S.J. Quinney College of Law, and a B.Sc. in Applied Management from Brigham Young University-Idaho. Connie’s work is currently focused on building and strengthening relationships with domestic and international RELAP5-3D license holders, leading the organization of the annual International RELAP5 User Group (IRUG) meeting, and providing program management support to the department manager and RELAP5 team.

Dr. Mauricio Tano Retamales

Staff Scientist

Dr. Mauricio Tano-Retamales is a Staff Scientist in the Thermal Fluids Systems and Methods Analysis group at Idaho National Laboratory (INL). He currently leads efforts for high- and intermediate-fidelity modeling and simulation of multiphysics phenomena in advanced nuclear reactors, which include the development of models coupling between neutronics, thermal-hydraulics, thermomechanics, salt chemistry, and corrosion. Additionally, Dr. Tano-Retamales is an active developer of INL’s coarse-mesh computational fluid dynamics (CFD) code Pronghorn and INL’s MOOSE-based subchannel code for advanced nuclear reactors. Dr. Tano-Retamales leads the coupling between RELAP5-3D and CFD codes and performs nuclear reactor analyses using RELAP5-3D.

Dr. Ishita Trivedi

Computational Scientist

Dr. Ishita Trivedi works as a computational scientist in the Advanced Reactor Design & Technology at Idaho National Laboratory (INL). She received her PhD in Nuclear Engineering from North Carolina State University focusing on Uncertainty Quantification and Propagation Methods for Safety Analysis of Lead-cooled Fast Reactors. Ishita also has a M.Sc. and B.Sc. in Nuclear Engineering from North Carolina State University and Pennsylvania State University, respectively. At INL, Ishita is involved in several advanced reactor multi-physics analysis projects including leading the MARVEL criticality benchmark development, nuclear thermal propulsion reactor modeling and simulation among other projects. Ishita also works in software quality assurance, validation & verification of INL applications.

Dr. Jan Vermaak

Senior Nuclear Multiphysics Engineer

Jan Vermaak joined INL in 2022 as a Senior Nuclear Multiphysics Engineer. He has 14 years of diverse experience in reactor engineering ranging from nuclear safety analysis to managing reactor engineering at the Texas A&M University TRIGA reactor. He holds a bachelor’s degree in mechanical engineering and a master’s and PhD in nuclear engineering. Jan’s current work is focused on code development and analysis in the fields of neutronics, computational fluid dynamics (CFD), thermal-hydraulics, mechanical stress, and nuclear fuel performance.