The American Society of Mechanical Engineers (ASME) Committee on Nuclear Quality Assurance (NQA) develops, manages and coordinates quality assurance requirements and implements guidance. The NQA-1 Level 3 designation means that RELAP5-3D can model all types of reactors reasonably well. However, it doesn’t meet the Nuclear Regulatory Commission requirements for any specific reactor unless additional commercial grade dedication is performed.

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 inl.gov/ncrc.

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.