​RELAP5-3D is a modeling and simulation software for predicting the coupled behavior of the reactor coolant system, the reactor core, and the secondary coolant loop during various operational scenarios, design basis transients, and postulated transients that may occur in a nuclear reactor. Although the software originated in 1979 as “Reactor Excursion and Leak Analysis Program” to study transients for commercial Light Water Reactors, the code has evolved to successfully model thermal and fluid performance of high-temperature gas-cooled, super-critical CO2, sodium fast reactors, and molten salt reactors. 

Via numerous international benchmarks and design bases analyses accepted by the Nuclear Regulatory Commission for reactor licensing, RELAP5-3D has established itself as the premier system analysis code and is often the standard by which other system codes are measured.  One of the strengths of RELAP5-3D is its considerable experiment database that validates the software’s modeling. A system analysis code employs correlations and models to​​ approximate the performance of a component or system and experiment data for the physical phenomena provides confidence that the system analysis code’s correlations/models are sufficient for capturing the fundamental response of the physical phenomena.

System analysis codes are intended to be part of an analyst’s toolset. These codes provide relatively fast analyses of a system.  More in-depth and higher-fidelity analyses are possible with a computational fluid dynamics (CFD) code. The ideal modeling scenario is to have the system analysis code predict flow, pressure and temperature in regions of the system where the flow is well-developed and understood and have the CFD code predict flow, pressure, and temperature in the more complex regions of the system.  To this end, considerable investments have been made to facilitate coupling RELAP5-3D to the openFOAM and STARCCM+ CFD codes.  The INL RELAP5-3D analysis team is contributing to a 2022 IAEA benchmark on system analysis code and CFD code coupling.

RELAP5-3D have a wide variety of users.  The software is used for reactor safety analysis, reactor design, simulator training of reactor operators, and for non-nuclear modeling and simulations.  The software is a Department of Energy 10 CFR 810 code, which means that there are guidelines to assure that nuclear technologies exported from the United States will be used only for peaceful purposes. RELAP5-3D is distributed via the INL Technology Deployment Office and requires a license agreement, export control review, and approved use by the INL Code Oversight Group.​​