RELAP5-3D review
A specialized code for 3D reactor thermal-hydraulics and accident analysis.
Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
RELAP5-3D is a thermal-hydraulics modeling and simulation code from Idaho National Laboratory for nuclear reactor behavior. It is aimed at teams working on reactor safety analysis, design, operator training, and reactor-system transients, rather than general-purpose CAE users. Its scope spans light-water, gas-cooled, molten-salt, and liquid-metal-cooled reactors, and it models normal operation as well as accident scenarios. The published plans show paid licensing, with no free plan.
The technical focus is three-dimensional thermal-hydraulics with integrated reactor kinetics. The code models 31 fluids and supports coupling with other codes through PVM, in-memory, socket, and Python methods. INL lists example coupled CFD codes including openFOAM and STAR-CCM+; that does not establish formal product integrations. This breadth across reactor types and coupling methods makes it relevant to nuclear analyses that need system-level transients or accident scenarios, but its specialization is a constraint for teams seeking a broader simulation suite.
Access and deployment depend on license level. According to the vendor's published plans, options range from Academic and single- or multi-user commercial licenses to Silver, Gold, and Platinum tiers. Level 1 provides HPC access; Level 2 provides a local executable; Silver offers restricted subroutine access through INL HPC; Gold and Platinum include source code access. Use requires an organizational license, export-control review, and approval for each user. Teams should choose RELAP5-3D when nuclear reactor thermal-hydraulics is central to the work and its licensing model fits; teams needing general-purpose CAE or unrestricted self-service access should consider alternatives.
RELAP5-3D pros and cons
- Where it wins
- Models normal operation and accident scenarios across multiple reactor types
- Supports 31 fluids and three-dimensional thermal-hydraulics modeling
- Offers remote INL HPC or local executable access by license level
- Where it doesn't
- Focused on nuclear and related modeling, not broad CAE workflows
- Organizational licensing and approval are required for access
- License fees vary by level and domestic or international status
RELAP5-3D fact sheet, pricing and score →
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