RELAP5-3D 4.x: Thermal-Hydraulic Nuclear Reactor Analysis, Licensing Opportunity

Technology Summary:

RELAP5-3D is a fully integrated system-level modeling code for simulating the behavior of nuclear power plants and other complex thermal-fluid systems. Originally developed for light water reactor safety analysis, RELAP5-3D supports detailed modeling of multiphase flow, heat transfer, reactor kinetics, and fluid-structure interactions. It is capable of performing both steady-state and transient analyses in one-, two-, or three-dimensional geometries, making it one of the most versatile nuclear systems analysis tools in use today.

Problem Addressed:

Designers, operators, and regulators of nuclear systems need validated tools to simulate a wide range of operating and accident scenarios, from routine transients to complex multi-physics events. Many commercial codes lack the physics detail or numerical flexibility needed to capture the behavior of advanced reactor designs, high-fidelity safety cases, or coupled systems involving multiple working fluids or complex geometries.

Proposed Solution:

RELAP5-3D enables high-fidelity simulations of nuclear thermal-hydraulics and safety behavior. Key capabilities include:

• Modeling of single- and two-phase flow using a two-fluid model


• Support for multiple fluids, flow regimes, and non-condensable gases


• Built-in reactor kinetics and neutron transport models (point kinetics or 3D neutron kinetics)


• Fully coupled heat structures and control system modeling


• Parallel processing and restart capabilities for large, complex runs


• Optional coupling with external tools for fuel performance, CFD, or systems-level co-simulation

RELAP5-3D is actively maintained and enhanced to support modern reactor designs, including small modular reactors, high-temperature gas-cooled reactors, and liquid metal-cooled reactors.

Key Advantages:

• Comprehensive Simulation Capabilities: Supports steady-state, transient, and accident analysis across multiple reactor types and configurations


• Multidimensional Modeling: Simulates flow and heat transfer in 1D, 2D, and 3D domains


• Built-In Neutronics: Includes tightly coupled neutron kinetics for integrated reactor core analysis


• High Fidelity and Flexibility: Validated models for a wide range of fluids, heat transfer modes, and operational conditions


• Interoperable Framework: Can be integrated with external modeling tools for coupled physics or system-level analysis


• Regulatory Acceptance: Used by U.S. national labs, industry, and regulators for licensing and safety evaluation

Market Applications:

• •Advanced Reactor Design and Licensing: Used by developers of SMRs, microreactors, and Gen-IV systems


• Safety Analysis: Supports regulatory submittals and design basis accident analysis for existing and new reactors


• Plant Performance Evaluation: Useful for plant optimization, transient analysis, and control strategy development


• R&D and Academic Use: Widely used in national laboratories and universities for education and research


• Coupled System Modeling: Applicable in multi-domain simulations, including hybrid energy systems and integrated energy parks

Licensing Opportunity:

INL’s Technology Deployment department focuses solely on licensing intellectual property and collaborating with industry partners who can commercialize our innovations.

We do not engage in purchasing, procurement, or hiring external services for technology development. Our objective is to connect with companies interested in licensing and bringing our technologies to market.