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

Overview

RELAP5-3D is a thermal-hydraulics code developed by Idaho National Laboratory (INL) to simulate how nuclear reactors perform during normal operations and accident scenarios. The code combines detailed 3D modeling with multiphysics capabilities to support analysis across a wide range of reactor types and system conditions. Backed by decades of development and trusted worldwide, RELAP5-3D helps reduce risk, accelerate design cycles and strengthen technical confidence—enabling teams to work more efficiently and make better decisions at every stage of the design process.

Designed to De-Risk Nuclear Projects from Day One

• Validates designs early: Simulate complex transients and accident scenarios during early-stage development to strengthen proposals and avoid costly

redesigns.

• Identifies system weak points before they become costly: Expose failure points under LOCA (Loss of Coolant Accident), RIA (Reactivity Initiated Accident), and other critical events before they show up in testing or operation.

• Simplifies modeling workflows: Built-in multi-physics and 3D capabilities reduce reliance on multiple tools, speeding up analysis and cutting integration overhead.

• Accelerates licensing approvals: RELAP5-3D is trusted by regulators worldwide, helping align safety cases with recognized evaluation frameworks.

Built by INL, Refined by Industry

• Proven across the nuclear ecosystem: Used by over 50 organizations in 25+ countries, including utilities, regulators, national labs, and developers of SMRs (small modular reactors) and microreactors.

• Collaborative development: INL works directly with licensees to expand RELAP5-3D’s capabilities and address evolving technical needs.

• Credibility that carries weight: RELAP5-3D’s long-standing trust with regulators and industry helps strengthen your case in proposals and evaluations.

• Expert support with ongoing innovation: Backed by INL’s technical team and continuous DOE investment, RELAP5-3D evolves alongside the industry while offering optional support and training for users.

Access

This software is a controlled nuclear code, subject to a special release process due to its sensitive nature. To request access, please visit the Nuclear Computational Resource Center and create an account.