Why ASYST Outperforms Legacy RELAP Codes
Compare ASYST to RELAP5-3D, RELAP-7, and NRC TRACE/RELAP5
The Nuclear Safety Code Landscape
The nuclear industry relies on system thermal-hydraulic codes for safety analysis, licensing, and design optimization. While legacy codes like RELAP5-3D, RELAP-7, and NRC TRACE have served the industry for decades, ASYST represents the next generation of nuclear safety analysis with capabilities specifically designed for both existing light water reactors and advanced reactor concepts.
At-a-Glance Comparison
See how ASYST compares to INL and NRC codes across key capabilities
ASYST vs RELAP5-3D (Idaho National Laboratory)
While RELAP5-3D has been the premier INL system code since 1996, ASYST addresses its key limitations
RELAP5-3D Limitations
- No integrated severe accident modeling - must couple with other codes like MELCOR
- NQA-1 Level 3 only - requires additional commercial grade dedication for NRC licensing
- No liquid fuel kinetics - cannot model MSR fuel behavior
- 3D flow mixing limitations - unreliable for complex 3D flow in large volumes
- Legacy FORTRAN77 codebase - complicates maintenance and updates
- No macOS support - limited to Linux and Windows only
ASYST Advantages
- Integrated SCDAPSIM module for complete severe accident analysis in one code
- Integrated Uncertainty Analysis (IUA) reduces user effects and training requirements
- Multi-fluid property options for molten salts, liquid metals, and supercritical water
- Three-fluid, 9-equation capabilities for complex flow scenarios
- Modern FORTRAN (Ver 4.x) for better portability and maintenance
- Interactive 3D visualization with 3Dgui, Adv3Dgui, and RHYS platform
About RELAP5-3D: Developed at Idaho National Laboratory since 1979, RELAP5-3D has over 1,000 individual users including Naval Nuclear Laboratories, TerraPower, BWXT, NuScale, and Westinghouse. While it excels at LWR thermal-hydraulics, it was not designed for integrated severe accident analysis or advanced reactor concepts.
ASYST vs RELAP-7 (Idaho National Laboratory)
RELAP-7 was announced as the "next generation" code in 2011 but has not achieved widespread adoption
RELAP-7 Status
- Development began in 2011 - over 13 years ago
- Limited development activity compared to RELAP5-3D
- Restricted access through Nuclear Computational Resource Center
- Designed for RISMC methodology - narrow focus
- Uncertain future development roadmap
RELAP-7 was designed to "take advantage of advances in computer architecture, software design, numerical methods, and physical models"
but INL continues to release RELAP5-3D updates (most recently in 2024 after a 5-year gap) as its primary thermal-hydraulic code.
Source: INL Software Marketplace
ASYST: Active Development
- 100+ organizations across 32+ countries in ADTP consortium
- Two active development branches (Ver 3.x and 4.x)
- Incorporates Fukushima Daiichi lessons learned
- Commercial support and professional training available
- Regular updates and continuous improvement
Ready for the Future
While RELAP-7 remains focused on traditional RISMC applications, ASYST is already supporting advanced reactor licensing for SMRs, MSRs, and HTGRs worldwide.
ASYST vs NRC TRACE/RELAP5
The NRC developed TRACE to consolidate legacy codes, but it remains focused on light water reactor regulation
NRC TRACE
The "TRAC/RELAP Advanced Computational Engine" is NRC's flagship thermal-hydraulics tool, designed to consolidate TRAC-P, TRAC-B, and RELAP for LWR analysis.
NRC RELAP5
The original RELAP5 developed for NRC is now considered legacy. NRC recommends TRACE for new analyses while maintaining RELAP5 for backward compatibility.
ASYST
Next-generation code supporting all reactor types with integrated severe accident modeling, uncertainty analysis, and advanced visualization tools.
Key Differentiators from NRC Codes
Advanced Reactor Support
NRC TRACE and RELAP5 are designed primarily for light water reactors. ASYST Version 4.x includes native support for molten salt reactors (MSR), high-temperature gas reactors (HTGR), supercritical water reactors (SCWR), and heavy liquid metal cooled reactors - critical for next-generation reactor licensing.
Integrated Severe Accident Analysis
Neither TRACE nor RELAP5 include severe accident modeling. NRC uses separate codes like MELCOR for these analyses, requiring complex code coupling. ASYST integrates SCDAPSIM for seamless analysis from normal operations through severe accidents.
Built-in Uncertainty Quantification
ASYST's Integrated Uncertainty Analysis (IUA) performs uncertainty analysis using both source code and input parameters, significantly reducing "user effects" - a known challenge with NRC codes where different analysts can produce varying results from the same scenario.
Commercial Support Model
NRC codes are freely available but come with limited support. ASYST includes commercial licensing with dedicated technical support, professional training programs, and direct access to code developers through the ADTP international consortium.
NRC Computer Codes: For more information about NRC's thermal-hydraulic codes, visit the NRC Computer Codes page.
When to Choose ASYST
ASYST is the clear choice for these applications
Advanced Reactor Development
Designing SMRs, MSRs, HTGRs, or other advanced concepts? ASYST 4.x provides native multi-fluid support that legacy codes simply don't have.
Severe Accident Analysis
Need to analyze beyond design basis accidents without coupling multiple codes? ASYST integrates thermal-hydraulics and severe accident modeling seamlessly.
BEPU Analysis
Best Estimate Plus Uncertainty analysis is built-in with ASYST's IUA capability, reducing user effects and delivering more consistent results.
Training & Education
Desktop simulator environments and 3D visualization tools make ASYST ideal for analyst training and educational programs.
International Projects
With users in 32+ countries, ASYST through the ADTP consortium offers international collaboration and regulatory acceptance beyond US-centric codes.
Commercial Support Needs
Require dedicated technical support and professional training? ASYST commercial licensing includes direct access to code developers.
Built by RELAP/SCDAP Pioneers
ASYST is developed by the same team that created the original SCDAP/RELAP5 code at Idaho National Laboratory. Dr. Chris Allison, ISS Technical Leader, directed the SCDAP/RELAP5 development program at INL from 1981-1996 before founding Innovative Systems Software to advance nuclear safety analysis technology.
This heritage means ASYST maintains full compatibility with RELAP/SCDAPSIM input models while incorporating decades of additional development, including lessons learned from the Fukushima Daiichi accident analysis and decommissioning R&D activities.
ASYST Development Heritage
SCDAP/RELAP5 at INL
RELAP5/SCDAPSIM at ISS
ASYST Development
External References
Learn more about the codes discussed on this page
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