ASME Section III, Division 5: 2025 Code Updates

The 2025 Edition of ASME Section III, Division 5 introduces a series of important updates to the design rules that support advanced reactor technologies operating at high temperatures. These revisions reflect lessons learned from historical testing programs, recent US Department of Energy and international research initiatives, and the growing need for a reliable framework to qualify innovative reactor designs such as high-temperature gas-cooled reactors, molten salt reactors, and sodium-cooled fast reactors.

A significant change is the refinement of creep-fatigue interaction rules. Earlier editions of ASME Section III, Division 5 provided conservative approaches for cyclic service at elevated temperatures, but the 2025 update incorporates more recent test data and modeling improvements. The rules for strain range partitioning and fatigue damage envelopes have been expanded to provide clearer, more accurate methods for evaluating damage accumulation during long-term cyclic operation with hold times. This ensures that design evaluations are technically defensible and reduce unnecessary conservatism while maintaining the safety margins expected in nuclear service.

The material property basis for Division 5 has also been strengthened. The 2025 Edition adds updated allowable stress data for key alloys of interest to advanced reactor developers, including Alloy 617, Alloy 800H/HT, and Grade 91 ferritic steels. These additions integrate decades of historical test results with new experimental programs, providing designers with a robust foundation for evaluating long-term creep, fatigue, and environmental degradation. The expansion of the property databases gives engineers more confidence in selecting materials and justifying performance under demanding service conditions.

The 2025 Edition of the Code also now places greater emphasis on the dual use of design-by-analysis and design-by-test approaches. Advanced reactor developers often rely on high-fidelity finite element modeling to assess novel geometries and operating regimes, but regulators and stakeholders also demand verification through experimental testing. The 2025 Edition of the Code explicitly acknowledges both pathways, providing flexibility for innovation while ensuring that rigorous verification remains at the core of the process. This balance is particularly valuable for first-of-a-kind designs, where established service histories are limited and traditional design rules may not be sufficient.

The 2025 edition also refines provisions for configuration management and verification of design bases. Designers are now required to more rigorously document configuration changes, maintain traceability of design verification, and evaluate the reuse of standardized or previously qualified designs. Updated rules for model test scaling laws ensure that data from mockup tests can be reliably applied to full-scale components, addressing a long-standing challenge in the qualification of large, complex systems.

Environmental and irradiation effects have been given greater emphasis in the updated design rules. The 2025 Edition provides improved guidance for addressing environmentally assisted degradation mechanisms, including carburization, decarburization, thermal embrittlement, and helium embrittlement at high temperatures. Provisions for irradiation creep and swelling in advanced reactor environments have also been expanded, reflecting the operating conditions expected for very high-temperature and fast-spectrum systems.

Finally, the Code now includes expanded property databases and design evaluation rules for graphite and composite materials, which are critical to certain high-temperature reactor cores. These updates incorporate recent international irradiation testing results and improve the structural evaluation framework for graphite, particularly with respect to fracture toughness and thermal shock resistance.

Many advanced designs, such as high-temperature gas-cooled reactors, molten salt reactors, and sodium fast reactors, rely on materials and operating conditions that challenge existing design frameworks. The changes introduced in 2025 offer additional technical tools, data, and regulatory alignment to support moving these projects forward with confidence.