Engineering Analysis

At the Core of Writing the Rules for Innovative Nuclear Engineering

How do you codify safety rules for next-generation nuclear installations when, by definition, there is no long-term operating feedback?

This question is particularly critical for an organization such as the French Alternative Energies and Atomic Energy Commission, the CEA. France derives the largest share of its electricity from nuclear power worldwide, which places the country among the global leaders in nuclear engineering, operations and safety regulation.

Within France’s nuclear sector, the CEA plays a central role. It is both an operator of research reactors and a project owner for innovative nuclear systems, positioning it at the heart of the evolution of the technical standards used across the sector.

Among these standards is RCC-MRx, a set of rules applicable to the design and construction of mechanical equipment for advanced nuclear installations. Created through the merger of two earlier codes, RCC-MRx now covers installations such as research reactors and fusion facilities. Its latest edition, published in 2025, illustrates how nuclear codification seeks to remain cautious while still accommodating innovation.

Challenges That Require Continuous Adaptation of Engineering Tools

This evolving framework requires tools capable of applying these rules as written and doing so efficiently.

In fusion projects, this need has emerged in very practical terms. At the ITER construction site, the Test Blanket Modules, which are designed to test tritium production and the behavior of structural materials in a fusion environment, are connected to the outside of the machine through a dense network of piping known as the “Pipe Forest.” These lines must carry fluids with extreme properties while withstanding severe thermomechanical loads.

As Stéphane Gazzotti, a research engineer at the CEA and lead author on the subject, explains:

“No RCC-MRx post-processing tool existed as such, which led the CEA, in collaboration with ITER and Assystem, to develop a specific flexibility analysis methodology.”

This lack of a ready-to-use tool highlights a recurring issue in innovative nuclear projects: codes often evolve faster than the software tools capable of applying them without approximation.

PIPESTRESS and the Importance of Native Code Integration

In this context, the CEA relied on PIPESTRESS, Hexagon’s piping analysis software, which has become a reference tool within the French nuclear sector.

Used for more than forty years in civil nuclear projects and maintained in accordance with nuclear quality assurance procedures, PIPESTRESS offers a key advantage. “Unlike other tools, PIPESTRESS integrates RCC-MRx and RCC-MX codes natively, using exactly the coefficients prescribed by the codes to calculate piping stresses,” Stéphane Gazzotti explains.

This point is far from trivial. In a highly regulated environment, the ability to demonstrate that calculations strictly follow the formulas and coefficients defined in the codes directly affects the credibility of engineering studies, their traceability and their acceptance during technical reviews.

Readability, Traceability and Faster Iterations

Beyond code compliance, PIPESTRESS addresses very practical needs encountered when working with complex piping networks. The software makes it possible to model large systems while performing simultaneous calculations across all piping elements, which significantly simplifies design iterations.

“It allows large networks to be modeled while facilitating iterations and the simultaneous calculation of all piping elements,” Stéphane Gazzotti adds. Geometries can be imported quickly using macros, routing, supports and components can be modified directly and the entire model can be edited in text format.

This level of editability is particularly important in nuclear projects, where every assumption must be reviewed, verified and justified. It creates continuity between engineering work, internal reviews and exchanges with partners and regulatory bodies.

PIPESTRESS also enables spectral analyses to be carried out with relative ease, which is essential for addressing seismic or dynamic loads that are common in fusion installations.

A Tool That Supports the Evolution of Nuclear Codes

The value of PIPESTRESS is not limited to executing calculations. By making it possible to apply RCC-MRx to pioneering projects, the software also contributes indirectly to testing the robustness of the code itself. Feedback from these applications then feeds into discussions on how the rules and material data should evolve.

In this process, the CEA acts as a reference body, accumulating data from real projects and qualification testing. This contribution helps explain why RCC-MRx is now recognized beyond France, particularly within the European context of innovative reactor development. Find more details here.