ITER project: Overcoming challenges in characterizing irradiated composite materials for nuclear fusion

Characteristics of the materials used in the ITER project

Terms of service and characterization requirements

ITER reactor materials must withstand extreme environments: cryogenic temperatures, neutron irradiation, and electromagnetic fields, while maintaining mechanical properties and electrical insulation. To reproduce these conditions in the laboratory, tests require:

• Precise control of cryogenic temperature.
• Uniform application of load without stress concentrations.
• Clamping systems that prevent slippage or breakage.
• Reliable instrumentation at low temperatures.

Special features of irradiated composite materials

Neutron irradiation alters the polymer matrix, increasing rigidity and reducing ductility. This makes it difficult to grip the test specimens, which are more prone to fracture or slippage. Therefore, the test methodology must be adapted compared to non-irradiated materials.

Technical challenges in cryogenic testing of irradiated materials

Specimen preparation per ASTM D3039 for cryogenic testing

Test specimen preparation was critical due to fiber orientation and tab quality, in accordance with ASTM D3039. Each material required a different strategy:

• Material A: test specimen and tabs machined in one piece.
• Material B: metal tabs attached with cryogenic glue.
• Material C: polymer matrix tabs glued to the main body.

Conventional cryogenic testing equipment

Initial tests were carried out in CERN cryostats, immersing the test piece and clamps in liquid nitrogen (77 K). Although the self-adjusting clamps worked in most cases, the brittleness of the irradiated test pieces pushed their capacity to the limit: too much pressure caused breakage, too little pressure caused slippage.

Specific problem of slippage in the grips

Material C presented the biggest problem due to the embrittlement of the matrix after irradiation. Slippage was exacerbated by thermal contraction in liquid nitrogen and the high investment in each specimen (only one per condition). This made it necessary to develop an alternative gripping system that would prevent slippage even in very brittle specimens.

Development of innovative solution: localized cryogenic containment system

System concept and design

To solve the slippage issue in irradiated specimens, Applus+ Laboratories developed a system that keeps the central area of the specimen submerged in liquid nitrogen (77 K) while the ends remain at room temperature, secured with conventional hydraulic clamps. The design includes a flexible cryogenic-compatible chamber, continuous LN₂ supply, and sensors to monitor the temperature.

System implementation and validation

The system was adapted to universal testing machines, allowing high clamping pressure without nitrogen leaks. Its open design facilitates the installation of additional sensors and reduces LN₂ consumption by cooling only the area of interest.

Advantages of the developed system

• Eliminates slippage thanks to standard hydraulic clamps.
• Compatible with different geometries and conventional equipment.
• Lower liquid nitrogen consumption compared to total immersion systems.
• Greater flexibility and test cadence for irradiated materials.

Experimental methodology implemented in low-temperature tests in the ITER project

Final test configuration

Two configurations were used:

• Materials A and B: tests in CERN's conventional cryostat, with complete immersion in liquid nitrogen and cryogenic clamps.
• Material C: system developed by Applus+ Laboratories, with the central area submerged in LN₂ and the ends secured with hydraulic clamps, monitoring the temperature at various points.

Optimized testing procedure

The test specimens were thermally conditioned to ensure uniformity at 77 K. The temperature was verified with distributed sensors before applying controlled load, adjusting the speed according to the material. During the test, load, displacement, and temperature were monitored in real time. After the test, a detailed failure mode analysis was performed to validate the results.

Results and discussion

The test campaign validated two methodologies:

• The conventional CERN system was suitable for most Material A and B specimens, offering reproducible results.
• The system developed by Applus+ Laboratories was essential for Material C, preventing slippage and allowing characterization to be completed.

The combination of both methods made it possible to obtain reliable data under post-irradiation cryogenic conditions, overcoming technical challenges and optimizing the use of irradiated material.

Detailed analysis and future prospects

Proposed solution

The localized cryogenic containment system developed by Applus+ Laboratories represents a key innovation: cooling only the test area while using conventional hydraulic grips. This resolved the conflict between high clamping pressure and material brittleness.

Importance of clamping systems in composite material testing

The tests carried out showed that the clamping system is a determining factor in the validity of the results, especially in materials that have been irradiated and subjected to cryogenic temperatures. Irradiation modifies stiffness and ductility, which increases fragility in the gripping areas and reduces tolerance to high pressures. Added to this is thermal contraction in liquid nitrogen, which alters the coefficient of friction and the interaction between the test piece, heels, and jaws.

In this context, the grip design must balance two opposing requirements:

• Apply sufficient pressure to prevent slippage, which would invalidate the test.
• Avoid premature damage to the heads, which would compromise the integrity of the test piece.

The project experience demonstrated that standard protocols are not sufficient for these extreme conditions. The solution developed by Applus+ Laboratories, which combines localized cooling with conventional hydraulic jaws, redefines the traditional approach, offering greater control and reliability. This learning underscores the need to consider the clamping system as an integral part of the experimental design, rather than a secondary element.

Implications for future testing protocols for nuclear fusion materials

The methodologies developed can be applied to other nuclear fusion projects, reducing dependence on large facilities and accelerating the qualification of materials under extreme conditions.

Conclusions 

This project has enabled significant progress in the characterization of irradiated composite materials for nuclear fusion applications, facing extreme conditions that combine cryogenics, irradiation, and high mechanical demands. Among the main achievements are:

• Development of methodologies adapted to faithfully reproduce the operating conditions of the ITER reactor, ensuring reliable results in environments where standard protocols are insufficient.
• Identification of slippage in clamps as the main technical challenge, which led to the creation of an innovative localized cryogenic containment system capable of combining selective cooling with conventional hydraulic clamps.
• Validation of three advanced composite materials under post-irradiation cryogenic conditions, providing critical data for the design of the ITER reactor heat shield.
• Generation of knowledge applicable to future protocols, which will enable the optimization of testing in fusion projects and other industries requiring characterization in extreme environments.

This work demonstrates the importance of technological collaboration. The synergy between ITER, CERN, and Applus+ Laboratories, together with the support of Fusion for Energy and Carlos III University of Madrid, has been key to overcoming seemingly insurmountable challenges. The solutions developed not only contribute to the advancement of the ITER project, but also open up new possibilities for democratizing access to specialized cryogenic testing, accelerating the qualification of materials for fusion energy.