What is Powder HIP? An explanation of the technology adopted for the cooling structure of mirrors inside the ITER vacuum vessel.

A cooling structure using Powder HIP (Powder Hot Isostatic Pressing) is applied to the Divertor Infrared Thermography, one of the diagnostic instruments installed inside the ITER vacuum vessel.

Iori, the representative director, in collaboration with the National Institutes for Quantum Science and Technology (QST), manufactured a metal mirror incorporating a three-dimensionally bent pipe to which Powder HIP technology is applied (the process is explained in the diagram below).

For details, please refer to the paper below.
https://doi.org/10.1016/j.fusengdes.2026.115639
(Fusion Engineering and Design, Volume 225, April 2026, 115639)

Advantages of applying Powder HIP cooling structures to ITER vacuum vessel equipment

• Ability to form complex cooling structures
• High cooling capacity can be achieved
• Integrity of welded joints, which form the pressure boundary, is maintained
• Designer's intent (results of thermal-structural analysis) can be applied as is

Why is a cooling structure necessary for the mirrors?

Mirrors installed inside the vacuum vessel of ITER (International Thermonuclear Experimental Reactor) need to perform measurements during plasma operation. Since the mirrors are exposed to radiative heat from plasma emission and nuclear heat generation caused by neutron bombardment during plasma operation, they require a cooling structure.

To ensure the optical performance of the optical system, which consists of a series of mirrors, the surface shape of each mirror must maintain its ideal form, and the cooling channels need to be optimised through thermal-structural analysis and other methods.

However, there is no precedent for diagnostic instruments like ITER, where optical systems extend for approximately 20 metres, and the precision required for the surface shape of each mirror is high, leading to complex cooling channels that are difficult to manufacture.

Furthermore, the cooling channels form the pressure boundary, separating the cooling medium (water) from the vacuum. Therefore, various tests and inspections, such as non-destructive testing to guarantee integrity based on nuclear codes like RCC-MR and ASME, are required for the welded joints at the boundary. To reliably achieve sound welds, it is desirable to apply full penetration butt welding.

Candidate cooling structures and their respective challenges

Possible candidates for mirror cooling structures include: 1) pipe clamps, 2) drilling, 3) pipe brazing, 4) solid HIP, and 5) powder HIP.

• Pipe clamps: Can be realised at low cost, but cooling performance is inferior to other methods.
• Drilled holes: Cooling performance is good, but it is difficult to create complex cooling channels. Additionally, plug welding is used to seal the drilled holes, but ensuring the integrity of the welded parts is difficult.
• Brazing: Can be realised at low cost, but it is difficult to guarantee adhesion and stable cooling performance cannot be consistently achieved. Complex cooling structures cannot be formed either.
• Solid HIP: Necessary cooling capacity and weld integrity can be achieved, but it is difficult to form complex channels.

Powder HIP allows us to realise only the advantages of these methods. It consists of pipes where the cooling structure is formed solely by butt welding, and then the metal powder, which will form the base material of the mirror, is formed by HIP so as to envelop the pipes. After HIP, it is machined into the shape of the mirror, thereby forming a complex cooling structure inside the mirror, while the pressure boundary, i.e. the welded parts, are also formed solely by butt welding of the pipes, thus ensuring the integrity of the welded parts.

Advantages in terms of design effort

Powder HIP also excels in its ability to directly manufacture the complex cooling structures optimised through thermal structure analysis. With methods other than Powder HIP, it is often difficult to achieve manufacturing due to manufacturing constraints, leading to multiple iterations between the design and manufacturing departments. Powder HIP minimises such indirect costs, and although the manufacturing cost itself may be slightly higher than other methods, the total cost is not significantly different.

Track record of application at the ITER Organisation

The formation of cooling channels using this Powder HIP has also been approved by the ITER Organisation, and this will be the first instance of Powder HIP being applied to equipment within the ITER vacuum vessel.

Further development using metal 3D printers

When forming cooling structures by bending pipes, not only the bending radius (bend radius) is limited, but also the accessibility of the welding torch can be a manufacturing constraint. In routings where pipes are closely spaced, the welding torch cannot be manoeuvred properly, increasing the likelihood of incomplete fusion in the weld, which tends to be avoided by welding practitioners.

By utilising metal 3D printers (metal additive manufacturing, AM), which have seen remarkable technological advancements in recent years, it is becoming possible to manufacture mirrors and other components incorporating more complex cooling structures. Compared to pipe bending, this method allows for smaller bending radii (e.g., approximately one times the pipe diameter, so-called 1D bending) and dense routing that does not require consideration of weldability.

Regarding strength, recent technological advancements have also led to reports that components manufactured using metal 3D printers can achieve mechanical properties equivalent to those of forged materials. Furthermore, it has been reported that applying HIP (Hot Isostatic Pressing) to materials fabricated by 3D printing can yield benefits such as further improvement in mechanical properties and surface roughness.

On the other hand, current nuclear codes such as RCC-MR do not explicitly address the applicability of metal 3D printing to pressure boundaries (to the best of the author's knowledge). Moving forward, as prototype reactor designs for power demonstration and fusion reactor development with a greater emphasis on cost-effectiveness progress, the simultaneous requirements for ensuring the integrity of pressure boundaries, efficient manufacturing, and high cooling capacity will become increasingly critical. Establishing methods for evaluating the integrity of components made from metal 3D printing remains an important future challenge.

Conclusion

Our company possesses expertise in nuclear codes, solid HIP/powder HIP, welding, and heat transfer engineering, and we will continue to contribute to the development of equipment capable of withstanding the thermally demanding environment within fusion reactor vacuum vessels.