Ongoing climate changes are forcing an energy transition aimed at reducing greenhouse gas emissions. A promising solution is fusion energy – a clean and safe alternative to fossil fuels. The fusion process does not generate long-lived radioactive waste or CO₂ emissions, and the energy obtained from this process far exceeds that produced by traditional methods.

The global context 

Nuclear fusion reactions release significantly more energy per unit of fuel mass than nuclear fission. Fission energy, in turn, is over a million times greater than that released from chemical energy during combustion of fossil fuels. Fusion energy also poses a lower safety risk since it does not allow for uncontrolled reactions. Nevertheless, scientists and engineers still face many challenges before fusion can be used for commercial energy production.
Currently, there are over 50 fusion research projects globally, the most well-known being ITER (International Thermonuclear Experimental Reactor) – a multinational initiative involving 32 countries and the successor to the now-retired JET (Joint European Torus). Other plasma-generation devices under development or modernization include tokamaks, stellarators, and magnetic bottles.
Experiments are being conducted by publicly funded entities (e.g., ITER, projects in China, Japan, South Korea, Germany) as well as private companies such as Lockheed Martin, Tokamak Energy (UK), and Commonwealth Fusion Systems (USA).
SPETECH^® is involved in numerous fusion-related projects, contributing both to conceptual work (e.g., weld joint concepts for the SPARC reactor) and engineering calculations (e.g., flange joints for KHNP in Korea), as well as supplying specialized seals for the ITER facility. SPETECH^® is also an active member of CeNTE – the New Energy Technologies Center – supporting the EUROfusion project.

The role of sealing in fusion devices 

Fusion reactions occur in vacuum chambers with complex constructions, where sealing is essential to maintain ultra-high vacuum (around 10⁻⁶ Pa) and isolate plasma from its surroundings.
These vacuum chambers, including that of ITER, are equipped with access ports for components such as heating antennas, diagnostics, and shielding modules. The seals of these ports must withstand extreme conditions: high temperatures, low gas permeability, and mechanical damage.

ITER is testing three various sealing types:

• Elastomeric seals 
• Metal seals
• Welded lip seals

Elastomeric seals – temporary but prone to degradation at high temperatures.
Initially, most ports will be sealed using Temporary Closure Plates (TCP) and elastomeric seals to achieve vacuum during commissioning and initial plasma production. However, elastomer seals are not recommended for high-temperature environments (like the ITER vacuum chamber) due to mechanical degradation, gas permeability, and outgassing under long-term use.

Metal seals – more durable and efficient, but require precision manufacturing and are costly.
A better solution is metal seals, which are temperature-resistant and have low gas permeability, enabling better vacuum levels. ITER is considering double C-ring metal seals with internal spiral springs, though this imposes strict flange quality requirements (e.g., flatness, roughness) and is expensive and challenging to manufacture at large scale.

Weld lip seals – most reliable, but with limited maintenance accessibility.
Welding remains the most reliable sealing method, but is impractical for maintenance. Thus, ITER will implement welded lip seals starting in the second research phase, expanding them gradually—except where detachable connections are required (e.g., for cryogenic or HNB systems).
 

Fig. 1 View of the ITER tokamak vacuum chamber with visible ports [1] https://fusionforenergy.europa.eu/downloads/procurements/itercalls/373/s..., p. 2

Fig. 2 Metal seals of the vacuum chamber ports: a) welded seal between the plug and the chamber shell, b) seal using metal gaskets and a U-profile with additional flanges.

In the case of using metal gaskets, the flatness of the mating surfaces must not exceed ≤ 0.4 mm per 1 meter along the sealing axis and 0.5 mm per 1 meter perpendicular to the main sealing axis. The surface roughness of the sealing face must be between Ra 0.4 and Ra 0.8. Additionally, attention must be paid to the orientation of machining marks, which must be aligned parallel to the sealing axis. For square flanges, this is not achievable using milling or disc grinding; therefore, manual grinding with the use of guides along the port perimeter, including the arcs, is recommended.

Material Requirements

For SPETECH^® seal deliveries to ITER pipelines, stringent quality control and chemical composition documentation were required, especially minimizing activation elements like Niobium (Nb), Tantalum (Ta), and Cobalt (Co) that could lead to long-term radiation.

Based on prior R&D in fusion and breeder reactor programs, 316L stainless steel is preferred due to radiation resistance, load-bearing capacity, and water contact compatibility. ITER uses a specific variant: 316L(N)±IGX, denoting:

• 316 – steel type
• L – low carbon
• (N) – controlled nitrogen
• IG – ITER Grade (customized for impurities, alloying elements, and delivery conditions)
• X – application-specific classification

Other suitable materials include nickel alloys, especially alloy 718, used in metal gaskets and bolted plug connections.

The Cryostat

Due to superconducting materials (cooled by liquid helium or nitrogen), isolation from 100 million °C plasma and ambient temperatures is critical. Magnetic traps isolate plasma from the torus walls, while vacuum insulation (~10⁻⁴ Pa) protects cryogenic magnets from heat ingress.

The ITER cryostat is a welded cylindrical structure (approx. 30 m diameter/height, 3850 tons). Advances in high-temperature superconductors (e.g., REBCO) now allow for more compact spherical tokamaks, reducing cryostat sizes accordingly.

SPETECH^® contributed to:

• Cryostat port seals – 9-meter components ensuring absolute vacuum integrity under demanding conditions.
• Cryogenic system seals – engineered for low-temperature use and extreme material performance.

Summary

Harnessing fusion energy for commercial use represents one of the greatest challenges in modern engineering. Projects such as fusion reactors challenge even fundamental engineering topics like sealing.
Through involvement in multiple fusion initiatives and high-level engineering services for global nuclear technology leaders (USA, Korea, France, India, Germany), SPETECH^® supports fusion advancement by delivering sealing solutions that meet the most rigorous technical standards. The company has proven that Polish engineering can contribute meaningfully to global high-tech projects.
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Fig. 3. Selected design concepts for the main connection of a spherical tokamak cryostat using a welded seal
a) a membrane in the shape of a semi-torus around the flange joint,
b) a connection using clamps,
c) a connection supported by an external (backing) ring.
 

References

1. Technical Specification - Mechanical Engineering Support for ITER Vacuum Vessel Port Structures and their Interfaces with others Plant Breakdown Systems
2. Preparation for Korean procurement scope of ITER sealing flanges. G.H.Kim. Fusion Engineering and Design 200 (2024) 114167 
3 Assessment and selection of materials for ITER in-vessel  components .G. Kalinin.  Journal of Nuclear Materials 283±287 (2000) 10±19
4. Conference materials from the Polish Forum on Sealing Design and Maintenance, Bystra, October 26–27th, 2023

Author: Jan Kasprzyk, President and Technical Director of SPETECH^®