Fusion’s neutron hammer: the impact on design

As fusion energy edges closer to commercial reality, a central engineering challenge is emerging: the extreme neutron flux bombarding the inner core of reactors, threatening the longevity of critical components, especially seals, structural materials, and vacuum vessels. For some first-generation reactors like SPARC, this could mean replacing key parts every year. But new designs such as ARC propose alternative solutions, like the FLiBe liquid blankets, to mitigate this issue.

SPARC: Fusion’s Neutron Hammer

SPARC, developed by Commonwealth Fusion Systems, is one of the most advanced high-field tokamak prototypes in the world. It is expected to achieve plasma breakeven (Q > 1) by 2026. However, due to its compact size and high magnetic fields (up to 12 T), the neutron flux is way beyond usual rates in the industrial nuclear installations used for electricity production.

Maintenance and Sealing: The Hidden Bottleneck

This intense flux leads to rapid activation of structural materials, embrittlement, helium swelling and a challenge to the integrity of seal functions. Maintaining or replacing seals under such conditions requires full shutdown and remote intervention, which is costly and time-consuming. Without a hot cell or robotic maintenance infrastructure, this becomes a critical limitation.

ARC: Overcoming the Neutron Flux with FLiBe

The follow-up to SPARC, ARC, is designed with this challenge in mind. It introduces a liquid immersion blanket made of FLiBe (a mixture of lithium fluoride and beryllium fluoride).

What FLiBe Does

• Neutron moderation and absorption: Protects structural materials and the vacuum vessel.
• Tritium breeding: Lithium in FLiBe breeds tritium under neutron bombardment. It means it can produce the tritium needed to fuel the fusion reaction.
• Heat transfer medium: Operates efficiently at 700–900°C without high pressures.

New Material Challenges Appear

Solving neutron challenges on the materials, FLiBe comes with its own engineering constraints:

• High operating temperature (~850°C) requires materials with excellent thermal and chemical resistance.
• Compatible materials include:
• Nickel-based alloys like Hastelloy-N or inconel for better corrosion resistance in molten salt.
• SiC/SiC composites: Neutron-resistant and FLiBe-compatible (though costly and brittle).
• Graphite or coated refractory metals (e.g., molybdenum, tungsten).
• Sealing materials must resist thermal cycling, chemical corrosion, and potential tritium permeation — pushing designers toward textured metal seals (like Technetics’ HELICOFLEX^® TEXEAL^®).

Maturity of FLiBe and Broader Adoption

Despite promising theoretical performance, FLiBe blankets are not yet industrially mature. Key areas of ongoing development include:

• Redox control systems to avoid salt decomposition and corrosion.
• Tritium extraction systems from molten salts.
• Remote handling and salt purification technologies.

However, the momentum is growing:

• ARC is the most advanced user.
• Xcimer Energy (laser fusion) is investigating FLiBe as a neutron shield.
• MoltexFLEX and TMSR-LF1 (China) are deploying similar salts in fission MSRs.

This trend shows a clear industry shift: liquid blankets are becoming the future of durable fusion reactor architecture, offering better uptime, shielding, and integration potential, at the cost of higher chemical and thermal complexity.

The fusion industry is facing a defining moment: will reactor cores withstand their own power? While SPARC exposes the engineering pain points of first-generation tokamaks, especially in maintenance, sealing, and material fatigue, next-gen designs like ARC embrace FLiBe to overcome them. But as FLiBe becomes central to fusion’s future, the industry must now solve corrosion, tritium management, and high-temperature material integration.