N'GENIUS for Nuclear Fusion

The quest for a viable 'holy grail' of power

Harnessing the energy of a human-engineered sun here on Earth is one of the most complex technological challenges for generations. But the belief that nuclear fusion is “always 30 years away” has never been more misguided. The global race for fusion energy is accelerating rapidly, and it appears to be a matter of when, not if, the power that drives the universe finally becomes a commercial reality.

Fusion is a key future technology for achieving net-zero emissions, producing clean and abundant energy by recreating the nuclear reaction that powers a star – where two light atomic nuclei collide and fuse together under extreme temperatures to form a heavier nucleus and release vast amounts of energy.

To mimic this process on Earth, hydrogen gases – deuterium and tritium – are heated to over 100 million °C to create a superheated plasma and release high-energy neutrons. The neutrons hit the reactor walls, generating heat which is used to produce electricity.

Since no material can withstand the intense plasma temperature, it is typically confined in a doughnut-shaped vacuum chamber inside a tokamak or stellarator, using strong magnetic fields to prevent it from touching the reactor walls.

High-temperature superconducting (HTS) magnets are important for the development of economically viable fusion reactors as they are stronger and more efficient than traditional low-temperature superconductors (LTS). Although still cryogenic, these devices typically operate at temperatures between 20 K (-253ºC) and 77 K (-196ºC) compared to LTS magnets which operate at approximately 4 K (-269ºC).

Due to their formability and weldability, non-magnetic behaviour and mechanical strength, ductility and toughness properties at low-temperatures, austenitic stainless steels are a critical material for both superconducting magnets and fusion reactor shielding components. In fact, they are the primary structural materials used throughout the ITER (International Thermonuclear Experimental Reactor) project in southern France.

Potential problems with conventional austenitic stainless steels

While some austenitic stainless steels such as 316L and 316LN are currently utilised in superconducting magnets there is a concern that many of the products manufactured from these grades do not possess a stable, single-phase austenitic microstructure. These conventional alloys contain a percentage of the ferrite phase content and demonstrate a susceptibility to strain-induced martensitic transformation, particularly at cryogenic temperatures and under high mechanical stresses – two of the primary engineering challenges associated with HTS magnet systems.

When this transformation occurs, the alloy's austenite phase converts into a ferromagnetic martensite phase, causing significant changes to the material's magnetic properties - the steel's magnetic permeability increases. In superconducting magnet systems, this can distort the magnetic field required for plasma containment and affect the stability and precision of the HTS coils.

Additionally, the material's ductility and fracture toughness is reduced which is a major cause for concern in cryogenic applications. At these low temperatures, martensitic phases are more prone to cracking and loss of ductility can compromise the structural integrity of components under stress.

Conventional grades containing Niobium, Vanadium or Titanium

Conventional grades of austenitic stainless steels that intentionally contain elements including Niobium, Vanadium or Titanium should not be considered as optimum for use in nuclear fusion applications as these elements promote precipitation within the microstructure, also adversely affecting ductility, toughness and suitability for use at cryogenic temperatures.

Furthermore, this would change the composition of the matrix locally in the vicinity of these precipitates, leading to impoverishment of certain critical elements which adversely affects the performance of these grades.

It is also clear that such conventional stainless steels would not possess a single-phase austenitic microstructure, potentially leading to strain-induced martensitic transformation.

Superconducting magnets generate immense electromagnetic forces during operation, causing significant mechanical stresses on the magnet structure. Austenitic stainless steels which possess higher strength properties than conventional 300 Series grades have better resistance to deformation, fatigue or failure under these loads, providing greater structural integrity to the magnet system.

As a result of their significantly higher strength in the solution heat treated condition, components manufactured in N'GENIUS Series™ grades will also require less cold-working to achieve the desired mechanical properties compared to those made from conventional austenitic stainless steels.

In addition, the nitrogen content in N'GENIUS Series™ grades ensures they possess a more stable single-phase austenitic microstructure, remain non-magnetic even under mechanical stress or cold work, and retain high mechanical strength at cryogenic temperatures.

Performance benefits of N'GENIUS Series™ alloys for nuclear fusion:

 

N'GENIUS Series™ alloys to be considered for Nuclear Fusion applications

The following N'GENIUS Series™ alloys can be considered for a range of Nuclear Fusion applications:

• N'GENIUS ICONIC 316LM4N (PRE_N ≥ 35)
• N'GENIUS INFINITY 316LM4N (PRE_N ≥ 37.5)

N'GENIUS 316LM4N™ Alloy Types and Variants can be selected and engineered for optimal performance in piping systems, forged and cast components.