Fusion Energy

Fusion energy is generated by combining one or more lighter elements to form a heavier element, releasing energy in the process. The plasmas required for the production of fusion energy can be contained using either magnetic confinement, inertial confinement, or a combination of both approaches. Depending on the type of fusion reactor being constructed, extreme temperatures, radiation exposure, and/or vacuum conditions may require the use of specialized engineering plastics and composites.

Disclaimer: DOE and IAEA guidelines for the plastics used in fusion energy applications including the references contained herein should be carefully reviewed before selecting a polymer for a particular design.

Plastics for UHV (Ultra-High Vacuum) Applications

Ultra-high vacuum seals require plastic materials with low gas permeability and low outgassing. UHV seals also need to have moderate compressive modulus so that they will be soft enough to conform to mating metal surfaces while also being stiff enough to maintain a seal. It is important that UHV seals have excellent creep and stress relaxation characteristics so they will operate reliably for the required part life. UHV seals used in devices with a wide operating temperature range must have low rates of thermal expansion and contraction so that they will have sufficient dimensional stability.

High performance plastics including DuPont® Vespel™ SP-1 and PEEK can be considered for vacuum seals for fusion applications.

The following reference provides useful information about plastics for use in UHV sealing applications.

Murari, A. & Vinante, C. & Monari, M.. (2002). Comparison of PEEK and Vespel® SP1 Characteristics as Vacuum Seals for Fusion Applications. Vacuum. 65. 137-145.

Norplex-Micarta’s CryoLAM™ glass/epoxy composites have low outgassing, which is why they are often specified for vacuum applications where mechanical strength is required.

Plastics for Tritium Applications

The beta radiation exposure associated with tritium service will eventually degrade all polymer sealing materials. Careful steps have to be taken to design regular preventative maintenance schedules that take this into account. It is also important to design valve mechanisms that close within a desired torque range to prevent leak paths and excessive deformation of the plastic seal. The U.S. Department of Energy and the International Atomic Energy Agency report that DuPont™ Vespel® is shown to be more radiation resistant than most polymers and that this material has successfully been used for valve stem tips in some tritium laboratories. DOE and IAEA guidelines for the plastics used in tritium applications including the references below should be carefully reviewed before selecting a polymer for a particular design.

International Atomic Energy Agency, Safe Handling of Tritium, Review of Data and Experience, Technical Reports Series No. 324, IAEA, Vienna (1991).

DOE-STD-6003-96, Safety of Magnetic Fusion Facilities: Guidance, Washington (1996).

DOE-HDBK-1129-2007, Tritium Handling and Safe Storage, Washington (2007).

Weaver, W. W. (1994). Guidelines for Valves in Tritium Service. Fusion Technology, 25(4), 428–433.

Plastics that Resist Degradation from Radiation

Plastic materials that are exposed to radiation will eventually exhibit reduced tensile strength and elongation, and loss of mechanical properties. Some polymer materials can also generate decomposition products during degradation that may be undesirable in fusion energy devices.

DuPont™ Vespel®, PEEK, Ultem®, and certain grades of G-10 glass/epoxy composite have better resistance to certain types of radiation compared with many other engineering plastics. These plastics and composites have been specified for use in a number of fusion energy devices.

Polyimide composite materials tend to have superior radiation resistance compared with epoxy-based composites. Norplex-Micarta’s P-95 sheet consists of a high temperature polyimide resin system reinforced with a woven glass fabric.

Detailed information about the resistance of plastics and composites to various forms of radiation can be found in the reference below.

International Atomic Energy Agency, Insulators for Fusion Applications: Report of a Consultants Meeting on Insulators for Fusion Applications Organized by the International Atomic Energy Agency Held In Karlsruhe, 25-25 June, 1986. IAEA-TECDOC-417 (1987).

Plastics for use at Cryogenic Temperatures

The superconducting magnets used in fusion reactors may require cooling to cryogenic temperatures. The plastic used in these devices need to maintain some degree of ductility and also be resistant to the effects of thermal cycling. The plastics also need to have low thermal conductivity and low rates of thermal expansion and contraction as well as sufficient mechanical properties to support the magnets.

PEEK, glass-filled PEEK, and DuPont™ Vespel® are often specified for applications that require performance at cryogenic temperatures. Norplex-Micarta’s CryoLAM™ glass/epoxy composite materials including NP500CR sheet and RT521M tube are also widely used for cryogenic applications. CryoLAM™ materials have enhanced resistance to fatigue failure due to thermal cycling.

Plastics for use at Elevated Temperatures

Fusion devices may require plastics capable of operating at elevated temperatures. There are a number of variables that need to be considered when selecting plastics for high temperature applications including:

• The strength and modulus of the material at the required temperature
• Creep and stress relaxation behavior throughout the operating temperature range
• Thermal expansion rates
• Resistance to the degradation of mechanical properties over long periods of time at the specified temperatures

Plastic materials including DuPont™ Vespel®, Ultem®, PEEK, glass-filled PEEK, and Torlon® PAI are often used for elevated temperature applications.

Plastics for Electrical Insulation

Fusion energy devices may require plastic or composite electrical insulators with the dielectric properties and arcing resistance to manage substantial voltages. The electrical insulating requirements are sometimes complicated by exposure to cryogenic conditions, thermal cycling, and radiation. NEMA or UL ratings may also be required for certain electrical insulating applications.

Plastic materials including DuPont™ Vespel®, Ultem®, PEEK, and Torlon® PAI often specified for applications that require electrical insulating properties. Norplex-Micarta’s CryoLAM™ glass/epoxy composite materials including NP500CR sheet and RT521M tube also have outstanding electrical insulating characteristics.

The reference below provides rich information about the electrical insulating properties of plastics and composites which can be considered for use in fusion energy applications.

DOE CONF 801237.  Proceedings of the Meeting on Electrical Insulators for Fusion Magnets. Held at the DOE headquarters in Germantown, Maryland, 2-3 December, 1980. (1981)

Plastics for Mounting Optical Lenses and Mirrors

The mirrors and lenses used for fusion energy devices need to be securely and precisely positioned without the use of hard metallic contact surfaces. DuPont™ Vespel® is often used for the contact pads for optics. This material has low outgassing, moderate hardness, and moderate compressive modulus to allow optics to freely expand and contract during thermal cycling.

Plastics for Magnet Supports

The plastics and composites used to support superconducting magnets need to have a number of performance characteristics including:

• Low rates of thermal expansion and contraction
• Resistance to damage from thermal cycling and fatigue
• Mechanical strength
• Some degree of ductility at cryogenic temperatures
• Low thermal conductivity
• The ability to operate in magnetic fields
• Low outgassing for use in vacuum environments
• Resistance to degradation from radiation

The most popular composite material for use in magnet supports is glass/epoxy composite sheet. Depending on the operating environment, special formulations of glass/epoxy may be required. Norplex-Micarta’s CryoLAM™ glass/epoxy composite materials including NP500CR sheet and RT521M tube are often used in magnet support applications when mechanical strength, low outgassing, and resistance to fatigue failure from thermal cycling are required.

Ultem®, PEEK, glass-filled PEEK, and DuPont™ Vespel® can also be considered for certain magnet support applications.