In general, thermoplastics are known for being corrosion resistant, robust, and usually easy to fabricate. However, the operating environment makes it challenging to find good polymer seals for nuclear applications. In this blog post, the focus will be on thermoplastic polymer sealing solutions as an alternative to elastomeric seals.

Nuclear Sealing Solutions

There are many different applications where seals are necessary for the nuclear industry, including transport flasks and port seals, waste containment packages, ventilation systems, repair systems, valve seats, robotics, manipulators, and hydraulic rams. Reliable seals are especially crucial to reactor coolant pumps (RCP) and self-actuated abeyance systems.

Harsh Environments for Nuclear Seals

One of the most challenging environments for nuclear sealing applications is a combination of halogenated material issues and high gamma radiation exposure levels.

Halogenated Material Complications

In many nuclear plant applications, the use of halogenated materials (particularly those containing fluorine or chlorine) is strictly prohibited because these materials can potentially leach aggressive chemicals such as halides. Halides in aqueous streams, for example, can cause such problems as stress corrosion cracking in stainless steel. This means that while a seal made from a halogenated material may perform well, it can cause severe damage downstream. In addition, halogenated materials may also release corrosive gasses resulting in serious safety issues.

Gamma Radiation

Gamma radiation can cause polymers to degrade and lose critical properties rapidly. This degradation typically takes the form of chain scission and cross-linking, which leads to brittleness and fractures. This has proven to be a severe problem for commonly used seal polymers such as PTFE, PFA, PA, and UHMW PE. And even though seals may not be directly exposed to gamma radiation, this type of radiation can penetrate materials and lead to indirect exposure.

In addition, free radicals can also be trapped within the polymeric structure when exposed to radiation, leading to continued degradation after exposure in a phenomenon known as post-irradiation degradation. High gamma radiation dose rates can also generate heat to complicate matters further.

Other Issues

Nuclear applications can also involve very high temperatures and pressures and wide pressure variation. Exposure to corrosive chemicals can also be problematic. And for a sealing solution to be effective, there are vital considerations such as qualified life and ability to withstand station black-out (SBO) conditions. Therefore, the importance of reliable seals cannot be overestimated.

Polymer Seal Materials for Nuclear Environments

For harsh nuclear environments, three particular polymers can be used as a seal jacket (depending on the application and relevant operating conditions). These are PEEK, FEP, and ETFE.

PEEK (Polyetheretherketone)

Polyetheretherketone is often known by brand names such as Fluorolon by Advanced EMC Technologies,  PEEK by Victrex, Ketron by Mitsubishi, and TECAPEEK by Ensinger. Of thermoplastic polymers suitable for nuclear applications, PEEK is the most commonly used material for rotary shaft seals. Among the key properties of PEEK are:

• Excellent performance at high temperatures up to 500°F
• Excellent chemical compatibility
• Self-lubricating
• Very low friction
• Excellent resistance to high-energy radiation on the order of 109 rads

PEEK is, however, sensitive to certain acids, carbon sulfides, fluorine, and chlorine.

FEP (Fluorinated Ethylene Propylene)

Fluorinated Ethylene Propylene is often recognized by brand names such as Teflon FEP by Dupont, Neoflon FEP by Daikin, and Dyneon FEP from Dyneon/3M. Critical properties of FEP include:

• Excellent performance at high temperatures up to 400°F
• Very low friction
• Excellent chemical compatibility
• Self-lubricating
• Good resistance to high-energy radiation on the order of 105 rads

In addition, FEP is melt-processable and therefore reasonably easy to use in the manufacture of seals. However, FEP’s limitations for nuclear applications are primarily its susceptibility to attack by acids.

ETFE (Ethylene Tetrafluoroethylene)

Ethylene Tetrafluoroethylene, or ETFE, is often referred to by trade names such as Tefzel by DuPont, Neoflon ETFE by Daikin, and Texlon by Vector Foiltec. Important properties of ETFE include …

• High melting temperature
• Excellent chemical compatibility
• Very low friction
• Non-stick and self-lubricating
• Self-cleaning 
• Excellent resistance to high-energy radiation on the order of 107 rads

Its limitations in nuclear applications include the fact that it is rated to only 300°F and is susceptible to attack by esters and aromatics. In addition, it is often cost-prohibitive.

Conclusion

Seals are a critical component in the nuclear industry. While elastomeric seals may be commonly used, there are high-performance polymer alternatives such as PEEK, FEP, and ETFE that provide fundamental properties needed. In addition, such materials can provide the required performance even when subject to gamma radiation, intense heat, and extreme pressures. 

If you are investigating polymer seals for nuclear applications, contact the sealing solution experts at Advanced EMC. We understand the specific challenges of the nuclear industry and will share our comprehensive knowledge of polymers to help you find the ideal sealing solution.