AMP 06 September 2023

15 ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 hydrocarbon precursors (C-H) are “protium” or 1H. Protium is an impurity in the fuel cycle (which uses deuterium and tritium) and needs to be removed. Using deuterated polymers (C-D) is a potential method to significantly reduce the protium impurities introduced to the fuel cycle. However, currently deuterated polymer precursors are only synthesized in small volumes because there is not significant demand. Research is ongoing into catalysts and methods that can be used to deuterate polymers to high percentages of deuterium via isotopic exchange or other methods (Fig. 3)[21]. Currently, research into polymer precursor deuteration has been able to achieve up to 99% conversion of polymers in batch synthesis over multiple days. Research is ongoing to see how polymer precursor deuteration can be made more effective through higher activity catalysis as well as utilizing continuous flow processing[22]. POLYMERS FOR JOINING AND SEALING Tritium-resistant polymers for fluid/gas seals. Tritium fuel cycles for defense applications have long avoided the use of polymeric materials because they are rapidly degraded by tritium. This has included the development of expensive all-metal vacuum pumps and seals. In a limited number of components such as valve seats where metal sealing options are not an option, ultra-high molecular weight polyethylene (UHMW) or Vespel have typically been used. However, the choice of these materials was done empirically with very little consideration of the effects that govern the material degradation mechanisms. Indeed, tritium exposures of polymers have yielded a catalog of materials performance. However, detailed studies on the degradation mechanisms are conspicuously absent. This is further complicated by the fact that some materials, specifically fluoropolymers, widely used for their chemical resistance, perform very poorly in a tritium envi- ronment. It is well known that fluoropolymers generally display poor stability in any radiation environment[23], however, in tritium service the problem is magnified due to the formation of tritiated hydrofluoric acid byproduct which is highly corrosive and toxic[24]. Figure 4 shows PTFE and Nafion fluoro- polymers that have been exposed to tritium during accelerated aging tests. Both polymers show significant signs of degradation due to the beta decay of tritium to Helium-3. With respect to fusion energy, there is a significant need for improved polymeric materials with higher resistance to tritium degradation due to the near continuous operation and economics of bringing power to the grid. Research groups are beginning to develop new polymers for fusion applications based on the needs in fusion energy systems. Initial efforts suggested that increasing the aromaticity of the carbon filler in commodity polymers can improve their resistance to degradation in a tritium environment. However, these materials are still not suitable for long-term service in a high concentration tritium environment. More recent work is focused on developing new polymer backbone formulations to further increase resistance to tritium degrad- ation. Figure 5 shows new polymer materials that exhibit improved resistance to tritium exposure. Further development of tritium-resistant polymers for fuel cycle applications is needed[25]. Other joining and sealing materials. A fully functional power plant would also involve use of a variety of soft materials such as potting materials, sealants, and adhesives within optical, diagnostic, mechanical, and electrical assemblies. These materials must be evaluated for functional survivability under the unique thermal and radiation environments to which they would be exposed based on their locations and shielding. Several studies have shown, for example, that commonly used polymers such as polydimethylsiloxanes, polyvinylformal, and polytetrafluoroethylene are susceptible to radiation-induced property changes such as reduced adhesion and failure loads as well as changes in modulus[26-30]. Understanding of the degradation conditions, as well as mechanisms involved, may en- able design and development of new damage-resistant soft materials that allow long-term performance under these extreme conditions. ~AM&P Note: For additional reading on high-Z layer beam mitigation, fast ignition, and 2PP ICF capsules, the LLNL staff recommends Ref. 31-36. Portions of this work were performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and partially funded by the LLNL IFE Institutional Initiative and the LLNL LDRD program under tracking code 23-FS-018 and 23-ERD-027. Fig. 3 — Hydrogen isotope exchange of polymer precursor molecules. Reproduced from Ref. 21. Fig. 4 — Effects of tritium exposure on polymer materials: (a) unexposed PTFE; (b) PTFE exposed to tritium; and (c) Nafion exposed to tritium. Fig. 5 — New polymeric materials in development that have shown improved irradiation resistance.

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