PNNL

The Science

Researchers studied the behavior of lithium silicates under ion irradiation at high temperatures to emulate the conditions after one year inside the SlimCS DEMO fusion reactor. They first fabricated pellets of lithium orthosilicate (Li₄SiO₄) with lithium metasilicate (Li₂SiO₃), then irradiated the pellets with Si⁺, He⁺, and D⁺ ions at temperatures up to 500°C. Analysis of the irradiated materials shows that deuterium (and inferred tritium) release from the pellet is very efficient during ion irradiation at 500°C. This property is well suited for fusion blanket applications. The study also discovered that Li₂SiO₃ may be more irradiation resistant than Li₄SiO₄. In addition, their research suggests that coating lithium ceramics with pure nickel can minimize lithium loss under irradiation conditions. 

The Impact

Tritium is a fuel for fusion reactors but must be produced on-site. This heavy isotope of hydrogen can be sourced from lithium—a naturally abundant element. However, finding a source material that displays good structural stability, rapid tritium release, and low lithium volatility under the extreme conditions of a fusion reactor remains a challenge. This research provides evidence of microstructural and compositional evolutions of lithium silicates under irradiation conditions. Furthermore, it shows that coating the ceramic with pure nickel may minimize lithium loss—indicating the combination of materials may be a suitable candidate for fusion applications. 

Summary

This study demonstrates the microstructural and compositional evolutions, behavior of deuterium release, and lithium loss from lithium silicates under emulated fusion conditions. Researchers fabricated pellets of biphasic Li₄SiO₄ and Li₂SiO₃ using hot pressing of milled powders. They then sequentially irradiated the samples with Si⁺, He⁺, and D⁺ ions at temperatures up to 500°C to emulate the burnup of ⁶Li after one year inside the conceptual SlimCS DEMO fusion reactor. 

Structural analysis results reveal that the crystalline Li₄SiO₄ phase was not observed in the irradiated depth region in this study. However, under the same conditions, Li₂SiO₃ remained crystalline in the damage peak region, suggesting that Li₂SiO₃ is more irradiation resistant than Li₄SiO₄. Data from this study also show that deuterium (and inferred tritium) release from the pellet is very efficient during ion irradiation at 500°C, accompanied by significant lithium loss. By thermal annealing the room-temperature irradiated pellets for 10 min at 500°C, the researchers achieved nearly complete deuterium release without an observable lithium loss. Additionally, they studied using nickel as a coating to minimize lithium loss without a significant impact on tritium diffusion and release. Their results indicate that nickel facilitates fast deuterium (and thus tritium) diffusion while providing a high energy barrier for lithium diffusion.

A portion of this work used the facilities of the Environmental Molecular Sciences Laboratory, a Department of Energy, Office of Science user facility located at Pacific Northwest National Laboratory.  

Contact

Weilin Jiang
Pacific Northwest National Laboratory
weilin.jiang@pnnl.gov 

Funding

The lithium silicate study was supported by the Department of Energy, Office of Science, Fusion Energy Sciences program (FWP 13784) and study on the effect of Ni coating was supported by the National Nuclear Security Administration.