Scientists have identified the specific chemical mechanism responsible for high levels of tritium retention in lithium-based plasma-facing components (PFCs). In a study published in *Nuclear Fusion*, researchers from General Atomics, Princeton Plasma Physics Laboratory (PPPL), and the University of Tennessee, Knoxville, demonstrated that deuterium plasma exposure causes the formation of lithium deuteride on the PFC surface. This compound, which would be lithium deuteride-tritide (Li(D,T)) in a D-T reactor, is then buried by subsequent layers of evaporated lithium, effectively sequestering the fuel isotope within the wall material. This finding clarifies a long-standing question about the interaction between lithium and hydrogen isotopes under fusion conditions. Source: General Atomics / DIII-D

The experiments were conducted at the DIII-D National Fusion Facility using the Divertor Materials Evaluation System (DiMES) probe. This diagnostic allowed researchers to expose solid and liquid lithium samples to the high-heat-flux conditions of the tokamak's lower divertor. Post-exposure analysis of the samples confirmed that the trapped deuterium was not merely dissolved but was chemically bonded within a stable deuteride compound. According to the published results, this chemical trapping is highly efficient, with lithium capable of absorbing deuterium up to a 1-to-1 ratio with lithium atoms. This high retention rate presents a significant challenge for tritium inventory control in future power plants. Source: General Atomics / DIII-D

The experiments were conducted at the [DIII-D National Fusion Facility](/programs/diii-d-national-fusion-facility) using the Divertor Materials Evaluation System (DiMES) probe.

Lithium is a compelling candidate for PFCs due to its ability to improve plasma performance. Its low atomic number (Z) minimizes plasma cooling from sputtered impurities, and its chemical reactivity helps pump residual hydrogen isotopes and oxygen, leading to cleaner and more stable plasmas. Previous experiments have shown that lithium walls can reduce recycling and enable access to higher-performance plasma regimes. However, the same chemical reactivity that provides these benefits also drives the tritium retention problem. Understanding the precise mechanism is the first step toward developing mitigation strategies, such as operating at temperatures high enough to decompose the hydride compounds or developing novel PFC designs. Source: General Atomics / DIII-D

This work has direct implications for the design and operation of future fusion devices that plan to use lithium, including the development of a viable tritium breeding cycle. An unacceptably high rate of tritium retention in the walls would complicate the fuel cycle, increase the required tritium inventory, and pose safety and regulatory hurdles. The DIII-D findings will inform materials science research aimed at controlling the surface chemistry of PFCs. Future work may focus on lithium alloys or engineered surfaces that retain the plasma-conditioning benefits of lithium while minimizing the formation of stable hydrides, ensuring a closed and efficient fuel loop for commercial fusion energy. Source: General Atomics / DIII-D