Fusion reactors have gotten a lot of attention over the last few years. The recent excitement seemed be kicked into high gear when Commonwealth Fusion Systems announced in late 2021 that they had closed on a $1.8B funding round led by Tiger Global Management. Over the next few years, many other fusion companies were born and followed suit, with large seed rounds and early-stage funding rounds. Nearly all of these fusion efforts were spinoffs from various university programs, although some more recent ones were not.

Most of these fusion efforts have also centered around the deuterium-tritium (D-T) approach for fusion. There’s a good reason for this. The D-T reaction has the highest cross section at the lowest energy, meaning it is the most likely fusion reaction to proceed at a given plasma temperature. But the D-T reaction has two big drawbacks. First of all, it is dependent on a fusion fuel (tritium) that is very rare and very expensive. Secondly, the D-T reaction releases 80% of its energy as a high energy (14.6 MeV) neutron that does not contribute to the heating of the plasma. Since neutrons are uncharged particles, they cannot be confined by electric or magnetic fields and sail right out of the plasma.

The strategy to mitigate these challenges is a “breeding” blanket in the fusion reactor, and in more recent D-T reactors designs, and especially those that plan to use high-temperature superconducting magnets, that breeding blanket is planned to be based on FLiBe salt. Dr. Charles Forsberg of MIT gave a very interesting talk about this in 2019 at Oak Ridge:

I took a fusion engineering class at Georgia Tech during my graduate studies, and knew about many of the challenges of magnetic confinement fusion, and of D-T fusion in particular. Since learning about molten-salt reactors, I had often wondered if lithium-beryllium fluoride salts had been considered as the breeding blanket for a fusion reactor. I reasoned that LiF-BeF2 already contained two of the important ingredients for that blanket. It had lithium, needed to breed new tritium, and it had beryllium, which could serve as a neutron-multiplying material. But it did not seem like LiF-BeF2 had been favored as a fusion blanket material.

After I watched Dr. Forsberg’s presentation, it seemed that not only would FLiBe be a suitable breeding blanket material, it might be the ONLY suitable breeding blanket material for these newer reactor designs that had higher magnetic field density.