Beryllium acts as a neutron multiplier: 9Be+n→24He+2n.
The consumable Be is supplied either as a solid metal layer, or as part of the molten salt (such as FLiBe) coolant, or both. The right amount of Be in the path of D-T neutrons theoretically allows the fusion reactor to have a Tritium Breeding Ratio (TBR) greater than one. That's the good news!
Speaking practically, this hasn't been shown to work yet, and my understanding is that it is a must-have for commercial D-T fusion. Recycling tritium within an operating D-T fusion plant (and beyond that, increasing global T inventory to bootstrap / enable the world to bring new fusion reactors online) is one of the key remaining engineering challenges to viable D-T fusion power, and is an active research area. The greater-than-unity breeding side of the problem is only the first challenge; separating, extracting, and capturing the freshly baked tritium is also an unknown. But we've known about this problem for a while, and there is room for optimism; ITER, for example is planning to test 6 different designs for tritium breeding blankets.
Beryllium acts as a neutron multiplier: 9Be+n→2 4He +2 n.
The consumable Be is supplied either as a solid metal layer, or as part of the molten salt (such as FLiBe) coolant, or both. The right amount of Be in the path of D-T neutrons theoretically allows the fusion reactor to have a Tritium Breeding Ratio (TBR) greater than one. That's the good news!
Speaking practically, this hasn't been shown to work yet, and my understanding is that it is a must-have for commercial D-T fusion. Recycling tritium within an operating D-T fusion plant (and beyond that, increasing global T inventory to bootstrap / enable the world to bring new fusion reactors online) is one of the key remaining engineering challenges to viable D-T fusion power, and is an active research area. The greater-than-unity breeding side of the problem is only the first challenge; separating, extracting, and capturing the freshly baked tritium is also an unknown. But we've known about this problem for a while, and there is room for optimism; ITER, for example is planning to test 6 different designs for tritium breeding blankets.