One of the ongoing research issues for commercial
fusion power development is the choice of material for the plasma-facing portions of the reactor vessel, also known as the first wall. Most reactors operate at the equivalent of a high vacuum and thus demand high-strength materials to resists the inward pressure of the magnets against the empty interior. Typical materials are those used in other chemical and atomic processes, like various steel alloys. Unfortunately, these same materials have a number of disadvantages when used in fusion reactors. One major problem is that when escaped fusion fuel hits the material it cools, returning to the fuel mass at a lower temperature and cooling the fuel as a whole. This is known as "recycling". The other is that these reactions can also spall off metal atoms, and due to their high atomic mass, or "high-Z", when they are heated they give off copious amounts of
X-rays which also cools the plasma fuel. One of the attractive features of a liquid lithium PFC is that it virtually eliminates recycling. This is because lithium has a high chemical reactivity with atomic
hydrogen, which is then retained in the PFC. In addition, lithium has a low atomic number, Z. This gives the lowest possible energy loss by radiation from PFC material that may end up in the plasma, because radiation increases strongly with increasing Z. Finally, flowing liquid lithium can also potentially handle the high power densities expected on reactor walls. ==Tokamak performance==