The blanket also has to have a high cross section for elastic collision with neutrons to efficiently capture their energy as heat. ITER is planned to have liquid lithium as this blanket because it has a high cross section and the odd neutron capture will result in helium and much desired tritium that can be extracted for use as the main fuel in the reactor. Unfortunatey, elementary lithium is a very temperamental material.
I heard a story a while ago about a facility working with a lot of molten sodium. The whole building had no running water at all, no bathrooms even, because water and sodium mixing is a problem.
Lithium doesn't seem a whole lot safer.. but I don't think a fusion power plant could be water free. As far as I'm aware there is no more efficient method of using heat to spin a generator than a steam turbine. A lithium/water heat exchanger seems pretty frightening. Do they have a better plan for that?
Haven't looked at the proposal, and not commenting either way on the risk of using water in this context, but the water/reactive-metal issues have been widely investigated in other liquid metal reactor designs and generally intermediate cooling loops with some benign medium are used.
Edit to expand; having now looked it seems some of the intermediate loop designs have the SAME material (e.g. sodium). This at least means the loop at risk of mixing with water is isolated from the core (where an explosion is difficult to manage). I thought I'd seen other designs, just can't find them now.
From somewhat outdated memory: lithium is thermodynamically less stable in the presence of water, but the reaction is slower than it is for sodium. (And so on: potassium and cesium react even more violently with water.)
But this is largely moot, as lithium in salt form (Li+) does not react with water. I would be more concerned about the beryllium salt dissolving in water and escaping. Beryllium is nasty.
If memory serves, lithium is more reactive than sodium. ITER will have some kind of heat exchanging system to cool the lithium. But I don't know what that secondary system uses.
MIT's design uses molten FLiBe salt as the blanket. It's chemically stable, and the beryllium multiplies the neutrons for breeding with the lithium.
The inner wall of the reactor is 3-D printed, and the reactor uses jointed superconducting tape that allows it to be opened up annually to replace the inner wall.
MIT's design also uses 40% of the world's annual mine production of beryllium, for a single reactor. Using the entire estimated Be resource in these reactors would supply maybe 1% of the world's primary energy demand.
That was once said about lithium and batteries. Typically once the market demand is there we suddenly find a bunch of viable reserves to tap. This is tragically happening with petroleum where we're tapping incredibly difficult reserves via deep sea drilling and fracking.
I don't think asteroids would actually be a good source of Beryllium. IIRC they have approximately the same overall elemental distribution as the earth does, just more evenly distributed - which is great for finding dense materials like gold or rare-earth metals that have sunk towards the earth's core, but not so great for one of the lightest metals in existence.
Gold and the like have sunk into the core not because they are heavy, but because they chemically partitioned into a phase (liquid iron/nickel) that was heavy.
Uranium, which is also heavy, partitions into light silicate minerals, and is enriched in the Earth's continental crust by 3 orders of magnitude above the average of the planet. Without this concentration fission energy would likely never have been considered as an energy source.
It is my understanding that beryllium is concentrated in the Earth's crust by about a factor of 100 over carbonaceous chondrites. Beryllium is actually very rare on a cosmic scale, since it is one of the "X-process" elements made by cosmic ray spallation, not in stars.
