@MolbOrg another thing i should probably mention about the lava tanks for TESS units - the pipe loops drawing heat for the turbines start 5 m from the walls and floor. Between there and the actual walls of the tank is packed regolith fines - which as you pointed out, have excellent insulation properties. The heat the tank structure is subjected to is not high, and heat loss is reduced by this method.
Sure, it makes the tanks even heavier - but that is what is so wonderful about working with 1/6 g. At the heat loads those walls need to bear, they can be made out of poured stone like most thi…