Using the robots shown? To get the angles needed as the down starts really curving inwards, the robots would seem to have to be like spiders, hanging out over the edge squirting out lava at an angle.
Note that in that video, all of the cement is laid down purely vertically. There are a few places where the cement is laid down over an existing substrate, metal mesh and lintels and the like, but nowhere does it show "sideways" printing. It's always directly atop something solid.
Depends on what you mean by nuclear. If you have a 100 kilowatt reactor, that's great and all, but how many kilos of sand per hour can you melt with the electricity you get out of that?
I kinda wonder if the better approach might be actual concrete. Water is of course at a premium, so perhaps it would be best to manufacture the domes in segments inside sealed chambers which can be kept warm and which can extract the water from the cured slabs. Then use a crane to lift them into place and cement them together (obviously losing *that* water) and then pile dirt on top of that.