You warm up a piece of metal and pour water on it. The water keeps it cool and makes steam.
Also, when dealing with that much high energy radiation, your metal has a tendency to wondrously become another metal, with all the problems that comes with.
we want a lot of water! That's a lot of energy!
> Also, when dealing with that much high energy radiation, your metal has a tendency to wondrously become another metal, with all the problems that comes with.
That's kinda problem with anything fusion or fission, wherever neutrons hit things get weird. Probably much worse for fusion tho.