We did a lot of work on this in the early 80's. Thought it was a great idea! Turns out it wasn't so hot, since you had to deal with a lot of holes, and the heat dissipation was a problem.
Nevertheless, we did a lot of analysis, and some drilling at Bruce. That's how I got to know the site so well.
I also did some dynamic modeling, since the extra heat might cause rock spall and rock bursts.
As we all know, I decided that the Bruce site could never support an underground structure, so we walked away from all this silliness.