I don't remember what I ate last night
but I can spout off obscure rules to all sorts of game like nobody's business!
We ran into this question last evening and I've been looking for this answer as well. Here was our situation.
Early in the game we had a hazmat explosion, I pulled a hot spot from our board supply and put it on that space. Three hazmat remained on the board.
Later in the game, we had exhausted the hot spot supply on the board and had another hazmat explosion. At this point, being unable to discern in the rules what we were supposed to do, I figured grabbing from the board was a mistake and replenished the board supply (back to one hot spot from the first explosion) and then put a new hot spot from the box on the space of the second explosion.
My thinking was this: With no hot spots available on the board, we could not complete the rules for a hazmat explosion, so perhaps they are not supposed to come from the board supply..
So.. if I was incorrect.. Do you skip that step when a hazmat explosion occurs once the supply has been depleted?
If a hazmat explodes and you have no more hotspots in the supply, skip that step; you don't have to place a hotspot down onto the board.