I've been inside those type natural draft water cooling towers. It is hard to imagine anything in there being the source of the explosion.
The hot water is sprayed in about 1/3 of the way up from the bottom. The spray continues down across a multitude of plates, dishes and other material designed to give the splashing water a convoluted path falling back to the bottom.
The splashing, spraying water warms the air, giving up heat from the water. The natural rising of the heated air draws in more air from the open bottom. The curving shape creates a speed up to the rising air drawing in air faster and giving more cooling.
In short, that is a big open concrete tube with a bunch of sprinklers in the bottom third. There is no containment vessel or sources of ignition inside it. There is one heck of a draft and an incredible amount of "fog".
That picture doesn't look like a stain to me, it looks like a place where water is seeping out from the inside spray.
Ahh. Water saturating right through the thin concrete. Interesting theory. Looks a little dark for that, but maybe. I saw pictures of the other cooling towers, and it looks like they can exhibit very dark streaks if it's been raining.
I didn't know that they actually fill up any of the interior volume of the tower with the evaporator material and spray heads; I thought they were all arranged around the outside perimeter of the tower, at ground level. Thanks for the information.
Someone told me (long ago) that the concrete at the narrowest part of the tower is only two inches thick or so. It's easy to see vertical rebar in the damaged tower.